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If the universe is expanding how can there be collisions of galaxies?

Edwin Hubble discovered that the universe is expanding by observing that all but the closest galaxies were moving away from us, using a phenomenon called the Doppler effect. Furthermore the farther a galaxy is the faster it moves away from us. This is very important because it shows that each galaxy would see the same effect, of galaxies moving away from them with an increasing speed with distance. Therefore we conclude the universe is expanding.

The expansion of the universe is due to the expansion of space itself, and this can be thought of as a force dragging galaxies away from each other. However, on small enough scales gravity wins over this force, and is able to cause collisions of galaxies. The same is true in galaxy clusters which are huge groups of galaxies which are gravitationally bound together, even though the expansion of the universe is trying to pull them apart. And of course the same can be said of smaller scales, such as inside our own galaxy or Solar System.

In fact the Milky Way and the Andromeda Galaxy (our closest large neighbour) are currently on a collision course, which will happen about 4-5 billion years from now.

If space is made of hydrogen would a tank of oxygen in space react to form water?

Space is actually not made of hydrogen. Space is essentially a vacuum meaning it is not made of anything, it only contains everything we are familiar with (all the matter and energy). Hydrogen is however the most common element in the universe as it makes up about 75% of normal matter (in terms of mass). Therefore most of the objects in the universe (for example stars, planets, gas clouds and galaxies) are made up of mostly hydrogen.

A tank of oxygen gas taken to space and opened would result in the oxygen escaping the tank quickly due to the lack of external pressure which on Earth is caused by the atmosphere. The oxygen molecules however will remain intact and won’t react with anything (at least initially).

What do observations of the Cosmic Microwave Background tell us about where the universe started?

The Cosmic Microwave Background (CMB) is radiation in the universe which was released about 13.8 billion years ago (approximately 380,000 years after the Big Bang). Because we see that this radiation is practically the same no matter which direction we look, we know this release occurred roughly simultaneously everywhere in the universe. Therefore on large enough scales the universe must be the same everywhere.

Part of what this shows us is that the expansion of the universe is not caused by galaxies moving away from each other, but rather by the expansion of space itself! As a simple analogy think of a deflated balloon with dots drawn on it. When the balloon fills with air the surface of it stretches which causes the dots to move away from each other. One can see that the dots don’t actually have any mobility of their own, but rather the expanding rubber caused them to move away from each other. Similarly in space, as it expands it carries all the galaxies with it.

Imagine now reversing the expansion of the universe back in time. Eventually we would reach a situation in which all of the points in space ‘fall’ right on top of each other. Even though this might seem to suggest that the universe originated from this one spot, because location is based on points in space, this event (the Big Bang) actually occurred everywhere in space.

The reason for this is that the Big Bang was the event that created space (and time). At that moment all of points in space were on top of each other and have been moving apart ever since.

Could some medium be slowing down light in space therefore affecting our measurements of distance and speed of galaxies?

The methods we use to measure distances and speeds in the universe don’t rely on the speed of light, and therefore would not be affected by such a medium.

For example, stars known as Cepheid variables appear to change brightness with very regular periods. Many observations of these types of stars have shown that the period with which their brightness changes is related to the total amount of light the star emits (its luminosity). Therefore we are able to use this relationship to determine the star’s luminosity simply by seeing how long it takes its brightness to change. Luminosity, when combined with brightness, tells us the distance to the star. If the speed of light happened to change while light was moving from the star into our telescope, all of the light from the star would be affected equally, and therefore we would still observe the same effect and obtain the same distance.

In order to determine the speed that a galaxy is moving away from us, we take a spectrum of it. Spectra are obtained by splitting the light from an object, in this case a galaxy, into the various wavelengths light come in (similar to splitting white light using a prism to obtain a rainbow). As light leaves the surfaces of stars it interacts with chemical elements in the outer layers of the stars themselves, in gas clouds and other locations in the galaxy. This interaction leaves a signature in the spectrum, which are unique to each element, and therefore are seen at specific wavelengths. If in the spectrum we see that the chemical signatures are all shifted compared with where they should be, we conclude the galaxy is moving. This effect is called Doppler shift. Once again a change in the speed of light as it makes its way into our telescope would not influence the location of the chemical signatures.

Since light has a limited speed, could it have a limited distance it can travel?

We have no evidence to suggest that it does. If this were true you’d expect that light wouldn’t just disappear once it reached a certain distance, but rather the effect would be gradual. Since light is energy one can think of the energy decreasing as light travels through space, causing the radiation to become less and less noticeable (effectively disappearing).

This idea was previously proposed as an alternative explanation for the observed energy loss of light as it travels to us through the expanding universe. We observe this effect due to light reaching our telescopes with lower energy than what it was emitted, and is caused by the expansion of space diluting the energy contained in the radiation as it travels to us. However some astronomers wanted to propose an alternative mechanism for this observed phenomenon which did not involve the expansion of space, since they did not believe it to be occurring. What they came up with is the Tired Light Hypothesis, which you can read about on Wikipedia.

However this theory was shown to be inconsistent with many observations we’ve made.

What is dark matter?

Dark Matter (DM) makes up about 80% of all the mass in the universe, however we don’t know what DM actually is. But we do have substantial evidence to suggest that this mysterious form of matter does exist.

DM does not interact with light, or it does extremely weakly, therefore we must rely on its gravitational influence to detect it. Here are two examples:

When we measure how fast stars orbit the centre of our galaxy we expect to find that their speed decreases with distance. This is because the force of gravity becomes weaker at larger distances, and therefore the speed that stars can move becomes smaller. This is the reason why planets farther away from the Sun move slower. However what we actually find is that the speed stays roughly constant with distance. Our conclusion is that there must be extra mass present which we are not seeing, but it is contributing to the gravitational force. This effect is observed in many galaxies, not just our own.

Another example comes from measurements of gravitational lensing effects, which is the apparent bending of the path of light as it moves close to a massive object. Around large groups of galaxies, called clusters, we expect to find distorted images of galaxies that are farther away. However when we use these distorted images to measure the mass of the cluster we obtain a much larger mass than can be accounted for by all of the stars and gas that we can see, often by as much as 10 time more!

This missing mass is what we call Dark Matter.

There are more observations which lead astronomers to conclude that the majority of the mass in the universe is of an unknown type. And although there are some ideas as to what it could be, ranging from micro black holes to particles called neutrinos, the reality is that it could be a new type of particle that we have never even thought about before.

What is dark energy?

Dark Energy (DE) makes up nearly 100% of all the energy in the universe. But if we include matter in the calculation, the number only drops to 70%. Meaning roughly 70% of the entire universe is some form of energy that we know very little about.

The evidence for the existence of DE is more subtle than for Dark Matter. Mainly it comes from observations of supernovae type Ia. These events are explosions of objects called White Dwarfs, which are the leftover cores from stars like the Sun. These explosions are incredibly energetic and can be seen nearly through the entire observable universe! But they also have an extremely useful feature. They all emit roughly the same amount of light. Therefore if we see such an explosion we can compare how bright it appears to us on Earth with how much light it actually emits and obtain the distance to the supernova. By comparing the distances of many such supernovae with theoretical models for the evolution of the universe we find that the universe is currently accelerating in expansion.

That is weird because given that gravity attracts everything, including space itself, the expansion of the universe should in fact be slowing down. This acceleration must be caused by a bizarre source of energy which ‘pushes’ space. We call this Dark Energy.

Unfortunately there are currently no widely accepted suggestions as to what makes up Dark Energy.

The expansion of the universe adds dark energy. What about conservation of energy?

It is true that according to our current understanding of dark energy, its density across the universe remains constant with time. Therefore as the universe expands more dark energy gets created (from where is unclear).

Unfortunately it is unclear whether dark energy is a form of energy in the same way that, for example, kinetic energy is. The name was given because dark energy shares certain properties in common with energy, at least on a universal scale. Also, since at the time of the discovery of dark energy dark matter has already been known, it was a natural progression to call this additional ‘dark’ component of the universe an energy.

All we can say is that dark energy acts as a pressure force on space and causes it to expand. And so it doesn’t necessarily need to follow any conservation laws.

Could the Big Bang be explained by extremely fast rotation of the universe, through relativistic effects?

This question deals with the effects predicted by Special Relativity, that the faster an object moves the larger its mass becomes and the smaller it gets. Therefore if the universe was originally spinning at a speed close to the speed of light, perhaps the mass of it grew to what we see in the universe today and the size was extremely small as is predicted regarding the early universe.

It is very important to understand that the above effects are relativistic, meaning they are only observed phenomena from the perspective of somebody looking at the moving object. For example the changing of length, which is called length contraction, would cause me to measure a meter stick you are holding while travelling very quickly to be shorter than a meter. However simultaneously the meter stick would appear perfectly normal to you, since it isn’t moving relative to you.

Similarly with mass, we can distinguish two different versions of mass in Special Relativity. There’s the rest mass of an object, which is the mass you would measure in the same reference frame as the object. This does not change, no matter how fast the object moves. It is basically an inherent property of the object. Then there is the relativistic mass, which changes depending on how fast the object is moving relative to you. Basically the reason there is a change is that we take into account the total energy of the object. That is equal to its rest energy (being converted from the rest mass) plus the kinetic energy and converting it to a mass using E = mc^2. Therefore it is not an actual measurement of mass.

Therefore, although the fast rotation would result in a high relativistic mass, there would still need to be the same amount of mass we have in the universe currently, since that’s a measure of the rest mass. And regarding the effect of length contraction, the universe would appear its natural size to anything rotating around with it, therefore this would not work for objects inside the universe.

Could a massive boundary around the universe cause the accelerated expansion of the universe?

As discussed in the answer to the question “What is dark energy?” (above) the universe is expanding at an accelerated rate. One might suggest that instead of an internal force pushing the universe to accelerate in expansion, which we believe is caused by dark energy, perhaps there is an external force pulling the universe outward.

Gravity is an excellent source of pulling forces, however there are some issues with the idea that some sort of massive “wall” surrounds the universe and pulls on space gravitationally to accelerate it. The main issue is that the gravitational force of a spherical mass distribution on something inside of it is zero. This is a fairly typical result in physics. It is used when we want to explain the conditions deep inside stars, and it allows us to simplify the discussion by only considering the gravitational force due to mass within the radius we are interested in, for example.

Further issues are that we have no observations to suggest such a boundary exists, it would without a doubt leave some sort of imprint on the universe which would be detectable. Also, the existence of such a wall contradicts our current understanding of the structure and history of the universe.


What is at the center of the Milky Way?

It is thought that at the center of our galaxy lies a Super Massive Black Hole (SMBH), with mass roughly 4 million solar masses.  Surrounding this extremely massive object is a small star cluster. One of the main pieces of evidence for the existence of this SMBH is the extremely small and fast orbits of the stars around their common centers of orbit. Where we cannot see any sources, such as stars, for the existence of this gravitational force.

Check out the following YouTube video of a simulation of the orbits of these stars, based on inferred orbits from observations.

Why do the stars in the Milky Way move so harmoniously together?

The stars in the galactic disk, which is the main component of the galaxy, and the one containing the spiral arms, all orbit the center of the galaxy. Therefore, just like in the Solar System, where the planets orbit around the sun harmoniously, so do all of the stars, nebulae, Giant Molecular Clouds, and other members of the stellar “family”.

The Milky Way galaxy, and as far as we can tell all other spiral galaxies as well, has two other components. The bulge and the halo. These two components contain older stars and Globular Clusters (giant groupings of stars all orbiting a common center). In these two latter components the orbits are no longer in a nice plane, they are all randomly distributed .


What contributions do amateur astronomers have to the field of astronomy?

Amateur astronomers have always been an important part of the astronomical community. They have contributed in a variety of fields, from the discoveries of comets to asteroids and supernovae. With the advancement of automated detectors in professional astronomy, which are able to find the above mentioned much faster than an amateur astronomer could possibly do, the potential contribution of amateur astronomers may appear to be diminished. However, there are plenty of areas where amateur astronomers can still contribute.

The discovery of comets, asteroids and supernovae is still possible by amateur astronomers, but the impact of this is smaller than it may have been in the past. One of the most significant areas of contribution is in the observations of variable stars (stars which periodically change in brightness). The American Association of Variable Star Observers maintains a database of over 20 million variable stars. It is impossible for professional astronomers to observe all of these stars, and measure the various periods they may be varying in brightness by. This is where amateur astronomers, by their sheer number, have a major advantage over professional astronomers. With the advancement of technology, relatively sensitive equipment such as SLR cameras and even infrared or ultraviolet detectors, are now within reach of many amateur astronomers. This allows them to observe a large number of variable stars around the Solar System, thereby increasing our understanding of the variability periods of various stars.

Another project which allows anybody, even if you don’t own a telescope, to contribute and help the astronomical community is Galaxy Zoo. This is a Hubble project where you have the opportunity to classify galaxies, based on a variety of features such as spiral arms, disk shape, bulge shape and many more. Because classifying galaxies is still done best with the eyes, it is important to get a consensus among as many people as possible, only then can we be sure we have made the correct classification. The task is made difficult by there being many “weird” galaxies, and most galaxies are not pictured as nicely as you would usually see from released Hubble pictures.

The potential contributions of amateur astronomers are not limited to those listed here, for example, amateur astronomers with technical skills also help improve professional technology. And the Astronomical Society of the Pacific still awards the Amateur Achievement Award on a nearly yearly basis.

So keep observing!

How are pictures featuring long circular arcs of star paths possible?

The Earth rotates around on its axis with a period of about 24 hours (actually 23 hours and 56 minutes). It therefore appears to us on Earth that the stars are moving around circles all centered on the rotation axis.

In order to obtain pictures like the one shown below one needs to take a long exposure photograph. That will show the movement of the stars. In this particular image the arcs are not complete, since the exposure was not 24 hour long, but if someone were to do that each star would complete a full circle.

This particular image is courtesy of NASA – Astronomy Picture of the Day, and was taken from Namibia (in Africa and is hence of the south celestial pole) with a roughly 11 hour exposure.

In similar images of the north celestial pole you will notice that Polaris, the north star, also creates a small arc. That is because it is not positioned exactly above the rotation axis of Earth, but fairly close.

How accurate were ancient astronomers in predicting various phenomena?

Many ancient civilizations kept track of astronomical phenomena such as eclipses (Solar and Lunar), solstices (summer and winter) and planet movements. Typically their interests were for religious reasons, but practical application were also found, for example predicting season changes to help with farming efforts.

It was very important and useful for them to be able to predict events. And the ancient astronomers were able to do this quite well given that the astronomical phenomena which interested them were all repetitive. Therefore by tracking the movements of the Sun, Moon, planets and stars they realized that everything they were seeing would repeat on a well defined period.

This is what allowed them to build such observatories as Stonehenge, Mayan pyramids and Egyptian pyramids, to name a few. Each of these has crevices and alignments of stones designed to either allow light from the sun to peek through at a certain time (such as summer solstice) or point to a specific event in the sky (such as the rising of a certain planet).

You can learn specific details about ancient observatories at the following website, maintained by Stanford University.

What is the celestial sphere?

The celestial sphere is the projected image of all stars visible from Earth onto an imaginary sphere centered on Earth. The existence of the celestial sphere is purely a matter of perspective, and does not represent any physical structure. Although stars may appear close to each other on the celestial sphere their physical distances from one another might be huge in three dimensional space. The appearance of a star on the celestial sphere depends on its visibility from Earth, and therefore depends on its intrinsic brightness (luminosity) and distance.

The celestial sphere is where we find all of the constellations. Given the tendency of the human mind to find patterns and structures resulted in the view that the stars form figures which were typically of cultural significance to the observer. But we now understand that it is just a matter of this two dimensional projection of the stars visible to the naked eye from Earth onto the celestial sphere, which makes it seem as if the stars are related to one another.

What are constellations and how are they determined?

Constellations, or asterisms as they are officially called, are groups of stars which appear near each other on the celestial sphere (see related question). Since constellations are based on our own visual perspective, the specific connections of stars and the resulting figures are arbitrary and personal. In fact the stars which make up the constellation are, in most cases, completely unrelated to one another in three dimensional space.

There are 88 constellations recognized by the International Astronomical Union (IAU), 48 of which are based on Ptolemy’s, a 2nd century Egyptian/Greek astronomer, recordings. These constellations include the zodiacal constellations most of us are familiar with, as they describe the zodiac signs used in astrology. According to the IAU a constellation is a region in the sky with specific boundaries, which typically is located around an asterism which is the specific stars which form the figure (what is typically call a constellation).

The main function of constellations today is to allow for easy communication regarding observed objects among astronomers. For example one astronomer might say that they are observing a planetary nebula in the constellation of Orion. The other astronomer would right away know what region of the sky is in question. Of course to locate the object would still require knowing its celestial coordinates.

Most of the constellations recognized by the IAU originate from Greek mythology, primarily the 48 “Ptolemy” constellations, the remaining ones are constellations described by various European explorers who saw these stars for the first time as they traveled to the southern hemisphere. However many cultures around the world observed their own constellations, based on their own sets of beliefs and cultures. For example the Chinese, the Native Americans, as well as many other cultures, since most had a fascination with the night sky.

For a list of all the constellations recognized by the IAU see the following link.

What options are available to do telescope observing in Toronto?

Here at the Department of Astronomy and Astrophysics at the University of Toronto we run a free public tour on the first Thursday of every month, except for January. The tours start off with an hour long lecture which covers a different topic each month, followed by an hour of telescope observing and planetarium shows. Please see the public tour website for more information.

There are plenty of other organizations around the Greater Toronto Area which offer free astronomy tours and/or observing nights. Here is a list, which is definitely not complete, of organization and their websites:

The David Dunlap Observatory offers observing nights –

The North York Astronomical Association –

The Stargazer’s Group of Mississauga –

The Royal Astronomical Society of Canada hosts various events. Here’s the link to their Toronto center –

The Toronto Astronomy Club –

The York University Astronomy Club –

Are satellites visible from Earth?

The number of satellites in orbit around the Earth is estimated to be several thousand, the exact number is hard to count as there are many satellites that are no longer operational and it is unclear whether they remain in orbit or not. Also some of the satellites have been launched in secrecy for various purposes (such as military). However many of them are visible from Earth. Be it because of lights on the satellite or due to reflective surfaces on them (solar panels or equipment for example).

If you go out on a clear night away from city lights among the stationary stars you will notice on occasion a moving source of light. These look somewhat similar to a plane in that it might have a light attached to it, but definitely you can distinguish it from a plane at least by the lack of noise. Given the large number of satellites in orbit you can spot one quite frequently, up to several an hour, depending on your location.

Another object of interest is the International Space Station (ISS) which is larger than typical satellites and is also closer (meaning it orbits the Earth about every 90 minutes). You can go on the NASA website to find information regarding when to go out and where to look in order to see the ISS.

Where is the best place to view Aurora Borealis (Northern Lights)?

Aurora Borealis (Northern Lights) happen when charged particles (such as electrons and protons), which fly towards the Earth from the Sun (released by solar flares, solar wind, or coronal mass ejections), are diverted towards the polar region by Earth’s magnetic field. There these particles collide with the atmosphere at high speed, and excite molecules such as oxygen and nitrogen gas. These excitations produce the light that we see.

Although Aurora Borealis occur on a regular basis, they can occur more frequently at the peak of the solar magnetic activity (see the question regarding solar flares), and the effect can increase if a strong solar flare is the source of the charged particles. Really strong Auroras could be seen from latitudes as low as 40 degrees North, or perhaps lower. In North American this means Southern Canada and perhaps Northern USA could be lucky enough to see them.

The biggest obstacle to seeing Auroras is light pollution from cities (of course cloud cover makes seeing them impossible). Therefore, to increase the chances of seeing them, and to improve their view, it is recommended to be as far away from cities as possible. And hope for clear skies.

What is the best time to view Aurora Borealis (Northern Lights)?

The straightforward answer to this question is that whenever a particularly strong solar flare or ejection is predicted to hit the Earth, the chances of seeing the aurora is higher. They also become visible from latitudes that normally wouldn’t see them. One thing to keep in mind is that because aurora are very faint, they cannot be seen during the day.

The Geophysical Institute at the University of Alaska Fairbanks tracks aurora activity, and they produce maps of where these can be visible. See their website here. Furthermore is a good source for predictions regarding solar activity which may result in aurora. Their predictions are based on observations made with various telescopes, for example the SOlar and Heliospheric Observatory (SOHO).

As discussed in the question above ejections of high energy particles from the Sun are responsible for the occurrence of Northern Lights. The frequency and strength of these ejections changes based on the Sun’s magnetic activity cycle, which has a period of about 11 years. In late 2013 and early 2014 the Sun will be going through its activity maximum, therefore the chance of seeing aurora is higher.

So do try to go out and see this spectacular display of dancing colours across the sky. Just remember to be patient, bundle up if going out in winter, and make sure to avoid looking at any sources of light (such as phones or flash lights) since you want your eyes to adjust to the darkness as much as possible. Good luck and enjoy!

Relativity and Gravity

Can the force of gravity become repulsive?

We are familiar with the gravitational force to be an attractive force. But unlike electromagnetic forces, gravity cannot be repulsive, even at small distances.

The reason the universe is expanding on very large scales, is not the repulsiveness of gravity. The universe is expanding due to its initial momentum it gained from the Big Bang.

Why do astronauts in the Space Station appear not to be influenced by gravity?

Gravity is a force with an infinite range, therefore no matter how far two objects are from one another they are still attracted gravitationally. Even though the strength of the force decreases with distance.

An object in orbit is attracted gravitationally to the central body, but because it has forward motion the force of gravity continuously bends the path of the object. A simple way of thinking about this is by imagining the orbiting object to be continuously free falling towards the central body, but due to its forward speed it keeps ‘missing’ the surface.

Astronauts in the Space Station are all ‘free falling’ along with the Space Station itself, and they are all doing so identically. It therefore seems to us as if they do not feel gravity because they are able to float around in the Space Station. Within this ‘free falling’ path around the Earth, an astronaut is able to push themselves around the Space Station without having much effect on the motion of the much more massive Station.

What is the reason light cannot escape the Event Horizon of a Black Hole?

In Newton’s picture (what we call classical mechanics), gravity acts as a force between two objects which have mass. And since photons (the particles of light) are massless they are not affected by gravity.

In General Relativity (GR) the picture is very different. Massive objects bend space around themselves in a manner which depends both on the mass of the object and the distance away from it. Other objects such as planets, stars, and photons move on paths that are straight in this bent space. For example the space around the Sun is bent in such a way that the Earth, following a straight path appears to be going around a circle centered on the Sun.

What happens at the Event Horizon is that space is bent in on itself such that any object trying to move outward away from the Black Hole ends up following a path which leads it back into the Black Hole. That is why the Event Horizon is black (and Black Holes get their name) because even light cannot escape.

An interesting fact is that if you were indestructible you could hide inside a Black Hole and no one would ever see you, although you would be able to see everyone. The problem is you would never be able to get out as well.

What is the force of gravity according to General Relativity?

Special Relativity tells us that space and time are a part of one entity which we call spacetime. This means that time acts as a fourth dimension to the space we live in, and is an active component in our universe. This is contrary to the earlier belief that space was where events happened and time was a separate entity which always moved forward constantly.

According to General Relativity mass deforms spacetime around itself. The degree of deformation depends on the amount of mass (the larger the mass the more spacetime is deformed). An object like Earth which moves in the spacetime around the Sun, is made to move in a bent path (its orbit) due to the shape of space.

A good analogy to this is to think of a rubber sheet. If there are no object on it the sheet is flat (which corresponds to flat spacetime with no mass in it). However if we put a ball on the sheet it will bend (corresponding to the deformation of spacetime due to a mass – the ball). We can imagine that if we push another ball sideways near the first ball it will be made to move around, and not in a straight line (this corresponds to a planet’s orbit). Please see this YouTube video (click the hyperlink) which demonstrates the effect of a warped rubber sheet on the path of a coin.

The above explanation deals with an object in orbit around another object. However we can use the same analogy to explain why two massive objects want to move towards each other due to what we would classically call the force of gravity. Each creates a bent spacetime around itself. The two objects then “roll” down the curvature of each other’s spacetime.

Please remember that the rubber sheet idea is just an analogy and the actual curvature of space works differently and in a far more complicated way. Primarily because all four dimensions of space are curved.

What is spacetime?

Spacetime is the combination of space and time into a four dimensional description of the universe.

Before Einstein made this discovery in the form of Special Relativity, space and time were considered to be separate. That meant events occurred in space (three dimensions), and time was a background entity which continuously moved forward and was the same for all observers. However we actually find that time is an active component in the universe, and should therefore be included in the dimensions of space.

As an example of how time is active consider the effect of Time Dilation. Let’s say you got onto a rocket and flew past me on Earth at a fast speed. If I compared the movement of my watch to yours, I would find that it is moving relatively slowly. However it is very important to note that simultaneously if you compared your watch to mine you would find the exact same thing, that my watch is moving relatively slowly. Furthermore it is important to realize that this is not some mechanical effect that causes the watch hands to appear to move slower and that’s it. It is an actual physical effect which causes time to appear  to slow down.

The reason both of us experience the same effect is due to the relative nature of the universe. Although in every day terms you would argue that you were the one in motion and I was stationary on the ground, in physical terms you were moving relative to me AND I was moving relative to you (except in the opposite direction). Motion is a matter of perspective.

Solar System

Why does the full Moon last longer closer to the equator?

The full Moon is caused by the near perfect alignment of the Earth, Sun and Moon whereby the Earth is located between the Sun and Moon. We are therefore able to see almost the whole lit half of the Moon, which is also lit by the Sun on only one half its surface, like the Earth. When a perfect alignment happens we see a Lunar Eclipse.

Therefore, although the full Moon phase is only an instant in time, when the alignment happens, it appears to be nearly full for a few days. This is true for everyone on Earth.

Can a satellite maintain an orbit such that it remains permanently on the far side of the Moon?

The Moon orbits the Earth in about 27.3 days. Therefore, given that its own rotation is synchronous with its orbit, the Moon takes 27.3 days to rotate around itself (which is why we only see one side of it).

Any satellite orbiting the Moon is also subject to Earth’s gravity, which has a substantial effect, even on the opposite side of the Moon. Therefore, if we want a satellite to remain permanently on the opposite side of the Moon, we need it’s orbit to be centered on the Earth, not the Moon, of course with both bodies within the orbit. And the satellite must have an orbital period of 27.3 days.

In order to achieve this we need to use Newton’s second law (which accounts for forces being applied on the satellite). The satellite experiences gravitational forces from both the Moon and the Earth (the distances to these objects are different however), and both of these forces account for the centrifugal (circular) force on the satellite, which keeps it in orbit.

Using this prescription, and after a bit of arithmetic we find that a satellite can orbit at a distance of 63,900 km above the center of the Moon (hence 448,300 km from the center of the Earth), and with an orbital speed of about 1.2 km/s (the Moon’s orbital speed is about 1.02 km/s), and remain always directly on the opposite side of the Moon from us.

In this derivation the assumption that the Moon’s and the satellite’s orbits are circular was made. The Moon’s orbit is in fact slightly elliptical. Accounting for this would introduce a slight refinement to our result, but at the expense of a vastly more difficult calculation.

When will the next solar flare be?

Solar flares are bursts of energetic and charges particles from the Sun, caused by magnetic activity in the sun. The strength of the magnetic activity in the Sun changes on an 11 year cycle (becoming periodically stronger and weaker). This cycle is easily observed by the total number of sun spots on the Sun, which are also caused by the magnetic field, and vary on an 11 year cycle.

Solar flares happen randomly, although the above mentioned cycle allows us to predict that at the height of the Sun’s magnetic activity (when the number of sun spots is a maximum), the likelihood of a solar flare happening is larger. Solar flares also do not have to be aimed at the Earth, and can send energetic particles in any direction.

Once a solar flare happens, we can see the light from it about 8 minutes later (since light takes time to reach us from the Sun). The particles are travelling slower than light, and therefore they take longer to reach us (about 30 minutes). That is the only way to predict a particular solar flare. As we are close to the solar maximum (in terms of its magnetic activity), which will happen in 2013, we can expect solar flares (and related solar activities) to occur more often.

Although solar flares cannot cause death on Earth (directly), they still have the potential to be damaging to electric instruments such as satellites and telecommunication in general, and also air planes on polar routes. They can also be dangerous to unprotected astronauts in space.

What types of eclipses are there?

The two types of eclipses are Solar eclipses, when the Moon blocks the Sun as viewed from Earth and Lunar Eclipses, when the Moon enters Earth’s shadow.

The Moon’s orbit is inclined about 5 degrees to the ecliptic (the plane in which the Earth orbits), that is why there isn’t a Solar eclipse every New Moon. Only when the Moon is in its New phase and also happens to be on the two spots in its orbit which intersects the ecliptic does a Solar eclipse happen.

It is a nice coincidence that the Sun and Moon have approximately the same sizes in the sky. If the Moon was significantly smaller only a portion of the Sun would get blocked, and if the Moon was larger the Sun might have been completely blocked, as opposed to a small part, the Corona, remaining visible.

There are two types of Solar eclipses, Annular and Total. Because the Moon’s orbit is not perfectly circular, its distance from the Earth change, and so does its size in the sky. Therefore there are times when the Moon moves between the Earth and the Sun, but it looks slightly smaller than the Sun. Therefore an outer rim of the Sun is still visible, that is called an Annular eclipse. If the Moon moves in between the Sun and Earth and is closer to the Earth it is able to cover the whole surface of the Sun, resulting in a Total Solar eclipse. In this case only the Corona of the Sun will be visible as a white halo, which is the outer layer of the Sun which is extremely hot, a few million degrees.

Only a small part of the Earth can see a total Solar eclipse at one time, however overall total Solar eclipses can happen anywhere on Earth.

Lunar eclipses happen when the Moon is on the exact opposite side of Earth compared with the Sun. In this case we would expect a Full Moon, but as the Moon moves into Earth’s shadow it becomes dark.

Remember to NEVER look at the Sun, whether eclipsed or not, with your naked eye. Always use proper observing equipment such as eclipse glasses or a telescope with a solar filter.

What causes the Moon to move away from the Earth? Will it ever stop?

The Moon moves away from the Earth due to what is known as a tidal force. To understand how this force works lets imagine the sphere of the Moon orbiting around the sphere of the Earth. Both of these objects exert a force of gravity on the other, and this force of gravity is weaker the farther away from the object you get. Therefore the Moon’s force of gravity is stronger on the side of the Earth closest to the Moon compared with the force on the farther side. This imbalance of forces causes the Earth to squish a little bit into an egg shape, with a bulge directed towards the Moon.

The Earth is rotating around itself. Because the rotation of the Earth is faster than the orbit of the Moon this bulge always moves ahead of the Moon. What happens as a result is that the Moon’s gravity pulls on the bulge backwards and slows down the Earth’s rotation. As an analogy think about trying to slow down a car. You need to push the brake pads into contact with the brake discs so that the rotating wheels slow down. The same is true here except unlike friction which is a contact force, gravity works at a distance.

In response to the slowing down of the Earth the Moon moves outward.

Eventually, billions of years from now, when the time it takes the Earth to rotate around becomes equal to the time it takes the Moon to orbit, this process will stop. That is because the bulge of the Earth will no longer be ahead of the Moon, but instead will face it directly. After that point both the orbit of the Moon and the rotation of the Earth will remain unchanged.

Why does the Moon appear not to be rotating on its axis?

Most celestial objects rotate, and in particular as far as we know all Solar System objects do.  For example the Earth’s rotation is responsible for the day (meaning the rising and setting of the Sun and so on).

As described in the answer to “What causes the Moon to move away from the Earth? Will it ever stop?” tides are causing the rotation of the Earth to slow down and the Moon to move away from Earth. Billions of years ago the same process caused the rotation of the Moon to slow down, as well as the Moon to move outward. That process continued until the time that it took the Moon to make one orbit around the Earth became equal to the time of a single rotation of the Moon. From that point on only a one side of the Moon ever faces the Earth, which is the side that we see today.

This configuration is called tidal locking, and the Earth is destined to become tidally locked to the Moon as well, but this will happen several billion years from now.

Could a tilt in Earth's rotation axis be the cause of Global Warming?

As you may know the Earth’s rotation axis is tilted (at an angle of 23.5 degrees) relative to the plane of its orbit around the Sun. This is the reason for the seasons. Although you might think that a tilting of the axis could cause the observed Global Warming, there are a few issues with that.

The larger this angle the more extreme seasons get. Therefore summers would become hotter and winters colder, as well as longer. Therefore although the summer in the North would become hotter, at the same time it is winter in the South which would be colder (and vice versa). This is something which is not observed. The result would not be a warming up of the whole globe.

Another issue is that we would easily notice that the Sun rises and sets differently. This is something that is well determined and depends on the position of the Earth in its orbit (i.e. date) and latitude on the Earth. If there was any change in this angle it would be easily detected. For example at Toronto’s latitude of 43.5 degrees the highest the Sun gets in the sky is 70 degrees above the horizon.

Perhaps most importantly there is nothing to cause changes in the angle of Earth’s axis tilt. The rotation axis is pretty stable at around 23.5 degrees, what does occur is the rotation axis wobbles like a spinning top, which happens with a period of about 26,000 years. This means that while the angle stays the same, the axis points in a different direction. For the rotation axis to move significantly there needs to be an external gravitational perturbation, something which we have due to Jupiter (the most massive planet in the Solar System), but our Moon helps Earth remain stable. There are no other significant perturbers in the Solar System, as we would have noticed their effect on the other bodies in the Solar System as well.

We must therefore look for other causes to the warming of the Earth.

Where did the water on Earth come from?

It is not 100% clear where the water on Earth came from, however there are several leading theories.

The cloud of gas the Solar System formed out of is believed to have contained a fairly large amount of water molecules which were well mixed throughout. Therefore as the cloud collapsed to form the Sun and planets it is expected that water should be present in all of them. However there are a few complicating factors to consider.

As the cloud collapsed its temperature increased, with the heating stronger towards the center. At high temperatures water remains in gaseous form and does not condense into liquid. Gaseous water is more difficult to stick to rocks than liquid water and at the early stages of the Solar System the temperature at the location of the Earth was greater than it is now. Therefore the rocks which collided to form the Earth had very little water in them. And water that may have been present at the time could have been easily ejected due to the strong collisions. Furthermore gaseous water is more easily pushed by radiation from the Sun out farther than where the Earth is.

There are a few potential sources for water. Comets which formed significantly far from the Sun contain large amounts of water, as is the case with asteroids from the outer parts of the asteroid belt. For a very long time the comets were a favourable source for the water on Earth. It is believed that there was a period of heavy bombardment by comets called the Late Veneer period that could have enriched the surface of the planet with water.

However recently some evidence suggests that asteroids are a more likely source for the majority of water on Earth. Not only are their numbers greater with some asteroids known to be quite large (in fact it is likely that even larger asteroids existed long ago but have since collided with the planets), but the chemical composition of the water on Earth matches better with that in asteroids than in comets. This is referring to the amount of deuterium (an isotope of hydrogen which is made of a proton and a neutron) being a component of water relative to regular hydrogen (just a proton). The ratio on Earth is about the same as in asteroids, whereas it is significantly smaller than in comets.

It is likely that several sources contributed to the total amount of water on Earth, but asteroids seem like the primary source (although the issue is far from settled).

What would happen to the Solar System if the Earth blew up?

Let’s assume for the sake of this scenario that the Earth exploded, what exactly caused this to happen is up to your imagination. The reality is that Earth is not a crucial component of the Solar System and hence the other planets will continue orbiting around the Sun as if nothing happened. Assuming the debris from the Earth will fly off symmetrically around it might hit some of the planets, however that would not be enough to cause serious damage to them.

The only component of the Solar System which will be affected is the Moon. Given that it has lost the object around which it orbits, the Earth, it will continue moving in the direction it was going when the Earth exploded. However it will not escape the Solar System, instead it will begin orbiting the Sun directly.

How can I confirm if I found a real meteorite?

A meteorite could be a very interesting find indeed, but it’s hard to tell whether you have a piece of space rock in your hand or actually just an ordinary Earth rock.

Natural science museums are a good place to turn for authentication services. For example the Royal Ontario Museum hosts session in which an expert can study a rock that you bring with you and tell you what it’s made of and so on, for free. See their website here for more information.

Is there life on Mars?

To answer this question we really need to be clear what we mean by “life”. It might seem like an easy concept to define, but it’s actually quite complicated.

To start we can list some properties we expect a living organism to possess. It needs to be able to: grow by absorbing energy from its environment, react to external stimulations, reproduce and mutate to adjust to a change in the environment or to improve in the current environment. And we can think of many more potentially important properties. The issue is that there are certain objects that have some of these properties, but are not living. For example crystals are to grow and react to their environment, however we would all agree crystals are not living organisms. On the other hand we don’t want to be too restrictive in our definition because life on other worlds may not necessarily be similar to what we are familiar with on Earth.

For the purpose of this question we can adopt the simple definition that something alive has the ability to reproduce, adopt, and react to its environment. So far we have yet to find such a living organism on Mars. In fact we haven’t fully confirmed a discovery of fossilized organisms, either.

Currently there are three rovers on Mars, Opportunity, Spirit and Curiosity. One of their main goals is looking for life (even fossilized). Opportunity was able to confirm that water used to flow on the surface of Mars, which is encouraging given that life as we know it depends on water (although it doesn’t necessarily have to).

Hopefully these rovers will find conclusive evidence that life did exist on Mars, and potentially still does.

Given tides raise water levels and heat up the Earth a bit, where does this energy come from?

Raising water levels and heating the interior of the Earth requires energy. Therefore, given that energy must be conserved, it must come from somewhere. In this case the source is the rotational energy of the Earth. As is explained in the answer to “What causes the Moon to move away from the Earth? Will it ever stop?” the Moon raises tides on Earth, which are easily seen in the changing ocean water levels. These tides cause the rotation of the Earth to slow down, which results in a decrease in its rotational kinetic energy. This energy goes into raising the water level, into warming up the interior of the Earth a little bit, and into the orbital energy of the Moon which is moving outward.

A similar process is occurring for Io, a moon of Jupiter. It is being tidally squished while it orbits Jupiter, which results in significant heating of its interior. In fact that is what causes Io to be the most geologically active object in the Solar System.

What makes our world move?

Our world, planet Earth, has two types of motions. It rotates around itself, as well it moves around the Sun. In order to understand why Earth performs these motions we need to talk about how Earth and the Solar System formed.

It all started out as a giant cloud of gas which started to collapse due to its own gravity. As it collapsed it began spinning faster and faster. When the Sun formed in the middle of this cloud and the planets formed around it everything continued to move in the same direction as the rotation of the cloud.

We see this in the Solar System today. The Sun rotates around itself, the planets orbit around the Sun and most planets rotate around themselves all counter clockwise, when looking from above the Solar System.

Are there comets in the Asteroid Belt between Mars and Jupiter?

Most likely there aren’t any. The reason is that comets and asteroids are different because of the location where they formed. Comets formed very far from the Sun, which is why they are frozen. Asteroids formed in the asteroid belt, where it is much warmer, and therefore they do not have ice on them.

How do comets orbit?

Comets orbit the Sun on orbits that look like an ellipse, which is a squished circle. This is different from orbits of planets which are shaped like circles. This orbit shape means a comet spends most of its time very far from the Sun, where we can’t see it. Sometimes a comet comes very close to the Sun which is where we are able to see it because the Sun’s hot rays cause the ice on the comet to evaporate.

Space and Aeronautics

What happens to the human body, if exposed to the vacuum of space?

Humans cannot survive in the vacuum of space, therefore it is extremely crucial for astronauts to wear properly fitted space suits.

The risks involved in being in space are mainly due to the vacuum. On Earth, we are constantly subjected to an atmospheric pressure of about 101 kilo Pascal, a pressure we conveniently call an “atmosphere”. Therefore there is a balance between the pressure of air in our lungs, and that of air outside, and we therefore don’t even notice the outside pressure.

It is not very clear what will happen in an explosive decompression event, where external air pressure drops substantially very quickly. There have been a small number of animal experiments, and a few unfortunate cases of accidental decompression of humans. However the collective insight from all of these is that a person could survive the vacuum of space for up to a couple of minutes, although they will lose conciousness fairly quickly.

As the air pressure outside dissapears, air in the lungs begins to expand, it is therefore important not to hold your breath, as that would cause severe damage to the internal organs. As air and water in the vains expand they form small bubbles, which cause blood flow to slow and eventually stop. The lack of oxygen flow to the brain first induces a loss of conciousness, and eventually death. Evaporation of water from the mouth and tongue might cause these to effectively freeze.

Contrary to common belief however, the expansion of the inner organs does not cause the body to explode, because the skin is strong enough to withstand the force, in fact even the eyes will not pop out.

In the case of a space suit rupture, the unfortunate astronaut must rely on a fellow astronaut for rescue, which is why NASA astronauts work at least in pairs when doing space walks, and strive to remain close to each other. If a fellow astronaut succeeds in reaching the decompressed astronaut, it is important not to recompress them too abruptly, as that could have grave consequences as well.

Another hazard in space is the intense radiation from the Sun, where there is no atmosphere to act as a filter. However, given the vacuum issue, this is only a real danger to astronauts in space suits, who need to avoid long exposure to the Sun.

Space is therefore a very dangerous environment, and astronauts, or future space travellers, must be well trained and very careful.

Stars and Planets

Has the star Sirius ever been aligned with the Orion Belt stars, as seen from Earth?

Alignment of stars as viewed from Earth depends on the positions of the Earth and the two stars in questions in three dimensional space. For this purpose we can ignore not only the position on Earth where the observation is made, but also the movement of the Earth around the Sun, therefore the alignment we are looking for is with the Sun and the stars.

The reason we can ignore these movements is because they have a minor effect on the position of stars in the sky. Although stars do move across the sky as viewed from Earth during our orbit around the Sun, an effect called parallax (which is used to measure distances to nearby stars), the angles by which stars move with respect to the distant background stars is very small. The closest star Proxima Centauri has a parallax angle of about 0.8 arcseconds (there are 3600 arcseconds in one degree), this is a very small angle, which cannot be noticed with the naked eye, and the farther a star is the smaller its parallax angle.

Sirius is located in the Canis Major constellation which is right besides the Orion constellation. However, it is very important to keep in mind that stars in a given constellation are not necessarily physically related to each other. And the three stars of Orion’s belt are completely separate in space, they are merely projected to be roughly in line with each other. In order for Sirius to have been aligned with any of the Orion Belt stars it would have had to physically move to be positioned between the star and the Sun.

Given the current proper motion of Sirius and the Orion Belt stars, if we trace their motions back in time, we find that these were never in line. In general there is a small likelihood that stars will align, as they are effectively point objects, and there are relatively a small number of them, as can be seen by the relatively empty night sky.

Why do stars twinkle?

We are all familiar with the twinkling of stars, it is in fact part of their charm. However, the twinkling is not a feature of the stars themselves. As light from stars (or from any object outside our atmosphere) passes through the atmosphere it gets deflected (or scattered) numerous times. This causes the resulting ‘image’ of the star to get blurred. Because the atmosphere is very turbulent this blurring is not constant, which is why we see the stars shifting. In fact, using a telescope and displaying the image of the star (which is actually supposed to look like a single dot) we can see it shifting around.

While the twinkling of stars is a very nice feature as far as the general population is concerned, it is a major problem for astronomical observations, and is referred to as ‘astronomical seeing’. As already mentioned, images of celestial bodies get blurred by the passage of light through the atmosphere, but in astronomy we strive to have the sharpest images we can get, that way the details we learn are greater. This is the main reason why all the major telescopes in the world currently are located in various remote locations on top of mountains. The goal there is to minimize the amount of atmosphere above the telescope, and the locations are also chosen to have as stable weather as possible, because strong winds increase this effect. An added effect there is the minimization of ‘light pollution’ (the glow of lights from cities which hinders observations).

Space telescopes are necessarily much smaller than the ones on the ground, as it is very difficult and expensive to put satellites in orbit. However, as they do not have an atmosphere to obstruct their view of the stars space telescopes obtain the best images, especially in the infrared, optical, ultraviolet, X-ray and gamma-ray bands of the electromagnetic spectrum. For beautiful examples check every single image taken with the Hubble Space Telescope.

Because space telescopes are so expensive yet we still strive to take the same quality images, astronomers have came up with a technique which can be used in ground telescopes, to improve the resulting images. This technique is called adaptive optics and it involves using bendable mirrors. These mirrors get bent based on the current atmospheric conditions above the telescope, to try to counter the effects of ‘seeing’. In order to measure the effect of the atmosphere lasers are used to shine a predetermined part of the atmosphere in order to create a source of light whose observable features are well known, and any deviation from it is due to the atmosphere. In one common example the lasers are chosen to be of such frequency to excite the sodium atoms in what is called the sodium layer of the atmosphere.

How can I help detect planets outside our Solar Syatem?

Since the launch of the Kepler space telescope the amount of planets we discovered outside the Solar System (called exoplanets) has gone up to over a thousand confirmed. Kepler uses what is known as the transit method for detecting exoplanets. It relies on the orientation of the orbit of a planet to be such that it goes in front of its parent star. When this happens the planet blocks some of the light of the star. This can be detected by the periodic dimming of the star’s light.

Although to some extent the detection process has been automated, the observations often have a lot of noise caused most often by fluctuations in the light of the star itself. Therefore it is still vital for humans to manually go over images and attempt to detect these dips in the brightness of stars.

This is where you can help. The amount of observations performed by Kepler is enormous, and it is difficult for astronomers on the Kepler team to go over each one carefully themselves. Furthermore, it is beneficial to have several people detect the same planet, as that acts as a confirmation of the existence of the planet.

By going to the website PlanetHunter you can look at real observation taken using the Kepler space telescope. You will then be able to determine whether a system contains a planet or not, and submit your suggestion. You may detect a new planet orbiting another star somewhere out there in the Milky Way galaxy.

The PlanetHunter website will guide you through the process of analyzing the stellar light, and knowing what to look for.