Hello astronomy enthusiasts! Welcome to your source for answers to all your astronomy related questions. Feel free to ask us any questions on any topic within astronomy. Your question will be sent to a Real Astronomer who will answer it as soon as possible on this page.
Have questions about a recent discovery announced in the news? Did you always want to know what is responsible for the phases of the moon? Or maybe you are curious about extrasolar planets? By clicking the link below, you can send us an email with your question. We will then post the question with the answer for everyone to learn something.Send us your questions
Please keep in mind that your question might get rephrased before being uploaded to the website, for the purpose of making it short and clear for everyone. Below is a list of topics, choose the one you are interested in, and click a question you don’t know the answer to.
Some of the questions might contain links to external websites, for interesting videos or pictures. Although we cannot guarantee these links to work, please let us know if one doesn’t, and we will try to find another one.
Browse from our list of topics
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.
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. http://solar-center.stanford.edu/AO/
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. http://www.iau.org/public/constellations/
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 analogy is just that, 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.
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 is the best place to view Aurora Borealis (Northern Lights)?
Aurora Borealis (Northern Lights) happen when charged particles (such as electrons), 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 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.
Why does the Moon appear not to be rotating on its axis?
Most celestial bodies have rotations about an internal axis, and in particular all Solar System bodies do, as far as we know. The Earth’s rotation is responsible for the day, for example.
The Moon has two main motions, one is the orbit around the Earth and the other is a rotation around its own axis. Due to tidal interactions the Moon’s rotation has become synchronized with its orbit. Therefore the two period are exactly the same. The result of this is that as the Moon orbits around the Earth its rotation ensures that only one side ever faces Earth. It therefore seems like the Moon is not rotating.
Similar tidal interactions are working to slow down the Earth’s rotation and simultaneously pushes the Moon away from us. This process takes several billion years to complete, but makes the day longer, eventually the day will become as long as the month (which is governed by the Moon’s orbit), that is also getting longer.
Could a tilt in Earth's rotation axis be the cause of Global Warming?
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.
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.