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.

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.

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.

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.

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.

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.

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.