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The principal challenge of Mercury is simply trying to see it. Although I have earlier described most of the planets appearing as particularly bright stars, Mercury is the exception because of its closeness to the Sun. Mercury is a mere 0.39 a.u. from the Sun while its neighbor Venus is almost twice that distance from the center of the Solar System. Mercury is so close to the Sun that it generally is only above the horizon during the daytime, which makes it the most elusive of all the planets.
A good place to find out when to look for Mercury and the other planets is at Sky and Telescope, but understanding Mercury's orbit is a useful first step to find the planet itself. First of all, Mercury, having the innermost orbit, moves more rapidly than any other planet. One Mercurian year lasts only 88 days while its synodic period is only 116 days. This means that about three times a year Mercury catches up with and overtakes the Earth. One might at first think that this catching up point, the inferior conjunction, would be the best viewing time of Mercury, except that at that time, the Sun, Mercury and Earth are in a straight line with the dark night side of Mercury facing us. How about half a synodic period later when it is at the superior conjunction? True, then we are directly facing the lit-up dayside of Mercury, but it will be so close to the Sun that the Sun’s brightness will overpower its meager reflected light. By the way, Mercury almost never actually crosses in front of or behind the Sun. It's orbit has an extremely high inclination of seven degrees to the general plane of the Solar System and so Mercury usually passes above or below the Sun. The rare occasions when Mercury passes directly in front of the Sun's disk are called transits. During a transit, one can see the small disk of Mercury pass across the Sun for several hours. Of course, the Sun must not be directly looked at, especially through binoculars.
Now that we know when we can’t see Mercury, when can we spot it? The best time is when Mercury has swung away from the Sun. If your draw two imaginary lines from the Earth, one to a planet and another to the Sun, the angle that the lines form at the Earth is the elongation of the planet. The elongation of a full Moon is 180 degrees, because it is always found on the opposite end of the sky from the Sun. A quarter Moon has an elongation of 90 degrees. If there existed another planet following the Earth’s orbit which was a quarter orbit away from the Earth, it woo would have an elongation of the 90 degrees which would be its maximum elongation, for at any other time it form a small angle with the Sun. Mercury, which is so close to the Sun, can never have a 90-degree elongation; its maximum elongation is only 28 degrees. Mercury’s maximum elongation is the most favorable time. Even then, Mercury won’t be far from the Sun. It will either set less than an hour after sunset or rise less than an hour before sunrise. Rarely will the part of the sky containing Mercury be completely black. So far we have discussed elongation, the angle between Mercury and the Sun. Another important angle is that between Mercury’s path and the horizon line itself. The steeper the angle is, the higher Mercury will be in the dark part of the sky, and the easier it will be to see. The Southern Hemisphere is blessed with the best views of Mercury, while those of us north of the equator can best see it either during spring sunsets or at autumn sunrises.
Once you know where and when to look, Mercury is easy to identify. It will appear as a star with a negative apparent magnitude. Through binoculars or a telescope, it will appear as a small white disk with no details. It's so difficult to see anything on Mercury's surface that until recently the length of time it takes for Mercury to rotate around its axis (59 days) was unknown. Thanks to the American spacecraft Mariner 10, we now know that Mercury’s surface is covered with Moon-like craters. Don’t try to find them; they're impossible to see even through the largest telescope. It was decided to name these craters after writers, musicians, painters and other artists. For instance, there are craters named Homer, Melville and Proust. Mercury is the smallest planet and less than half the size of the Earth. Both Mercury and Venus pass through phases as seen from the Earth, similar to those of the Moon. These will be discussed in the next section.