A Little Bit About Mercury
Mercury is the planet with the closest orbit to the sun, making it, along with Venus, an inferior planet. Compared to other planets very little is known about Mercury as only two space probes have been sent to investigate it and take photographs. Mercury has a large iron core for its small size, which constitutes for much of its mass.
Mercury can be seen with the naked eye and can actually become very bright. It has visible phases, similar to the Moon which can be seen with a telescope. We do know that at least visually, its surface is very similar to that of the Moon. It has a heavily cratered surface, is gray in color, and has practically no atmosphere. Like the rest of the inner terrestrial planets (Venus, Earth, and Mars), Mercury is rocky and full of minerals and metals. Unfortunately the view can be more difficult to see than some of the other planets for several reasons. First it has a very small apparent size in the sky due to the fact it is a very small planet and that it is often further away than Venus. The second reason is because it must be observed at dusk or dawn and therefore is very low in the sky. The obscuring affects of our atmosphere are most pronounced for objects low in the sky and distort the light that reaches us, creating a fuzzy image. (Think of the sun when it setting and appears reddish orange and very hazy).
Of the five planets that can be seen without optical aids, Mercury is the most difficult to observe due the fact that, being an inferior planet, it is always in close proximity to the sun from our view here on Earth. It can however, be seen either in the early morning just before sunrise or the early evening just before sunset depending where it is in its orbit. The best time to view Mercury is when it has reached the point in its orbit where it appears to be furthest from the sun in the sky (called greatest elongation). At any other time the planet drifts too close to the Sun and gets faded out by its light. Mercury’s orbit is highly elliptical and therefore some “greatest elongations” put Mercury further away in the sky from the Sun than others. To see when Mercury is at its greatest elongation check out the Astronomical Calendar page.
Facts About Mercury
Average Distance from the Sun: 0.387 AU (57,900,000 km)
Orbital Period: 87.9 days
Length of Day: 58.6 days
Mass: 3.30×1023 kg (0.055 Earth masses)
Diameter (equatorial): 4878 km (0.382 times the Earth)
Surface Temperature Range: 350°C to -170°C / 662°F to -247°F