Order from Sun: 1
Named For: Roman messenger of the gods
Diameter: 3030 miles
Volume: .056 Earth
Gravity: 0.38 Earth
Distance from Sun: 36 million miles
Orbital Period: 88 Earth days
Orbital Inclination: 7 degrees
Orbital Eccentricity: .205
Length of Day: 59 Earth Days
Temperature: -280F to 800F
Composition: Rock, silicates
Atmospheric Pressure: trace
Atmospheric Composition: oxygen, sodium, hydrogen
Notable Exploration: Mariner 10 (1974-5), Messenger (2011-present)
Of all the planets, Mercury is the one most people never see. The great astronomer Nicholas Copernicus, who finally rediscovered the idea that the sun is the center of the solar system, reportedly never saw Mercury. The reason that Mercury is so difficult to spot is that it is so close to the Sun. The greatest possible elongation only takes Mercury about 28 degrees away from the sun. Because the ecliptic is rarely vertical, Mercury at greatest elongation actually appears much lower that 28 degrees in the sky most of the time. Because Mercury is an inferior planet, it is only seen in the early morning just before sunrise and early evening just after sunset. Mercury is best seen in spring evenings and on fall mornings when the ecliptic is nearly vertical, allowing Mercury to appear highest in the sky. When seen, Mercury averages out to be about a zero magnitude object near the horizon. Even though it is bright, because it is so close to the Sun, Mercury is often difficult to spot. Binoculars cure this problem. Because the sky needs to dim before the planet can be seen, any time that Mercury appears about ten degrees above the horizon is considered a good appearance. If you see Mercury, you will join an exclusive club of people who have seen the planet nearest to the Sun.
When looking an Mercury with binoculars, it still looks like a bright star without any special features.
In the higher power of telescopes, Mercury appears to go through a complete set of phases from new to full and back again, just like the Moon. While interesting to watch in the present, in the past, the phases of Mercury (and Venus) conformed the theory that the planets do go around the Sun, not vice versa.