The smallest of the 4 terrestrial (rocky) planets and the planet that is most similar in its outer appearance to Earth's Moon. Mercury is very close to the Sun and never exceeds an angular distance of 28° from the direct line of sight of the Sun. Mercury can only be seen at most 2 hours on any given night or morning that is favorable for viewing.

Mercury less than 1/3 of the distance the Earth is from the Sun (0.31 A. U.). The planet can reach a scorching 700 K (800°F) during the day to 100 K (-280° F) at night.

Mercury rotates on its axis very slowly. The orbital period of Mercury is equivalent to 88 Earth days and its rotation rate is 59 Earth days. Mercury is considered speedy because it moves quickly around the Sun (less than ¼ of an Earth year) and Mercury was named after the Roman god of commerce and thievery. Although the entire surface of Mercury has not been observed yet it has many craters one of which is the result of a major impact of an asteroid that sent shock waves through the core of the planet. This impact formed the Caloris Basin that eventually filled with lava from below.