Mercury: The Fastest Planet and the Mind's Messenger

What is Mercury?

Mercury is the smallest planet and the closest to the Sun: a rocky world 4,879 km across, about 40% wider than the Moon. It orbits the Sun at an average of 57.9 million km (0.39 au) and goes around in 88 days, the shortest year in the solar system, which makes it the fastest planet: about 47 km every second. With almost no air to hold heat, its surface swings from about 430 °C in the day to −180 °C at night. Because its orbit lies inside Earth’s, we always see it near the Sun, never more than about 28° away, low in the evening or morning twilight.

Where Mercury is in the sky

The ecliptic is the path the Sun appears to follow across our sky over a year, the plane of Earth’s orbit seen from Earth. Positions along it are measured in degrees, from 0° at the March equinox to 360°. The zodiac divides it into twelve equal 30° parts named after signs: Aries covers 0° to 30°, Taurus 30° to 60°, and so on. The signs are counted from the equinox, not from the star patterns they were named after; those constellations now sit about 24° further along, because Earth’s axis slowly wobbles (precession).

Mercury moves along the ecliptic at up to about 2° a day and usually spends two to four weeks in a sign, longer when it turns retrograde (above).

Mercury in astrology

In astrology Mercury stands for the mind and messages: thinking, talking, writing, learning, short trips and trade. It rules Gemini and Virgo. Its retrograde periods are read as a time to review, recheck and revisit rather than launch.