Mercury

Mercury is a planet of extremes: the smallest planet, the closest to the Sun, the fastest around it, and the one with the biggest swing between daytime and nighttime temperatures. After Earth, it is also the densest planet in the solar system.

Mercury is the closest planet to our Sun. It is named for the ancient Roman god of trade and profit. Legend says Mercury's winged sandals gave him super speed. Mercury the planet is super fast, too. It zips around the Sun every 88 days - faster than any other planet. No wonder it got its name from the quickest of the ancient gods.

Mercury is just over a third as far from the Sun as the Earth is; on average it is 0.387 AU (about 58 million km) from the Sun. Mercury's orbit is the most eccentric of any planet: at aphelion (the point in the orbit farthest from the Sun) Mercury is about 70 million km from the Sun, and at perihelion it is only about 46 million km away.

Mercury has no real seasons. Seasons are caused by the tilt of a planet's axis relative to its orbit, and Mercury's axis is barely tilted at all (only a few hundredths of a degree), so it spins almost exactly upright.

Since Mercury is so close to the Sun, you will only see it low in the sky shortly before sunrise or shortly after sunset, depending on which side of the Sun the planet is on.

Like the Moon, Mercury shows phases as seen from Earth. It looks about half lit around its greatest elongations, when it appears farthest from the Sun in our sky. That separation ranges from about 18° (when Mercury is near perihelion) to about 28° (near aphelion). At greatest western elongation Mercury rises as early as it can before the Sun, so it is a morning object; at greatest eastern elongation it sets as late as it can after the Sun, so it is an evening object.