This rapid motion may have led to it being named after the Roman deity Mercury, the fast-flying messenger to the gods. Since it has almost no atmosphere to retain heat, Mercury's surface experiences the greatest temperature variation of all the planets, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day. Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 of a degree), but it has the largest orbital eccentricity. At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface is heavily cratered and similar in appearance to Earth's Moon, indicating that it has been geologically inactive for billions of years.