The planet Mercury is named after the Greco-Roman messenger of the gods, because it circles the Sun faster than the other planets, completing its circuit in 88 Earth days. Because it travels so close to the Sun, Mercury is often difficult to observe. It is only visible as a "morning" or "evening" star, hugging the horizon just before or after the Sun rises or sets. Like Venus, Mercury also has phases. Being so close to the Sun, temperatures during the day on Mercury are hot enough to melt many metals. At night they drop to -291°F, making the temperature range the greatest of all the planets. The gravitational pull of the Sun has "stolen" any atmosphere that Mercury had to protect itself against these extremes.