Mercury, the planet closest to the Sun, is very difficult to observe from Earth. The planet is only one-third the diameter of the Earth, and can be studied by telescope only during the day or when the Sun is rising or setting. When Mercury is near the horizon, the interference created by Earth's atmosphere reduces the visibility even with the best telescopes.
Telescopic view of Mercury created using the best 60 images by Baumgardner, Mendillo, and Wilson in 1998.
Map of Mercury prepared by E.M. Antoniadi in the 1920's.