Mercury, the smallest of the'planets and the one closest to the sun, is difficult to see.' Certainly it is necessary to choose the right time.' On occasions the planet can appear as a bright spot low down near the horizon. Under favour- able conditions it may be visible for as long as an hour at dawn or dusk.
Mercury can never : ppear more than 28°from the sun, and its distance may sometimes be as little as 180 from it. Good conditions are therefore necessary if we are to see it without the help of a telescope.The best times occur in the spring shortly after sunset, and in the early morning during the autumn; but clear skies near the horizon are often rare, and many amateur observers have never seen Mercury. It is said that Copernicus never succeeded in seeing it.
Just as the moon is held tightly in the gravitation of the earth, and always keeps the same face turned towards us, so Mercury is tightly bound to the sun and the same face is always exposed to the sun's burning rays. Mer- cury's actual distance from the sun varies between 29 and 43 million miles, the mean being about 36 million, or little more than a third of the earth's distance. Every square mile on the sunny side of its surface receives seven times as much heat as a square mile on the earth.
Apart from the sun itself, the sunny side of Mercury must be the hottest place in the solar system. There the temperature is so high that a lump of lead would simply melt in the sunshine.No ordinary liquids could exist, for they would all boil away.
But the side of Mercury which is turned away from the sun remains in eternal darkness, with a temperature only a few degrees above absolute zero. This is the tempera- ture at which bodies are considered to have"no heat left" in them. It is about-273°C. This side is probably even colder than the furthest planet Pluto.
One side of Mercury is very hot, and the other is ex-tremely cold.No atmosphere has been found on the pla-net.We can be fairly certain that no life exists on Mer-cury.
