Third, by watching a planet closely for a few hours it will be found to have moved a little. To note this change of position the planet and some fixed star near it must be closely watched and their distances compared from time to time.
Fourth, and last, the surest way of finding the different planets is by using an almanac which will tell you which planets can be seen at certain times of the year and in what part of the sky they are to be found.
Fig. 51.—Sizes of Planets Compared.
Seeing Mercury.—Mercury is so near the Sun that it can only be seen with the naked eye at certain times. Mercury should be looked for just above the eastern horizon for about an hour before the Sun rises in the spring; and above the western horizon for about an hour after the Sun sets in the autumn. You will have no trouble in knowing Mercury if you can only see him, for he is very bright and will be near the horizon. His pale ash color will also help you to single him out from the stars about him. Mercury goes through phases like our Moon, but these cannot be seen with the naked eye.
Mercury is a curious planet in that his day and his year are of exactly the same length, just like our Moon; this means that he turns on his axis once in exactly the same length of time it takes him to travel round the Sun. This causes one side of Mercury to be always turned toward the Sun, and of course this side is hot and light, while the other side is always turned away from the Sun and, consequently, it is dark and cold. Three views of Mercury in different phases as seen through a telescope are shown in [Fig. 52].
Mercury is 36 millions of miles from the Sun.
- His diameter is 3,000 miles.
- His day is 88 of our days long.
- His year is 88 of our days long.
Fig. 52.—Three Views of Mercury.