[Illustration: 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. [Illustration: FIG. 52.—THREE VIEWS OF MERCURY.] =Seeing Mars.=—We hear more of _Mars_ than of any other planet for two reasons: first, because great lines can be seen on his surface which are thought to be canals, and second, since Mars sometimes comes almost as close to the earth as Venus, there has been a great deal of talk since the invention of wireless telegraphy about our signaling to him. That people could live on Mars is possible, for the Red Planet is like our Earth, in that it has land, water and air, weather and seasons, with a warm equator and ice-covered poles. Seen through a telescope he looks like Fig. 53, though much smaller. [Illustration: FIG. 53.—MARS AS SEEN THROUGH A TELESCOPE.] But Mars cannot always be seen, for sometimes he disappears for a couple of years, but when he finally does return he is a good planet, for he stays a long time, and you cannot mistake him, for he will shine ruddy and bright with never a merry twinkle. Mars is 141 millions of miles from the sun. His diameter is 4,200 miles. His day is 24½ of our hours long. His year is 687 of our days long. =Seeing Venus.=—_Venus_ is so much brighter than any of the other stars or planets that you will know her the instant you see her. Indeed, when Venus is the brightest and the Sun is far enough away from her, she can often be seen with the naked eye in the daytime if the sky is clear. Three views of the phases of Venus are shown in Fig. 54. [Illustration: FIG. 54.—THREE VIEWS OF VENUS.]