Familiar talks on astronomy, with chapters on geography and navigaton . s one com-plete revolution about the sun in six hundredand eighty-seven days. So while its day isabout the same as ours, its year is equal tonearly two of ours. In the diagram (Fig. 9) let S represent thesun, E the earth, and M, M, M, M, the planetMars, in four positions in its orbit. We willsuppose the earth to be at E, at rest in its orbitEE. When Mars is at M, it is said to be in opposi-tion (that is, opposed to the sun) ; and when atM, in conjunction (that is, conjoined with thesun). At M, M, it is in quadrature. It is
Familiar talks on astronomy, with chapters on geography and navigaton . s one com-plete revolution about the sun in six hundredand eighty-seven days. So while its day isabout the same as ours, its year is equal tonearly two of ours. In the diagram (Fig. 9) let S represent thesun, E the earth, and M, M, M, M, the planetMars, in four positions in its orbit. We willsuppose the earth to be at E, at rest in its orbitEE. When Mars is at M, it is said to be in opposi-tion (that is, opposed to the sun) ; and when atM, in conjunction (that is, conjoined with thesun). At M, M, it is in quadrature. It isreadily seen that when Mars is in opposition itis nearer the earth than at any other time. Thedifference in its distance at opposition and con-junction is the diameter of the earths orbit; orabout one hundred and eighty-five millions ofmiles! 128 Familiar Talks on Astronomy, etc. Again, we see that when it is in conjunction,it must rise and set with the sun; whereas whenit is in opposition, it must rise as the sun sets;and it will be upon the meridian about M Fig. is, of course, the best time to observe it,and astronomers take advantage of it. It wasin this position (opposition) when the observa-tions I have told you of were taken at Green-wich and the Cape of Good Hope to determineits parallax. Its angular diameter at oppositionis seven times as great as in conjunction, as wemight expect. Mars is the only one of the superior planetsthat exhibits a marked phase. It sometimesappears gibbous. The Planet Jupiter. 129 Mr. Lockyer says: Mars has not only land,water, and snow, like the earth, but also cloudsand mists. The land is generally reddish whenthe planets atmosphere is clear; this is due tothe absorption of the atmosphere, as is thecolor of the setting sun with us. Hence thefiery red light by which Mars is distinguished inthe heavens. The water appears of a greenishtinge. JUPITER. The next planet outside of Mars is Jupiter. Itis not necessary to draw diagram
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