. the background of the sky, and let s be the sun. At equal intervals of time the earth will be at points marked i, 2, 3, 4, 5, 6, spaced out equally on its path. At the same time Mars will be at corresponding points on its orbit, marked I to 6 on the larger circle. When the earth and Mars are at their points i, Mars will appear to us to be at the corresponding point i on the sky-circle. Similarly for all the other points. The diagram shows us that we get a forward move- ment from I to 3, followed by a backward from 3 to 4, and then a forward from 4 to 6, and onwards until the next loop is com
. the background of the sky, and let s be the sun. At equal intervals of time the earth will be at points marked i, 2, 3, 4, 5, 6, spaced out equally on its path. At the same time Mars will be at corresponding points on its orbit, marked I to 6 on the larger circle. When the earth and Mars are at their points i, Mars will appear to us to be at the corresponding point i on the sky-circle. Similarly for all the other points. The diagram shows us that we get a forward move- ment from I to 3, followed by a backward from 3 to 4, and then a forward from 4 to 6, and onwards until the next loop is commenced. If we imagine the circles as lying slightly out of the plane of the paper, and inclined to it at different angles, the path traced out becomes a loop. The outer planets alwavs appear brightest when they are in the middle of the backward portion of the loop, for then they are always in a line with the earth and the sun, or exactly opposite the sun as seen by us—a configuration to which we give the technical name " ; Their brilliancy varies con- siderably at different oppositions. In the case of the earth and Mars the orbits are comparatively near each other at one part, and are much farther away at other parts. WTien an opposition takes place with the planets at the former points, Mars is nearer the earth than it is at any other time, and so appears much brighter. But if the opposition takes place with the earth and Mars at some of the latter points, the distance is so much greater that the maximum brilliancy of the planet is far less than it was in the previous case. This variation of brightness is more marked in the case of Mars than in that of any other planet. The best possible oppositions are those that occur at the end of August, when the planet loops its loop under the Great Square of Pegasus. These maximum opposi- tions happen once in about fifteen or seventeen years ; the last was in 1907, and the next will be in 1924, and will be absolute
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