JUPITER
Rises on the 1st at 6:48 p. m., and sets on the 2d at 8:06 a. m.; rises on the 15th at 5:48 p. m., sets at 7:12 a. m. on the 16th; rises at 5:47 p. m. on the 28th and sets the next day at 5:17 a. m. On the 1st, at 2:07 a. m., he is 4° 9´ north of the moon; on the 19th, at 2:00 a. m., in opposition to the sun, that is, on the opposite side of the sun from the earth; on the 28th, at 6:43 a. m., he is again in conjunction with the moon, being 4° 27´ north of our satellite. During the month his diameter increases two-tenths of a second, and he has a retrograde motion of 3° 24´ 8´´. The statement that Jupiter retrogrades some 3½° may puzzle some of our younger readers, who have doubtless been instructed in what is a fact, that not one of our planets has a retrograde motion; but that all move from west to east about the sun as a center. What we mean by retrograde is really only apparent retrograde; and it was something very puzzling to the early astronomers, particularly to those who thought that the earth and not the sun was the center of our system; that the sun and all the heavenly bodies revolved each day about our earth. When it was discovered that the earth revolved each day on its axis, and all the planets revolved about the sun, the retrograde motions were comparatively easy to understand. Let us see if we can obtain a clear idea of Jupiter’s actions for this month. As we view him on the night of the 1st he appears about five degrees east and 1° 2´ south of the bright star Regulus, which can be seen almost the entire night as the brightest of the six stars forming the sickle in the constellation Leo. Noting his position again on the night of the 28th, we find that he has moved westward about 3½°, and is only about 1½° east and 17´ north of Regulus; thus, as we say, having retrograded about 3½°. To assist us in understanding this, let us take an orange to represent the sun, a grain (of mustard, for example) to represent the earth, a pea to represent Jupiter, and a point of some kind for Regulus. Now place these objects on a stand in the following order: In one line, at the beginning, the orange; two inches distant, the grain; eight inches farther, the pea. Next draw a line through the center of the orange so as to make an angle of five degrees with the line through the orange, grain and pea, and at as great a distance as convenient, stick a pin to represent Regulus. Now move the grain and pea (the former about two and one-fourth times as fast as the latter) about the orange as a center, in the direction of the movement of the hands of the clock (that is, from left to right). We can readily see that on account of the more rapid motion of the grain, together with its being nearer the orange, that the pea will fall behind; and if we sight along the line of the grain and pea, the latter will be seen nearer the line joining the orange and the pin; and should we continue the moving of the grain and pea, making similar observations, we should find the pea approaching nearer and nearer, and perhaps even passing the line through the orange and pin. These relative motions we can see will continue until the grain makes nearly one-fourth of a circumference, after which the pea appears to make a movement in exactly the opposite direction. Now the foregoing represents tolerably well the relative positions and movements for this month of the bodies named. The earth, Jupiter and Regulus are on the same side of the sun; the earth nearest, Jupiter next (about five times as far as the earth), and Regulus next (at a distance of say 20,000,000,000,000 miles), and five degrees west of the line joining the earth and Jupiter. (These bodies we know move at the average rate of 18.38 and 8.06 miles per second respectively.) Our standpoint is the earth, and as we move eastwardly so much more rapidly than Jupiter, we find him dropping back each day, and apparently approaching nearer to Regulus, till at the end of the month we find him as before stated, only about 1½° east of that star. Should we watch him through March and April, we should find him retrograding during the former month and twenty-two days of the latter, on the 23d of April being 1½° west of Regulus; and on the same date, as the earth would be going directly away from him, he would appear stationary; and immediately afterward would seem to start again toward the east. Jupiter, as we know, is one of the superior planets, and an explanation of his retrograde motion explains that of all the others of his kind. A little ingenuity, putting the earth for Jupiter and Mercury or Venus for the earth, will show what is meant by the retrograde motion of the inferior planets.