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that of the earth. "When the earth is at E (Fig. 22), the planet at L is said to be in opposition to the sun. It is then at its greatest distance from him— 180°. The planet is on the meridian at midnight while the sun is on the corresponding meridian on the opposite side of the earth; or the planet may be rising when the sun is just setting. When the planet is at N, it is in conjunction, and being lost in the sun's rays is invisible to us.

Retrograde motion of a superior planet.—Suppose the earth to be at E and the planet at L, and that we move on to G while the planet passes on to O— the distance EG being longer than LO (just the reverse of what takes place in the movements of the inferior planets); at E, we should locate the planet at P on the ecliptic, in the sign Cancer; but at G, it would appear to us at Q, in the sign Gemini, having apparently retrograded on the ecliptic the distance PQ, while it was all the while moving on in the direct order of the signs. Now, suppose the earth moves on to I and the planet to U, we should then see it at the point W, further on in the ecliptic than Q, which indicates direct motion again, and at some point near Q the planet must have appeared without motion. After this, it will continue direct until the earth has completed a large portion of her orbit, as we shall easily see by imagining various positions of the earth and planet, and then drawing lines as we have just done, noticing whether they indicate direct or retrograde motion. The greater the distance of a planet the less it will retrograde, as we shall perceive by drawing another orbit outside the one represented in the cut, and making the same suppositions concerning it as those we have already explained.

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RETROGRADE MOTION OP A SUPERIOR PLANET.

Sidereal And Synodic Eevolution.—The interval of time required by a planet to perform a revolution from one fixed star back to it again, is termed a sidereal revolution (sidus, a star).

1. The interval of time between two similar conjunctions of an inferior planet with the earth and sun is termed a synodic revolution. Were the earth at rest, there would be no difference between a sidereal and a synodic revolution, and the planet would come into conjunction twice in each revolution. Since, however, the earth is in motion, it follows that after the planet has completed its sidereal revolution, it must then overtake the earth before they can both come again into the same position with regard to the sun. The faster a planet moves, the sooner it can do this. Mercury, travelling at the greater speed and on an inner orbit, accomplishes it much quicker than Venus. The synodic period always exceeds the sidereal.

2. The interval between two successive conjunctions or oppositions of a superior planet is termed a synodic revolution. Since the earth moves so much faster than any superior planet, it follows that after it has completed a sidereal revolution it must then overtake the planet before they can come again into the same position with regard to the sun. The slower the planet moves, the sooner it can do this. Uranus, making a sidereal revolution in eighty-four years, can be overtaken more quickly than Mars, which makes one in less than two years. It consequently requires over a second revolution to catch up with Mars, -fa of one to overtake Jupiter, and but little over of one to come up with Uranus. Indeed, the earth repasses Neptune in two days after it has finished a sidereal revolution.

Planets As Evening And Morning Stars.—The inferior planets are evening stars from superior to inferior conjunction, and the superior planets from opposition to conjunction. During the other half of their revolutions they are morning stars.

Mercury, evening star 2 months.

Venus, "" 9£"

Mars, "" 13

Jupiter, "" 6£"

Saturn, "" 6

Uranus, "" 6

To avoid filling the text with a multiplicity of figures, many interesting items are condensed in tables at the close of the volume.

VULCAN.

Supposed Discovery.—Le Verrier, having detected an error in the assumed motion of Mercury, suggested, in the fall of 1859, that there may be an interior planet, which is the cause of this disturbance. On this being made public, M. Lescarbault, a French physician, and an amateur astronomer, stated that on March 26 of that year he had seen a dark body pass across the sun's disk, and that this might have been the unknown planet. Le Verrier visited him, and found his instruments rough and home-made, but singularly accurate. His clock was a simple pendulum, consisting of an ivory ball hang

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ing from a nail by a silk thread. His observations were on prescription paper, covered with grease and laudanum. His calculations were chalked on a board, which he planed off to make room for fresh ones. Le Verrier became satisfied that a new planet had been really discovered by this enthusiastic observer, and congratulated him upon his deserved success. On March 20, 1862, Mr. Lummis, of Manchester, England, noticed a rapidly-moving, dark spot, apparently the transit of an inner planet. Many other instances are given of a somewhat similar character. As yet, however, the existence of the planet is not generally conceded. The name Vulcan and the sign of a hammer have been given to it. Its distance from the sun has been estimated at 13,000,000 miles, and its periodic time (its year) at 20 days.

MEECUEY.

The fleetest of the gods. Sign, a, his wand.

Description.—Mercury is nearest to the sun of any of the definitely known planets. When the sky is very clear, we may sometimes see it, just after the setting of the sun, as a bright sparkling star, near the western horizon. Its elevation increases evening by evening, but never exceeds 30°.* If we watch it closely, we shall find that it again ap

* This distance varies much, owing to the eccentricity of Mer cury's orbit

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