RETROGRADATION

, or Retrogression, in Astronomy, is an apparent motion of the planets, by which they seem to go backwards in the ecliptic, and to move contrary to the order or succession of the signs.

When a planet moves in consequentia, or according to the order of the signs, as from Aries to Taurus, from Taurus to Gemini, &c, which is from west to east, it is said to be direct.—When it appears for some days in the same place, or point of the heavens, it is said to be stationary.—And when it goes in antecedentia, or backwards to the following signs, or contrary to the order of the signs, which is from east to west, it is said to be retrograde. All these different affections or circumstances, may happen in all the planets, except the sun and moon, which are seen to go direct only. But the times of the superior and inferior planets being retrograde are different; the former appearing so about their opposition, and the latter about their conjunc- | tion. The intervals of time also between two Retrogradations of the several planets, are very unequal:

Again, Saturn continues retrograde 140 days, Jupiter 120, Mars 73, Venus 42, and Mercury 22; or nearly so; for the several Retrogradations of the same planet are not constantly equal.

These various circumstances however in the motions of the planets are not real, but only apparent; as the inequalities arise from the motion and position of the earth, from whence they are viewed; for when they are considered as seen from the sun, their motions appear always uniform and regular. These inequalities are thus explained:

Let S denote the sun; and ABCD &c the path or orbit of the earth, moving from west to east, and in that order; also GK &c the orbit of a superior planet, as Saturn for instance, moving the same way, or in the direction GKLG, but with a much less celerity than the earth's motion. Now when the earth is at the point A of its orbit, let Saturn be at G, in conjunction with the sun, when it will be seen at P in the zodiac, or among the stars; and when the earth has moved from A to B, let Saturn have moved from G to H in its orbit, when it will be seen in the line BHQ, and will appear to have moved from P to Q in the zodiac; also when the earth has got to C, let Saturn be arrived at I, but found at R in the zodiac, where being seen in the line CIR, it appears stationary, or without motion in the zodiac at R. But after this, Saturn will appear for some time in Retrogradation, viz, moving backwards, or the contrary way: for when the earth has moved to D, Saturn will have got to K, and, being seen in the line DKQ, will appear to have moved retrograde in the zodiac from R to Q; about which place the planet, ceasing to recede any farther, again becomes stationary, and afterward proceeds forward again; for while the earth moves from D to E, and Saturn from K to L, this latter, being now seen in the line ELR, appears to have moved forward in the zodiac from Q to R. And so on; the superior planets always becoming retrograde a little before they are in opposition to the sun, and continuing so till some time after the opposition: the retrograde motion being swiftest when the planet is in the very opposition itself; and the direct motion swiftest when in the conjunction. The arch RQ which the planet describes while thus retrograde, is called the arch of Retrogradation. These arches are unequal in all the planets, being greatest in the most distant. and gradually less in the nearer ones.

In like manner may be shewn the circumstances of the Retrogradations of the inferior planets; by which it will appear, they become stationary a little before their inferior conjunction, and go retrograde till a little time after it; moving the quickest retrograde just at that conjunction, and the quickest direct just at the superior or further conjunction.

Retrogradation of the Nodes of the Moon, is a motion of the line of the nodes of her orbit, by which it continually shifts its situation from east to west, contrary to the order of the signs, completing its retrograde circulation in the period of about 19 years: after which time, either of the nodes, having receded from any point of the ecliptic, returns to the same again.—Newton has demonstrated, in his Principia, that the Retrogradation of the moon's nodes is caused by the action of the sun, which continually drawing this planet from her orbit, deflects this orbit from a plane, and causes its intersection with the ecliptic continually to vary; and his determinations on this point have been confirmed by observation.

Retrogradation of the Sun, a motion by which in some situations, in the torrid zone, he seems to move retrograde or backwards.

When the sun is in the torrid zone, and has his declination AM greater than the latitude of the place AZ, but either northern or southern as that is (last fig. above), the sun will appear to go retrograde, or backwards, both before and after noon. For draw the vertical circle ZGN to be a tangent to the sun's diurnal circle MGO in G, and another ZON through the sun's rising, at O: then it is evident, that all the intermediate vertical circles cut the sun's diurnal circle twice: first in the arc GO, and the second time in the arc GI. So that, as the sun ascends through the arc GO, he continually arrives at farther and farther verticals. But as he continues his ascent through the arc GI, he returns to his former verticals; and therefore is seen retrograde for some time before noon. And in like manner it may be shewn that he does the same thing for some time after noon. Hence, as the shadow always tends opposite to the sun, the shadow will be retrograde twice every day in all places of the torrid zone, where the sun's declination exceeds the latitude.

But the same thing can never happen without the tropics, in a natural way.

, or Retrogression, in the Higher Geometry, is the same with what is otherwise called contrary flexion or flexure. See Flexure, and Inflexion. |