STATIONARY

, in Astronomy, the state of a planet when, to an observer on the earth, it appears for some time to stand still, or remain immoveable in the same place in the heavens. For as the planets, to such an observer, have sometimes a progressive motion, and sometimes a retrograde one, there must be some point between the two where they must appear Stationary. Now a planet will be seen Stationary, when the line that joins the centres of the earth and planet is constantly directed to the same point in the heavens, which is when it keeps parallel to itself. For all right lines drawn from any point of the earth's orbit, parallel to one another, do all point to the same star; the distance of these lines being insensible, in comparison of that of the fixed stars.

The planet Herschel is seen Stationary at the distance of from the sun; Saturn at somewhat more than 90°; Jupiter at the distance of 52°; and Mars at a much greater distance; Venus at 47°, and Mercury at 28°.

Herschel is Stationary days, Saturn 8, Jupiter 4, Mars 2, Venus 1 1/2, and Mercury 1/2 a day: though the several stations are not always equal; because the orbits of the planets are not circles which have the sun in their centre.