APSES

, in Astronomy, are the two points in the orbits of planets, where they are at their greatest and least distance, from the sun or the earth. The point at the greatest distance being called the higher apsis, and that at the nearest distance the lower apsis. And the two apses are also called auges. Also the higher apsis is more particularly called the aphelion, or the apogee; and the lower apsis, the perihelion, or the perigee. The diameter which joins these two points, is called the line of the apses or of the apsides; and it paffes through the centre of the orbit of the planet, and the centre of the sun or the earth; and in the modern astronomy this line makes the longer or transverse axis of the elliptical orbit of the planet. In this line is counted the excentricity of the orbit; being the distance between the centre of the orbit and the focus, where is placed the sun or the earth.

The foregoing definitions suppose the lines of the greatest and least distances to lie in the same straight line; which is not always precisely the case; as they are sometimes out of a right line, making an angle greater or less than 180 degrees, and the difference from 180 degrees is called the motion of the line of the apses: when the angle is less than 180 degrees, the motion of the apses is said to be contrary to the order of the signs; on the other hand, when the angle exceeds 180 degrees, the motion is according to the order of the signs.

Different means have been employed to determine the motion of the apses. Dr. Keil explains, in his Astronomy, the method used by the ancients, who supposed the orbits of the planets to be perfectly circular, and the sun out of the centre. But since it has been discovered that they describe elliptical orbits, various other methods have been devised for determining it. Halley has given one, which supposes to be known only the time of the planet's revolution, or periodic time. Seth Ward has also given a determination from three different observations of a planet, in any three places of its orbit: but his method being founded on an hypothesis not strictly true, Euler has given one much more exact in vol. 7. of the Petersburgh Commentaries. See various ways explained in the Astronomy of Keil and Mounier.

Newton has also given, in the Principia, an excellent method of determining the same motion, on the supposition that the orbits of the planets differ but little from circles, which is the case nearly. That great philosopher shews, that if the sun be immoveable, and all the planets gravitate towards him in the inverse ratio of the squares of their distances, then the apses will be fixed, or their motion nothing; that is, the lines of greatest and least distance will form one right line, and the apses will be directly opposite, or at 180 degrees distance from each other. But, because of the mutual tendency of the planets towards each other, their gravitation towards the sun is not precisely in that ratio; and hence it happens, that the apses are not always exactly in a right line with the sun. And Newton has given a very elegant method of determining the motion of the apses, on the supposition that we know the force which is thereby added to the gravitation of the planet towards the sun, and that this additional force is always in that direction.

APUS or Apous, Avis Indica, in Astronomy, a constellation of the southern hemisphere, situated near the south pole, between the triangulum australe and the chameleon, and supposed to represent the bird of paradise. Also supposed to be one of the birds named Apodes, as having no feet.

The number of stars contained in this constellation, are 11 in the British Catalogue, in Bayer's Maps 12, and a still greater number in La Caille's Catalogue; the principal star being but of the 4th or 5th order of magnitude. See Cœlum Australe Stelliferum, and the Memoires de l'Acad. for 1752, pa. 569.