SIDEREAL

, or Siderial, something relating to the stars. As Sidereal year, day, &c, being those marked out by the stars.

Sidereal Year. See Year.

Sidereal Day, is the time in which any star appears to revolve from the meridian to the meridian again; which is 23 hours 56′ 4″ 6‴ of mean solar time; there being 366 Sidereal days in a year, or in the time of 365 diurnal revolutions of the sun; that is, exactly, if the equinoctial points were at rest in the heavens. But the equinoctial points go backward, with respect to the stars, at the rate of 50″ of a degree in a Julian year; which causeth the stars to have an apparent pro- gressive motion eastward 50″ in that time. And as the sun's mean motion in the ecliptic is only 11 signs 29° 45′ 40″ 15‴ in 365 days, it follows, that at the end of that time he will be 14′ 19″ 45‴ short of that point of the ecliptic from which he set out at the beginning; and the stars will be advanced 50″ of a degree with respect to that point.

Consequently, if the sun's centre be on the meridian with any star on any given day of the year, that star will be 14′ 19″ 45‴ + 50″ or 15′ 9″ 45‴ east of the sun's centre, on the 365th day afterward, when the sun's centre is on the meridian; and therefore that star will not come to the meridian on that day till the sun's centre has passed it by 1′ 0″ 38‴ 57′′′′ of mean solar time; for the sun takes so much time to go through an arc of 15′ 9″ 45‴; and then, in 365^da 0^h 1′ 0″ 38‴ 57′′′′ the star will have just completed its 366th revolution to the meridian.

In the following table, of Sidereal revolutions, the first column contains the number of revolutions of the stars; the others next it shew the times in which these revolutions are made, as shewn by a well regulated clock; and those on the right hand shew the daily accelerations of the stars, that is, how much any star gains upon the time shewn by such a clock, in the corresponding revolutions.

This table will not differ the 279936000000th part of a second of time from the truth in a whole year. It was calculated by Mr. Ferguson; and it is the only table of the kind in which the recession of the equinoctial points has been taken into the calculation.

SIDUS Georgium, a new primary planet, discovered by Dr. Herschel at Bath, in the night of March 13, 1781. It is sometimes also called the Georgian Planet, and the New Planet, from its having been newly or lately discovered, also Herschel's Planet, from the name of its discoverer, and the Planet Herschel, or simply Herschel, by which name it is distinguished by the astronomers of almost all foreign nations. The planet is denoted by this character , a Roman H as the initial of the name, the horizontal bar being crossed by a perpendicular line, forming a kind of cross, the emblem of Christianity, meaning thereby perhaps that its discovery was made by a Christian, or since the birth of Christ, as all the other planets were discovered long before that period.

This planet is the remotest of all those that are yet known, though not the largest, being in point of magnitude less than Saturn and Jupiter. Its light, says Dr. Herschel, is of a blueish-white colour, and its brilliancy between that of Venus and the moon. With a telescope that magnifies about 300 times, it appears to have a very well defined visible disk; but with instruments of a small power, it can hardly be distinguished from a fixed star of between the 6th and 7th magnitude. In a very fine clear night, when the moon is absent, a good eye will perceive it without a telescope.

From the observations and calculations of Dr. Herschel and other astronomers, the elements and dimenfions &c of this planet, have been collected as below.