SIDUS, GEORGIUM, in astronomy, a new primary planet, discovered by William Herschel, Esq; of Bath, in the year 1781, for which he obtained from the Royal Society the honorary recompence of Sir Godfrey Copley's medal; and so called in honour of his majesty king George III. who has taken Mr Herschel under his patronage, and granted to him an annual salary. "From many calculations of our best
astronomers and mathematicians, says Mr Herschel, I have collected the following particulars, as most to be depended upon.
| Place of the node | 2h 11m 49s 30o |
| Inclination of the orbit | 43o 35' |
| Place of the perihelion | 172d 13' 17" |
| Time of the perihelion passage | Sep. 7, 1799 |
| Eccentricity of the orbit | .82034 |
| Half the greater axis | 19,079.04 |
| Revolution | 83,336.4 sidereal years. |
From my own observations on this planet's apparent diameter, which I have found cannot well be less than 4", nor indeed much greater, we infer that its real diameter is to that of the earth as 4,454 to 1; and hence it appears to be of very considerable bulk, and, except Saturn and Jupiter, by far the largest of the remaining planets. Its light is of a bluish-white colour, and in brilliancy between that of the Moon and of Venus. With a telescope which 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 sixth and seventh magnitude. In a very fine clear night, when the moon is absent, it may also be seen by the naked eye."