4 INTRODUCTION'. 



round him revolve the seven planets, the names of which are, be- 

 ginning with the nearest to the sun, Mercury, Venus, the Earth, 

 Mars, Jupiter, Saturn, and the Georgium Sidus. The last of these 

 was first discovered by Dr. Herschel with his telescope of great 

 size and power, 40 feet in length, and 4| in diameter, in the year 

 1781. For this discovery he received from the Royal Society the 

 honorary recompense of sir Godfrey Copley's medal. Though the 

 Georgium Sidus was not till then known as a planet, there are many 

 reasons to suppose it had been seen before, but had been considered 

 as a fixed star. But from the steadiness of its light, from its diame- 

 ter being increased by high magnifying powers, and from the 

 change he had observed in its situation, Dr. Herschel first con- 

 cluded it was a comet ; but in a little time he, with others, deter- 

 mined that it was a planet, from its vicinity to the ecliptic, the direc- 

 tion of its motion, and its being stationary in the time and in such 

 circumstances as correspond with similar appearances in other 

 planets. When the moon is absent, it may be seen by the naked 

 eye ; and the discovery of six satellites attending it confers upon it a 

 dignity, and raises it to a conspicuous situation among the great 

 bodies of our solar system.* 



The magnitudes, annual periods, and mean distances from the 

 sun, of the several planets, are given in the table subjoined. Their 

 relative distances may likewise be thus stated in a manner more 

 compendious and easy to be remembered. If the distance of the 

 earth from the sun be supposed to be divided into ten parts, the 

 distance of Mercury will be four such parts, that of Venus seven, 

 that of Mars fifteen, that of Jupiter fity-two, that of Saturn ninety- 

 five, and that of the Georgium Sidus one hundred and ninety. 



Besides these seven planets, there are eighteen others which 

 move round four of these in the same manner as they do round the 

 sun. Of these our Earth has one, called the Moon ; Jupiter has 

 four ; Saturn seven, two of these having been lately discovered by 

 Dr. Herschel ; and the Georgium Sidus six, discovered, as well as 

 the planet itself, by the same excellent astronomer. These are call- 

 ed moons, from their resemblance to our moon ; and sometimes 

 secondary planets, because they seem to be attendants of the Earth, 

 Jupiter, Saturn, and the Georgium Sidus, about which they move, 

 and which are called primary. 



The orbits described by the planets are not exact circles, but 

 ellipsis or ovals : hence the same planet is not always at the same 

 distance from the sun ; and the distance which is exactly between 

 the greatest and least distance is called the mean distance. 



* Another new planet was discovered on the 1st of January 1801, by M. 

 Plazz.i, astronomer-royal at Palermo in Ciciiy, who called it Ferdinand ea, in 

 honour of his Sicilian majesty. It is also called Ceres, or Ceres Ferdinandea. 

 It has its orbit between those of Mars and Jupiter. According to the latest 

 observations, its period is four years 222 days. Its distance from the sun is to 

 that of the earth as-267to]100, consequently above 250 millions of miles. It is not 

 visible to the naked eye ; and so small, that glasses of a very high magnifying power 

 will not show it with a distinctly -defined diameter : Dr. Herschel has however 

 estimated its diameter at 160 English miles. 



Another planet has also been discovered by Dr. Olbers of Hamburgh : it is 

 likewise situated between Mars and Jupiter. It has been named Pallas. Its 

 distance from the sun is to that of the earth as 280 to 100, or nearly 2/0 mil- 

 lions of miles. It is extremely small ; its diameter being estimated by Dr. Her- 

 schel at only 110 miles. 



