The distances of the Planets, and periods round the Sun, their diameters and velocities round their own axis, according to modern computations, are as follows:
|Revolves about the | Distance in |Sun in the space of| Miles | Y. D. H | | | _Saturn_ | 29:167:22 | 777.000.000 _Jupiter_ | 11:314:12 | 424.000.000 _Mars_ | 1:321:23 | 123.000.000 _Earth_ | 0:365: 6 | 81.000.000 _Venus_ | 0:224:16 | 59.060.000 _Mercury_ | 0: 87:23 | 32.000.000
_Moon_} Round the { D. H. M. | } Earth. { 27: 7: 43 | 240.000
| Periods round | Diameters | their own axis.| in Miles. | D. H. M. | _Sun_ | 25: 6: 0 | 763.000 _Saturn_ | | 61.000 _Jupiter_ | 0: 9: 56 | 81.000 _Mars_ | 1: 0: 40 | 4.440 _Earth_ | 0: 23: 56 | 7.970 _Venus_ | 24: 8: 0 | 7.900 _Mercury_ | | 4.240 _Moon_ | 27: 7: 43 | 2.170
The cause of _Eclipses_ and _Phases_ of the Moon, and some other phænomena not here explained, shall be shewed when we come to give a Description of the _Orrery_.
Besides the Planets already mentioned, there are other great bodies that sometimes visit our system, which are a sort of temporary Planets; for they come and abide with us for a while, and afterwards withdraw from us, for a certain space of time, after which they again return. These wandering bodies are called _Comets_.
The motion of Comets in the Heavens, according to the best observations hitherto made, seem to be regulated by the same immutable law that rules the Planets; for their orbits are elliptical, like those of the Planets, but vastly narrower, or more excentric. Yet they have not all the same direction with the Planets, who move from West to East, for some of the Comets move from East to West; and their orbits have different inclinations to the Earth’s orbit; some inclining Northwardly, others Southwardly, much more than any of the Planetary orbits do.
Altho’ both the Comets and the Planets move in elliptic orbits, yet their motions seem to be vastly different: For the excentricities of the Planet’s orbits are so small, that they differ but little from circles; but the excentricities of the Comets are so very great, that the motions of some of them seem to be almost in right lines, tending directly towards the Sun.
Now, since the orbits of the Comets are so extremely excentric, their motions, when they are in their _Perihelia_, or nearest distance from the sun, must be much swifter than when they are in their _Aphelia_, or farthest distance from him; which is the reason why the Comets make so short a stay in our system; and when they disappear, are so long in returning.