Why the Moon appears bigger than any of the Planets.
From hence it appears, that the same body at different distances, will seem to have very different magnitudes. Thus the diameter BD will appear from the point E, to be twice as large as from the point A. It also follows, that a small body, when at no great distance from us, may appear to be equal, or even to exceed another at a great distance, tho’ immensely bigger. Thus b d appears under the same angle, and consequently of the same bigness from the point A, that the line B D doth, tho’ one vastly exceeds the other. And this is the reason, why the Moon, which is much less than any of the Planets, appears to us vastly bigger than either of them, and even to equal the Sun himself, which is many thousand times greater in magnitude.
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 Sun in the space of Y. D. H. | Distance in Miles | |
|---|---|---|
| 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 |
| Round the Earth. D. H. M. | ||
| Moon | 27: 7:43 | 240.000 |
| Periods round their own axis D. H. M. | Diameters in Miles. | |||
|---|---|---|---|---|
| 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.
Plate 2.
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.