The theory of Copernicus deprived the Earth of its special position as the immovable centre of the universe, but raised it to the rank of a planet. It is therefore a heavenly body, yet needs no telescope to bring it within our ken; bad weather does not hide it from us, but rather shows it to us under new conditions. We find it to be a globe of land and water, covered by an atmosphere in which float changing clouds; we have mapped it, and we find that the land and water are always there, but their relations are not quite fixed; there is give and take between them. We have found of what elements the land and water consist, and how these elements combine with each other or dissociate. In a word, the Earth is the heavenly body that we know the best, and with it we must compare and contrast all the others.
Before the invention of the telescope there were but two other heavenly bodies—the Sun and the Moon—that appeared as orbs showing visible discs, and even in their cases nothing could be satisfactorily made out as to their conditions. Now each of the five planets known to the ancients reveals to us in the telescope a measurable disc, and we can detect significant details on their surfaces.
THE MOON is the one object in the heavens which does not disappoint a novice when he first sees it in the telescope. Every detail is hard, clear-cut, and sharp; it is manifest that we are looking at a globe, a very rough globe, with hills and mountains, plains and valleys, the whole in such distinct relief that it seems as if it might be touched. No clouds ever conceal its details, no mist ever softens its outlines; there are no half-lights, its shadows are dead black, its high lights are molten silver. Certain changes of illumination go on with the advancing age of the Moon, as the crescent broadens out to the half, the half to the full, and the full, in its turn, wanes away; but the lunar day is nearly thirty times as long as that of the Earth, and these changes proceed but slowly.
The full Moon, as seen by the naked eye, shows several vague dark spots, which most people agree to fancy as like the eyes, nose, and mouth of a broad, sorrowful face. The ordinary astronomical telescope inverts the image, so the "eyes" of the Moon are seen in the lower part of the field of the telescope as a series of dusky plains stretching right across the disc. But in the upper part, near the left-hand corner of the underlip, there is a bright, round spot, from which a number of bright streaks radiate—suggesting a peeled orange with its stalk, and the lines marking the sections radiating from it. This bright spot has been called after the great
Mean distance from Sun. Period Velocity
Class. Name. Earth's In millions of revolution. in orbit. Eccentricity.
distance of miles. In years. Miles per
= 1. sec.
Terrestrial Mercury 0.387 36.0 0.24 29.7 0.2056
Planets Venus 0.723 67.2 0.62 21.9 0.0068
Earth 1.000 92.9 1.00 18.5 0.0168
Mars 1.524 141.5 1.88 15.0 0.0933
Minor Eros 1.458 135.5 1.76 15.5 0.2228
Planets Ceres 2.767 257.1 4.60 11.1 0.0763
Achilles 5.253 488.0 12.04 8.1 0.0509
Major Jupiter 5.203 483.3 11.86 8.1 0.0483
Planets Saturn 9.539 886.6 29.46 6.0 0.0561
Uranus 19.183 1781.9 84.02 4.2 0.0463
Neptune 30.055 2791.6 164.78 3.4 0.0090
Mean diameter. Surface. Volume. Mass.
Name. Symbol. In miles. [Earth]=1. [Earth]=1. [Earth]=1. [Earth]=1.
Sun [Sun] 866400 109.422 11973. 1310130. 332000.
Moon [Moon] 2163 0.273 0.075 0.02 0.012
Mercury [Mercury] 3030 0.383 0.147 0.06 0.048
Venus [Venus] 7700 0.972 0.945 0.92 0.820
Earth [Earth] 7918 1.000 1.000 1.00 1.000
Mars [Mars] 4230 0.534 0.285 0.15 0.107
Jupiter [Jupiter] 86500 10.924 119.3 1304. 317.7
Saturn [Saturn] 73000 9.219 85.0 783. 94.8
Uranus [Uranus] 31900 4.029 16.2 65. 14.6
Neptune [Neptune] 34800 4.395 19.3 85. 17.0
Light
Gravity. and heat Albedo;
Density. Fall in received i.e. re-
[Earth] Water [Earth] feet per from Sun. Time of rotation flecting
Name. =1. =1. =1. sec. [Earth]=1. on axis. power.
d. h. m.
Sun 0.25 1.39 27.65 444.60 ... 25 4 48 ± ...
Moon 0.61 3.39 0.17 2.73 1.00 27 7 43 0.17
d. h. m. s.
Mercury 0.85 4.72 0.43 6.91 6.67 88 (?) 0.14
Venus 0.89 4.94 0.82 13.19 1.91 23 21 23 (?) 0.76
Earth 1.00 5.55 1.00 16.08 1.00 23 56 4 0.50 (?)
Mars 0.71 3.92 0.38 6.11 0.43 24 37 23 0.22
h. m.
Jupiter 0.24 1.32 2.65 42.61 0.037 9 55 ± 0.62
Saturn 0.13 0.72 1.18 18.97 0.011 10 14 ± 0.72
Uranus 0.22 1.22 0.90 14.47 0.003 9 30 (?) 0.60
Neptune 0.20 1.11 0.89 14.31 0.001 (?) 0.52
Danish astronomer, "Tycho," and is one of the most conspicuous objects of the full Moon.