

Back MatterAppendix.Sizes and Distances of the Planets.At the end of this book about planets and stars, it will be convenient to set down a few numbers which give their sizes and their distances. These numbers are not to be read, but sometimes you can refer to them if you like. They are generally as accurate as astronomers know how to make them, though sometimes the round number, not the accurate one, is given. The Earth.The earth is a globe flattened at the poles. Its largest diameter (at the equator) is 41,852,404 feet = 7,926.6 miles. Its shortest diameter (at the poles) is 41,709,790 feet = 7,899.6 miles. The earth turns around its axis in 23 hours 56 minutes 4.090 seconds of mean solar time (the time kept by our watches and clocks). The earth goes once around the sun in a year of 365 days 5 hours 48 minutes 45.51 seconds. The month (from one new moon to the next new moon) is 29 days 12 hours 44 minutes 2.684 seconds. The earth moves in its orbit every second, 18 1/2 miles. The sun weighs 333,470 times as much as the earth. The earth is 5 1/2 times as heavy as a globe of water of the same size. The weight of the earth is 12,000,000,000,000,000,000,000,000 pounds. The Moon.The month (from new moon to the next new moon) is 29 days 12 hours 44 minutes 2.684 seconds. The moon's diameter is 2,163 miles. The earth weighs 81 times as much as the moon. The earth's bulk (size) is 50 times as much as that of the moon. The moon weighs 3.4 times as much as a globe of water of the same size. The moon turns completely around its own axis once in 27 days 7 hours 43 minutes. For every 100 units of sunlight that fall on the moon's surface it reflects back into space 17 units only (about as much as a moon made out of gray sandstone would do). Mercury.Mercury's diameter is 3,030 miles. The sun weighs 10,530,500 times as much as Mercury. Mercury is 56/1000 of the bulk (size) of the earth. Mercury weighs 31 times as much as a globe of water of the same size. A piece of iron that weighs a pound on the earth would weigh 2/10 of a pound on Mercury. Mercury is 36,000,000 miles distant from the sun. Mercury goes completely around the sun once in 87.96926 days. (This is the year of Mercury.) Mercury moves in its orbit every second, about 27 miles. For every 100 units of sunlight that fall on Mercury 13 units are reflected back into space. Venus.The diameter of Venus is 7,700 miles. The sun weighs 408,000 times as much as Venus. Venus is 920/1000 of the bulk (size) of the earth. Venus weighs 4 8/10 times as much as a globe of water of the same size. A piece of iron that weighs one pound on the earth would weigh 8/10 of a pound on Venus. Venus is 67,200,000 miles distant from the sun. Venus goes completely around the sun once in 224.7008 days. (This is the year of Venus.) Venus moves in its orbit every second, about 22 miles. For every 100 units of sunlight that fall on Venus 50 units are reflected back into space. Mars.The diameter of Mars is 4,230 miles. The sun weighs 3,093,500 times as much as Mars. Mars is 15/100 of the bulk (size) of the earth. Mars weighs four times as much as a globe of water of the same size. Mars turns once on its axis in 24 hours 37 minutes 22.67 seconds. (This is the day of Mars.) A piece of iron that weighs one pound on the earth would weigh 4/10 of a pound on Mars. Mars is 141,500,000 miles distant from the sun. Mars goes completely around the sun once in 686.9505 days. (This is the year of Mars.) Mars moves in its orbit every second, 15 miles. For every 100 units of sunlight that fall on Mars 26 units are reflected back into space. Satellites of Mars.I. Phobos.—It is probably about 7 miles in diameter only. It is 5,850 miles distant from the center of Mars. It revolves about Mars once in 7 hours 39 minutes 15.1 seconds. II. Deimos.—It is probably only about 5 miles in diameter. It is 14,650 miles distant from the center of Mars. It revolves about Mars once in 1 day 6 hours 17 minutes 54.0 seconds. Jupiter.The diameter of Jupiter is 86,500 miles. The sun weighs 1,048 times as much as Jupiter. Jupiter is 316 times as heavy as the earth. Jupiter is 1,309 times the bulk (size) of the earth. Jupiter weighs 1.33 times as much as a globe of water of the same size. Jupiter turns on its axis once in 9 hours 55 minutes. (This is the day of Jupiter.) A piece of iron that weighs one pound on the earth would weigh 2.65 pounds on Jupiter. Jupiter is 483,300,000 miles distant from the sun. Jupiter goes completely around the sun once in 11.86 of our years. (This is the year of Jupiter.) Jupiter moves in its orbit, every second, 8 miles. For every 100 units of sunlight that fall on Jupiter it reflects 62 units back into space. Satellites of Jupiter.V. The fifth satellite of Jupiter was discovered by Professor Barnard, at the Lick Observatory, in 1892. It is very small, probably not more than 100 miles in diameter, and it revolves round the planet in 11 hours 57 minutes 22.7 seconds. The other moons of Jupiter are much larger bodies, and they were discovered by Galileo in 1610. They are called I, II, III, IV. I. Its diameter is 2,500 miles. It is 261,000 miles distant from the center of Jupiter. It revolves about Jupiter once in 1 day 18 hours 27 minutes 33.5 seconds. It is sometimes called Io. II. Its diameter is 2,100 miles. It is 415,000 miles distant from the center of Jupiter. It revolves about Jupiter once in 3 days 13 hours 13 minutes 42.1 seconds. It is sometimes called Europa. III. Its diameter is 3,550 miles. It is 664,000 miles distant from the center of Jupiter. It revolves once about Jupiter in 7 days 3 hours 42 minutes 33.4 seconds. It is sometimes called Ganymede. IV. Its diameter is 2,960 miles. It is 1,167,000 miles distant from the center of Jupiter. It revolves once about Jupiter in 16 days 16 hours 32 minutes 11.2 seconds. It is sometimes called Callisto. Saturn.The diameter of the ball of Saturn is 73,000 miles. The sun weighs 3,490 times as much as Saturn. Saturn is 95 times as heavy as the earth. Saturn is 760 times the bulk (size) of the earth. Saturn weighs 7/10 as much as a globe of water of the same size. Saturn's ball turns on its axis once in 10 hours 14 minutes 24 seconds. (This is the day of Saturn.) A piece of iron that weighs one pound on the earth would weigh 1 2/10 pounds on Saturn. Saturn is 886,000,000 miles distant from the sun. Saturn goes completely round the sun once in 29.46 of our years. (This is the year of Saturn.) Saturn moves in its orbit every second, 6 miles. For every 100 units of sunlight that fall on Saturn 52 units are reflected back into space. Satellites of Saturn.I. Minas.—Its diameter is about 600 miles. Its distance from Saturn is about 117,000 miles. It revolves around Saturn once in about 22 1/2 hours (22h. 37m. 5.7s.). II. Enceladus.—Its diameter is about 800 miles. Its distance from Saturn is about 157,000 miles. It revolves once around Saturn in about 33 hours (32h. 53m. 6.9s.). III. Tethys.—Its diameter is about 1,100 miles. Its distance from Saturn is about 186,000 miles. It revolves about Saturn once in about 1 day 21 hours (1d. 21h. 18m. 25.6s.). IV. Dione.—Its diameter is about 1,200 miles. Its distance from Saturn is about 238,000 miles. It revolves about Saturn in about 2 days 18 hours (2d. 17h. 41m. 9.3s.). V. Rhea.—Its diameter is about 1,500 miles. Its distance from Saturn is about 332,000 miles. It revolves around Saturn in about 4 days 12 hours (4d. 12h. 25m. 11.6s.). VI. Titan.—Its diameter is about 3,500 miles. Its distance from Saturn is 771,000 miles. It revolves around Saturn in about 16 days (15d. 22h. 41m. 23.2s.). VII. Hyperion.—Its diameter is about 500 miles. Its distance from Saturn is 934,000 miles. It revolves around Saturn once in about 21 days (21d. 6h. 39m. 27.0s.). VIII. Japetus.—Its diameter is about 2,000 miles. Its distance from Saturn is 2,225,000 miles. It revolves around Saturn once in about 79 days (79d. 7h. 54m. 17.1s.). IX. Unnamed. —Discovered by Professor Pickering at the Harvard College Observatory in March, 1899, by photography. Its time of revolution is about 17 months. The Rings of Saturn.The outer ring of Saturn (A) is 168,000 miles in diameter, and about 10,000 miles wide. It is separated from the next ring by a circular division about 1,600 miles wide. The next ring (B) is about 16,500 miles wide. A and B are the two bright rings. The faint innermost ring (C) is about 10,000 miles wide. The thickness of each ring is about 100 miles. All of the rings are made up of little satellites in swarms. Uranus.The diameter of Uranus is 31,900 miles. The sun weighs 22,600 times as much as Uranus. Uranus weighs 15 times as much as the earth. Uranus is 65 times the bulk (size) of the earth. Uranus weighs 1 2/10 times as much as a globe of water of the same size. We do not know how long it requires for Uranus to turn on its axis. A piece of iron that weighs a pound on the earth would weigh 9/10 of a pound on Uranus. Uranus is 1,781,900,000 miles distant from the sun. Uranus goes completely round the sun once in 84.02 of our years. (This is the year of Uranus.) Uranus moves in its orbit every second, 4 miles. For every 100 units of sunlight that fall on Uranus 64 are reflected back into space. Satellites of Uranus.I. Ariel.—Its diameter is about 500 miles. Its distance from Uranus is 120,000 miles. It revolves around Uranus once in about 2 days (2d. 12h. 29m. 21.1s.). II. Umbriel.—Its diameter is about 400 miles. Its distance from Uranus is 167,000 miles. It revolves around Uranus once in about 4 days (4d. 3h. 27m. 37.2s.). III. Titania.—Its diameter is about 1,000 miles. Its distance from Uranus is 273,000 miles. It revolves around Uranus once in about 9 days (8d. 16h. 56m. 29.5s.). IV. Oberon.—Its diameter is about 800 miles. Its distance from Uranus is 365,000 miles. It revolves around Uranus once in about 13 1/2 days (13d. 11h. 7m. 6.4s.). Neptune.The diameter of Neptune is 34,800 miles. The sun weighs 19,380 times as much as Neptune. Neptune weighs 17 times as much as the earth. Neptune is 85 times the bulk (size) of the earth. Neptune weighs 1 1/10 times as much as a globe of water of the same size. We do not know how long it requires for Neptune to turn on its axis. A piece of iron that weighs a pound on the earth would weigh 9/10 of a pound on Neptune. Neptune moves in its orbit every second, 3 miles. For every 100 units of sunlight that fall on Neptune 46 are reflected back into space. Neptune is 2,791,600,000 miles distant from the sun. Neptune goes completely round the sun once in 164.78 of our years. (This is the year of Neptune.) Satellite of Neptune.Its diameter is about 2,000 miles. Its distance from Neptune is 225,000 miles. It revolves around Neptune once in about 6 days (5d. 21h. 2m. 44.2s.). Comets that Belong to the Solar Family.Most comets do not belong to the sun's family. A dozen or more comets do, however, move around the sun just as the planets do. The comet nearest the sun (Encke's comet) takes 3.307 of our years to move once completely around the sun. The comet farthest from the sun (Halley's comet) takes 76.37 years. Distances of Stars.We know the distances of about thirty stars with some exactness. Alpha Centauri is the nearest star to us, and its light reaches the earth 4 1/3 of our years after it has left the star. 61 Cygni is a famous star and one of the nearest ones. Its light takes about 7 years for its journey. The bright star Sirius is a near star, too. Its light takes 8 1/2 years to reach us. The light from the polestar—Polaris—only reaches us 47 years after it has left the star. Most stars are very much farther off, and their light only reaches us hundreds or even thousands of years after it is sent out from the stars. 
