Mercury, Venus, and Mars—How to make a Drawing of our System—The Planet Mercury—The Planet Venus—The Transit of Venus—Venus as a World—The Planet Mars and his Movements—The Ellipse—The Discoveries made by Tycho and Kepler—The Discoveries made by Newton—The Geography of Mars—The Satellites of Mars—How the Telescope aids in Viewing Faint Objects—The Asteroids, or Small Planets.
MERCURY, VENUS, AND MARS.
We can hardly think of either the sun or the moon as a world in the sense in which our earth is a world, but there are some bodies called planets which seem more like worlds, and it is about them that we are now going to talk. Besides our Earth there are seven planets of considerable size, and a whole host of insignificant little ones. These planets are like ours in a good many respects. One of them, Venus, is about the same size as this earth; but the two others, Mercury and Mars, are very much smaller. There are also some planets very much larger than any of these, namely, Jupiter, Saturn, Uranus, and Neptune. We shall in this lecture chiefly discuss three bodies, namely, Mercury, Venus, and Mars, which, with the earth, form the group of “inner” planets.
The planets are all members of the great family dependent on the sun. Venus and the earth may be considered the pair of twins, alike in size and weight. Mercury and Mars are the babies of the system. The big brothers are Jupiter and Saturn. All the planets revolve round the sun, and derive their light and their heat from his beams. We should like to get a little closer to some of our fellow-planets and learn their actual geography. Unfortunately, even under the most favorable circumstances, they are a very long way off. They are many millions of miles distant, and are always at least a hundred times as far as the moon. But far as the planets may be, astronomers have been familiar with their existence for ages past. I can give you a curious proof of this. You remember how we said the first and the second days of the week were called after the sun and the moon, Sun-day and Moon-day, or Monday, respectively. Let us see about the other days. Tuesday is not quite so obvious, but translate it into French and we have at once Mardi; this word means nothing but Mars’ day, and our Tuesday means exactly the same. Wednesday is also readily interpreted by the French word Mercredi, or Mercury’s day, while Venus corresponds to Friday. Jupiter’s day is Thursday, while Saturn’s day is naturally Saturday. The familiar names of the days of the week are thus associated with the seven moving celestial bodies which have been known for uncounted ages.
HOW TO MAKE A DRAWING OF OUR SYSTEM.
I want every one who reads this book to make a little drawing of the sun and the planets. The apparatus that you will need is a pair of compasses; any sort of compasses that will carry a bit of pencil will do. You must also get a little scale that has inches and parts of inches divided upon it; any carpenter’s rule will answer. The drawing is intended to give a notion of the true sizes and positions of the fine family of which the earth is one member. The figure I have given ([Fig. 46]) is not on so large a scale as that which I ask you to use, and which I shall here mention. Try and do the work neatly, and then pin up your little drawings where you will be able to see them every day until you are quite familiar with the notion of what we mean by our solar system.
Fig. 46.—The Orbits of the Four Inner Planets.
First open the compasses one inch, and then describe a circle, and mark a dot on this as “Mercury,” in neat letters, and also write on the circle “88 days.” At the centre you are to show the “Sun.” This circle gives the track followed by Mercury in its journey round the sun in the period of 88 days. Next open your compasses to 1¾ in., which you must do accurately by the scale. The circle drawn with this radius shows the relative size of the path of Venus, and to indicate the periodic time, you should mark it, “225 days.” The next circle you have to draw is a very interesting one. The compass is to be opened 2½ in. this time, and the path that it makes is to be marked “365 days.” This shows the high road along which we ourselves journey every year, along which we are, indeed, journeying at this moment. If you wanted to obtain from your figure any notions of the true dimensions of the system, the path of the earth will be the most convenient means of doing so. The earth is 93,000,000 miles from the sun, and our drawing shows its orbit as a circle of 2½ in. radius. It follows that each inch on our little scale will correspond to about 37,000,000 miles. As, therefore, the radius of the orbit of Mercury has been taken to be one inch, it follows that the distance of Mercury from the sun is about 37,000,000 miles.
We have, however, still one more circle to draw before we complete this little sketch. The compass must now open to four inches, and a circle which represents the orbit of Mars is then to be drawn. We mark on this “687 days,” and the inner part of the solar system is then fully represented. You see, this diagram shows how our earth is in every sense a planet. It happens that one of the four planets revolves outside the earth’s path, while there are two inside. By marking the days on the circles which show the periods of the planets, you perceive that the further a planet is from the sun, the longer is the time that it takes to go round. Perhaps you will not be surprised at this, for the length of the journey is, of course, greater in the greater orbits; but this consideration will not entirely explain the augmentation of the time of revolution. The further a planet is from the sun, the more slowly does it actually move, and therefore, for a double reason, the larger orbit will take a longer time. From London to Brighton is a much longer journey than from London to Greenwich, and, therefore, the journey by rail to Brighton will, of course, be a longer one than by rail to Greenwich. But suppose that you compared the railway journey to Greenwich with the journey, not by rail, but by coach, to Brighton, here the comparative slowness of the coach would form another reason besides the greater length of the journey for making the Brighton trip a much more tedious one than that to Greenwich. Mars may be likened to the coach which has to go all the way to Brighton, while Mercury may be likened to the train which flies along over the very short journey to Greenwich.