Читать книгу The Friendly Stars - Martha Evans Martin - Страница 8

The distance between the two pointers is about five degrees; and this is an important thing to keep in mind. These known five degrees may be used as a unit of measure in finding other stars as described in this book. If one cannot carry the distance between the pointers in his mind from one part of the skies to another and thus make comparisons of distances that will be approximately correct, he can take a stiff piece of card-board, or an adjustable ruler, and, holding it out at arm's-length, adjust it so as to exactly cover the space between the two pointers. He will then have a fairly accurate measure of five degrees in the sky and will probably be surprised to find that it will require a little less than three inches on his ruler to cover it. This will be of great assistance in marking off approximate distances on the surface of the heavens as it appears to us, and will enable one to follow the directions given in these pages. It will be a still further convenience to make a measure from two to four times as long, thus covering ten or twenty degrees. In adjusting these measures it will be found necessary to sight with one eye only, keeping the other closed. One cannot cover a star from both eyes with a ruler at only an arm's-length away.

While these rough guides are conveniences to one entirely unaccustomed to observing the skies, a very little experience will soon enable one to estimate distances from star to star by the eye alone with sufficient accuracy to identify any conspicuous object in the heavens.

In looking for the brightest stars one may sometimes find himself confused by four bright stars which are not, after all, true stars, but are the planets Venus, Jupiter, Saturn, and Mars. Stars are suns and shine by their own light, as our sun does. Planets are dark bodies and shine only by light reflected from the sun. If the planets were as far away as the stars, they would be entirely invisible, and all the differences between them and the stars which I am about to enumerate are due entirely to their comparative nearness to us.

There are seven planets besides the earth, but the four already mentioned are the only ones likely to be mistaken for true stars and thus prove confusing to the ordinary observer. Of the other three, two (Neptune and Uranus) can be seen only with a telescope, and the other (Mercury) on account of its nearness to the sun is rarely seen at all by the ordinary observer, though when it is visible it appears as a rather bright star.

The first two, Venus and Jupiter, are at certain times brighter than the brightest stars. Venus sometimes shines so that it casts a shadow and sometimes can be seen, like the moon, in daylight. Saturn is usually as bright as a first magnitude star, and Mars varies so that it sometimes equals Saturn in brilliancy and sometimes is not so bright. They all move about in the heavens with seeming irregularity, and when they are in the region of any bright star may easily be mistaken for it by an untrained observer. A little careful observation, however, will show that they are not as the other stars. They are not fixed in their places, and a few nights of watching will make this apparent. They do not twinkle as the stars do, but shine with a steadier light. They are not mere points of light, as the stars are, but show as actual bodies and sometimes at their brightest even as rotund solid bodies, though small in comparison with the other two bodies of which we can see the outlines: the sun and the moon. A very little watching will make one familiar with their appearance so that he can always tell Venus and Jupiter at a glance and usually Saturn and Mars.

Their positions in the heavens are hard to describe definitely because the planets are constantly changing. They do, however, keep within a certain limited path, which is within a few degrees of the path that the moon follows as it threads its way among the stars each month, and at some time during the month it passes near each planet. This is sufficient to indicate that the planets are never found very far south or very far north.

Venus has still the further limitation of never being more than about forty-five degrees away from the sun. When it is in the evening sky it sets never much more than three hours after the sun, and when it is seen in the morning it rises never more than about three hours earlier than the sun. The other planets within their limited path already described rise and set at various times within twelve hours of the rising and setting of the sun.

The planets have each a distinctive color, which will assist somewhat in identifying them. Jupiter is white; Venus is a little tinged with yellow, Saturn is decidedly yellow, and Mars is fiery red.

There are many interesting things to observe about the planets, which are a study in themselves. This brief outline of their appearance and locations in the sky will be sufficient to prevent one from confusing them with the stars, which are the special objects of our quest at present; and with the North Star and the Dipper well in mind, we are now ready to be introduced to the brightest of them.