The luminosity of meteors is due to the intense heat caused by the resistance of the air to their passage, and in support of this theory it is found that meteoric stones are always covered, either wholly or in part, with a crust of cement that has recently been melted.
THE FIXED STARS
IN THE HEAVENS
We shall now study the so-called fixed stars, those stars, namely, which preserve the same relative position and configuration from night to night, only varying, and that with perfect regularity, in the times at which they reach the meridian. For this reason they have been known from the dawn of astronomy as fixed stars, in contrast with the planets or wandering stars.
The observer who watches the nightly changes in the sky with close attention will soon perceive that all these fixed stars appear to move in circles or parts of circles. Some of them describe larger circles than others, and the further south a star is when it passes the meridian, the larger circle will it describe.
It cannot be too often repeated that this motion of the stars is only apparent, being due to the real rotation of the earth, along with the observer on its surface, in the contrary direction. It is estimated that there are about three thousand stars visible to the naked eye in our latitude, though not all these are visible at the same time, many of them being below the horizon, while others are elevated in the sky at different times and seasons.
THE MAGNITUDES AND GROUPING
OF THE STARS
In beginning our study of the stars, let us put ourselves in the position of the earliest observers. Let us first, like them, watch the stars, and see how they appear from night to night.
We see, at the first glance, that the stars vary much in brightness. The brightest ones—like Sirius, Capella, Arcturus, and Vega—are called stars of the first magnitude. Those less brilliant, like the six brightest of “the Dipper,” are said to be of the second magnitude. All the stars which can be seen with the unaided eye are thus divided into six classes or magnitudes, according to their brightness.
Constellations. We also see that the stars are not uniformly distributed over the sky. They seem to be arranged in groups, some of which take the form of familiar objects. Every one knows the seven bright stars which are called “the Dipper.” Another group resembles a sickle, another a cross, and so on. All the stars in the heavens have been divided into groups called constellations. Many of these were recognized and named at a very early period.
We should become familiar with these constellations in order to study the stars with any profit.