To represent the positions of the stars, as tronomers imagine circles on the celestial sphere corresponding to the circles of longitude and latitude on the earth. As we imagine north and south meridians drawn on the earth from one pole to another, to measure terrestrial longitudes, so we imagine in the heavens cir cles drawn on the sphere from the north celes tial pole to the south one. As the longitude of a place on the earth is expressed by the angle which its meridian makes with the merid ian of Greenwich, so the corresponding quan tity for a star is the angle which the circle through it makes with a certain prime merid ian on the celestial sphere. This quantity for the stars is not called longitude, but right ascension, and the celestial meridians which de termine it are called hour circles.
In the same way as we have on the earth a great circle spanning it, everywhere equally distant from the two poles, and called the equa tor, so we imagine a circle spanning the heavens, everywhere equally distant from the north and south celestial poles, which is called the celestial equator, or the equinoctial. At any one place this circle will be apparently fixed in its position, always intersecting the horizon at its east and west points, and, in our lati tudes, intersecting the meridian south of the zenith by a distance equal to our distance from the equator. For example, to a dweller in latitude the highest point of the celestial equator will be 40° from the zenith, and above the horizon. From this point it spreads toward the east and west until it intersects the horizon as just stated. As a traveler journeys south, the position of the celestial equator be comes more and more nearly vertical; at the equator it rises vertically and passes through the zenith ; south of the equator it passes north of the zenith.
As the latitude of a place is measured by its angular distance from the equator north or south, so the corresponding number for a star is measured by its mean angular distance from the celestial equator, whether north or south. This is called the star's declination. Thus the right ascension and declination of a star deter mine its position on the celestial sphere just as longitude and latitude determine the position of a city on the earth. All star charts are constructed with this system of circles as a basis; in all modern star catalogues it is the right ascension and declination that are tabu lated.
We now have to consider the effect of the annual motion of the earth round the sun. If we watch the heavens at a certain hour every evening, say eight o'clock P.M., we shall find
that the stars are every night a little farther advanced in their diurnal motion then they were the night before. If they are in a certain posi tion at eight o'clock on one evening, they will pass the same position four minutes before eight on the next night, eight minutes before eight on the next night, and so on. In the course of a year these continually accumulat ing changes make up the whole 24 hours, so that a star which is in the zenith this evening will be on the meridian at eight o'clock in the morning six months hence, while at eight in the evening it will be at its greatest distance below the horizon. If we could see the sun among the stars, what we should notice would be that our luminary always forges a little farther east day after day, and in the course of a year seems to make a complete revolution among the stars. The result is that while the sun rises and sets 365 times, the stars rise and set 366 times. Since the latter are always in the same absolute direction, and seem to rise and set in consequence of the earth's rotation on its axis, we infer that the direction of the sun from the earth goes through a complete revolution in the course of a year. In other words, the sun appears to us to make an annual revolution around the celestial sphere among the stars. Since the time of Copernicus it has been known that this appearance is due to the actual revolution of the earth around the sun.
The apparent path of the sun among the stars can be mapped out by astronomical ob servation. When carefully observed, it is found to be a great circle of the sphere, called the ecliptic. We thus have two imaginary cir cles of fundamental importance spanning the heavens. One is the celestial equator, the other the ecliptic in which the sun seems to travel. These circles do not coincide, but intersect each other at two opposite points at an angle of This is called the obliquity of the eclip tic. The result of it is that during one-half the year the sun is south of the celestial equa tor, and during the other half is north of it. In the northern half of its course we have summer in the northern hemisphere and winter in the southern; in the southern half we have summer in the southern hemisphere and winter in the northern. Thus the changing seasons are due to the obliquity of the ecliptic. If the latter coincided with the equator, we should have no such annual round of seasons as that with which we are familiar.