Page:Astronomy for Everybody.djvu/52

equinox, a point to be defined in the next chapter. This takes a place in the sky corresponding to Greenwich on the earth's surface.

The position of a star on the celestial sphere is defined in the same way that the position of a city on the earth is defined, by its latitude and longitude. But different terms are used. In astronomy, the measure which corresponds to longitude is called right ascension; that which corresponds to latitude is called declination. We thus have the following definitions, which I must ask the reader to remember carefully.

The declination of a star is its apparent distance from the celestial equator north or south. In the figure the star is in declination twenty-five degrees north.

The right ascension of a star is the angle which the hour circle passing through it makes with the first hour circle which passes through the vernal equinox. In the figure the star is in three hours right ascension.

The right ascension of a star is, in astronomical usage, generally expresssed as so many hours, minutes, and seconds, in the way shown on Figure 3. But it may equally well be expressed in degrees as we express the longitude of places on the earth. The right ascension expressed in hours may be changed into degrees by the simple process of multiplication by 15. This is because the earth resolves 15° in an hour. Figure 3 also shows us that, while the degrees of latitude are nearly of the same length all over the earth, those of longitude continually diminish, slowly at first and more rapidly afterwards, from the equator toward the poles. At the