Page:Astronomy for Everybody.djvu/62

the vernal equinox is the first meridian, from which right ascensions are counted as already described. The two at right angles to it are called the solstitial colures.

Let us now show the relation of the constellations to the seasons and the time of day. Suppose that to-day the sun and a star passed the meridian at the same moment; to-morrow the sun will be nearly a degree to the east of the star, which shows that the star will pass the meridian nearly four minutes sooner than the sun will. This will continue day after day throughout the entire year when the two will again pass the meridian at about the same moment. Thus the star will have passed once oftener than the sun. That is to say: In the course of a year while the sun has passed the meridian three hundred and sixty-five times, a star has passed it three hundred and sixty-six times. Of course if we take a star in the south it will have risen and set the same number of times.

Astronomers keep the reckoning of this different rising and setting of the stars by using a sidereal day, or star day, equal to the interval between two passages of a star, or of the vernal equinox, across the meridian. They divide this day into twenty-four sidereal hours, and these into minutes and seconds according to the usual plan. They also use sidereal clocks which gain about three minutes and fifty-six seconds per day on the ordinary clocks, and thus show sidereal time. Sidereal noon is the moment at which the vernal equinox crosses the meridian of the place. The clock is then set at 0 hours, 0 minutes, and 0 seconds. Thus set and regulated, the sidereal