Page:Popular Astronomy - Airy - 1881.djvu/199

Rh in the same plane as the equator of the earth, whose axis (perpendicular to its equator, or making an angle of 90 degrees with every part of the equator) is directed to the Pole. Therefore, as the earth's equator changes its intersections with the plane of the ecliptic, the celestial equator also changes its plane and changes its intersections with the ecliptic. These intersections, as I mentioned, are called the first point of Aries and the first point of Libra: and one of the co-ordinates by which the place of a star or other body is defined, technically called the Right Ascension, is the interval of time between the passage of the first point of Aries over the meridian, and the passage of the star or other object over the meridian. But as this first point of Aries travels to the right, it passes the meridian every successive year earlier (with respect to the stars) than it would have done if it had been stationary; and therefore, the right ascensions of stars (for the most part) increase a little every successive year.

Secondly, as the place of the Celestial Pole changes from year to year, the North Polar distances of the stars change from year to year; some of them increase and some diminish. The annual amount of precession, although a formidable quantity in delicate astronomical observations, is a very small quantity for ordinary observers.

The annual motion of the first point of Aries is about fifty seconds in a year: it will require about 26,000 years to perform the entire revolution. The change in the distance of a star from the North Pole does not in any case amount to twenty-one seconds in a year. But these are quantities so large that we must be perfectly acquainted with their laws and magnitudes when we treat of small changes in the places of stars not exceeding one or two seconds.