The vernal equinox is an important point of reference on the celestial sphere. As the position of a point on the earth's surface is determined by its longitude and latitude so the position of an object on the celestial sphere--star, sun, planet--is determined by its Right Ascension and Declination. The Declination of a celestial object is its distance north or south of the celestial equator, measured in degrees, minutes and seconds of arc, on a great circle of the celestial sphere passing through the object and north and south poles of the heavens. These great circles are called hour circles and they correspond to the meridians or circles of longitude on the earth's surface. The declination of an object in the heavens corresponds to the latitude of a point on the earth's surface. The Right Ascension of a point on the celestial sphere corresponds to the longitude of a point on the earth's surface. It is measured--as longitude is measured--in degrees, minutes and seconds of arc or in hours, minutes and seconds of time--eastward along the celestial equator from the hour circle passing through the vernal equinox to the foot of the hour circle passing through the object. The hour circle passing through the vernal equinox is the zero meridian for the celestial sphere just as the meridian of Greenwich is the zero meridian on the earth's surface. * * * * * The mean distance of the earth from the sun is 92,900,000 miles and is called the astronomical unit. The sun with its satellites advances through the universe at the rate of 4 astronomical units in a year or approximately one million miles a day. The parallax of a star is the angle at the star subtended by the radius of the earth's orbit, 92,900,000 miles, or the astronomical unit. It is, in other words, the angular distance between the earth and sun as viewed from the star. The larger the parallax the nearer the star. The largest known stellar parallax is that of Alpha Centauri and its value is 0".75. The light-year is the distance that light travels in one year. It is equal to about 63,000 astronomical units or nearly six trillion (6,000,000,000,000) miles. The velocity of light is 186,000 miles per second. The parsec is equal to 3.26 light-years. It is the distance of a star that has a parallax of one second of arc. The apparent magnitude of a star is its apparent brightness estimated on a scale in which a difference of one magnitude corresponds to a difference in brightness of 2.51, or the fifth root of one hundred. A difference of five magnitudes corresponds to a difference one one hundredfold in brightness, of ten magnitudes to ten thousandfold in brightness. In exact measurements on this scale magnitudes are estimated to tenths. Stars that are one magnitude brighter than stars of the standard first magnitude are of the zero magnitude and stars still brighter are of negative magnitudes.