measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the Earth rotates. The line which passes through the highest point in the sky, called the meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360° (degrees of arc), of a circle is measured as 1h of right ascension, or 15°; of a circle is measured as 1m of right ascension, or 15 minutes of arc (also written as 15′); and of a circle contains 1s of right ascension, or 15 seconds of arc (also written as 15″). A full circle, measured in right-ascension units, contains , or , or 24h. Because right ascensions are measured in hours (of rotation of the Earth), they can be used to time the positions of objects in the sky. For example, if a star with RA = is at its meridian, then a star with RA = will be on the/at its meridian (at its apparent highest point) 18.5 sidereal hours later. Sidereal hour angle, used in celestial navigation, is similar to right ascension but increases westward rather than eastward. Usually measured in degrees (°), it is the complement of right ascension with respect to 24h. It is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures the angular distance of an object westward from the local meridian. Symbols and abbreviations Effects of precession The Earth's axis traces a small circle (relative to its celestial equator) slowly westward about the celestial poles, completing one cycle in about 26,000 years. This movement, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates (including right ascension) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. Right ascension for "fixed stars" on the equator increases by about 3.1 seconds per year or 5.1 minutes per century, but for fixed stars away from the equator the rate of change can be anything from negative infinity to positive infinity. (To this must be added the proper motion of a star.) Over a precession cycle of 26,000