  |
|
| Table of
Contents View the Stars External Links
|
The celestial equator
divides the sky into northern and southern hemispheres, just as the Earth's equator
divides the Earth into two hemispheres. Thus, all one has to do to envision the
celestial sphere is to extend out Earth's major geologic features such as the equator,
north and south poles. Declination measures how high a star is in the sky. Declination of an object is the angular distance north or south of the celestial equator. For example, a star that appears on the north celestial pole has a declination of 90o degrees and a star on the celestial equator has a declination of 0o whereas a star on the south celestial pole has a declination of -90o. Right ascension measures where a star is positioned around the celestial sphere. The right ascension of an object is the angular distance from the vernal equinox eastward. Right ascension is measured in hours, minutes and seconds. One hour is 15 degrees. In order to locate an object in the sky you must know both its right ascension and its declination. This is analagous to finding a city on the Earth using latitude and longitude. For example, the location of Bloomington is 39.2o N latitude and 86.5o W longitude. Now that we know about the celestial sphere we need to understand the celestial motions of the objects in the night sky.The equivalent on the celestial sphere of the Earth's prime meridian (the line on the Earth that passes through Greenwich, England from which longitude is measured) is where the Sun crosses the celestial equator moving to the north. |