![]() ![]() ![]() ![]() System for observing the motion of a star is shown below: Hence our coordinate system for events consists of a location plus a time otherwiseįor an observer standing on the surface of the earth, the simplest possible coordinate Still nothing happens because you have not specified a time, only a location. That occurs at a specific time in a specific place.įor instance, if you text message your friend the following: Events don't occur without a coordinate system because an event is something Without a coordinate system and measurements don't mean anything without a coordinate All rights reserved.Motion has no meaning with out a coordinate system. Copyright © 2023, Columbia University Press. The Columbia Electronic Encyclopedia, 6th ed. Because the vernal equinox is not always visible in the night sky (especially in the spring), whereas the sigma point is always visible, the hour angle is used in actually locating a body in the sky. The angular distance from the sigma point to a star's hour circle is called its hour angle it is equal to the star's right ascension minus the local sidereal time. The right ascension of the sigma point is equal to the observer's local sidereal time. By definition, the vernal equinox is located at right ascension 0 h and declination 0°.Īnother useful reference point is the sigma point, the point where the observer's celestial meridian intersects the celestial equator. Because a star's position may change slightly (see proper motion and precession of the equinoxes), such tables must be revised at regular intervals. The right ascensions and declinations of many stars are listed in various reference tables published for astronomers and navigators. Right ascension and declination together determine the location of a star on the celestial sphere. This angle is called the declination of the star and is measured in degrees, minutes, and seconds north or south of the celestial equator, analogous to latitude on the earth. Next the observer measures along the star's hour circle the angle between the celestial equator and the position of the star. (There are 360 degrees or 24 hours in a full circle.) The right ascension is always measured eastward from the vernal equinox. This angle is called the star's right ascension and is measured in hours, minutes, and seconds rather than in the more familiar degrees, minutes, and seconds. The astronomer then measures the angle between the vernal equinox and the point where the hour circle intersects the celestial equator. This is the star's hour circle, analogous to a meridian of longitude on earth. To designate the position of a star, the astronomer considers an imaginary great circle passing through the celestial poles and through the star in question. An important reference point on the celestial equator is the vernal equinox, the point at which the sun crosses the celestial equator in March. It divides the celestial sphere into the northern and southern skies. The great circle on the celestial sphere halfway between the celestial poles is called the celestial equator it can be thought of as the earth's equator projected onto the celestial sphere. If the earth's axis is extended, the points where it intersects the celestial sphere are called the celestial poles the north celestial pole is directly above the earth's North Pole, and the south celestial pole directly above the earth's South Pole. It represents the entire sky all celestial objects other than the earth are imagined as being located on its inside surface. ![]() The celestial sphere is an imaginary sphere with the observer at its center. Equatorial coordinate system, the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere. ![]()
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