|
Search over 100 encyclopedias and dictionaries: |
Research categories | Follow us on Twitter |
Research categories
View all topics in the newsView all reference sources at Encyclopedia.com |
|||
precession of the equinoxes
precession of the equinoxes westward motion of the equinoxes along the ecliptic . This motion was first noted by Hipparchus c.120 BC The precession is due to the gravitational attraction of the moon and sun on the equatorial bulge of the earth, which causes the earth's axis to describe a cone in somewhat the same fashion as a spinning top. As a result, the celestial equator (see equatorial coordinate system ), which lies in the plane of the earth's equator, moves on the celestial sphere , while the ecliptic, which lies in the plane of the earth's orbit around the sun, is not affected by this motion. The equinoxes, which lie at the intersections of the celestial equator and the ecliptic, thus move on the celestial sphere. Similarly, the celestial poles move in circles on the celestial sphere, so that there is a continual change in the star at or near one of these poles (see Polaris ). After a period of about 26,000 years the equinoxes and poles lie once again at nearly the same points on the celestial sphere. Because the gravitational effects of the sun and moon are not always the same, there is some wobble in the motion of the earth's axis; this wobble, called nutation , causes the celestial poles to move, not in perfect circles, but in a series of S-shaped curves with a period of 18.6 years. There is some further precession caused by the gravitational influences of the other planets; this precession affects the earth's orbit around the sun and thus causes a shift of the ecliptic on the celestial sphere. The precession of the earth's orbital plane is sometimes called planetary precession, and that of the earth's equatorial plane (caused by the sun and moon) is called luni-solar precession; the combined effect of the moon, the sun, and the planets is called general precession. Planetary precession is much less than luni-solar precession. The precession of the equinoxes was first explained by Isaac Newton in 1687. |
|
|
Cite this article
"precession of the equinoxes." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. "precession of the equinoxes." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1E1-precequi.html "precession of the equinoxes." The Columbia Encyclopedia, 6th ed.. 2011. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-precequi.html |
|
precession of the equinoxes
precession of the equinoxes The motion of the equinoxes along the ecliptic, arising from the combined motion of the equator (lunisolar precession) and the ecliptic (planetary precession); also known as general precession. It was first detected by the Greek astronomer Hipparchus in about 130bc from the apparent increase in the observed celestial longitudes of stars. It amounts to about 50″.3 per year. Hence the equinoxes move westwards on the celestial sphere by 1 ° in about 72 years, and take 25 800 years to complete one circuit.
|
|
|
Cite this article
"precession of the equinoxes." A Dictionary of Astronomy. 1997. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. "precession of the equinoxes." A Dictionary of Astronomy. 1997. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O80-precessionoftheequinoxes.html "precession of the equinoxes." A Dictionary of Astronomy. 1997. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O80-precessionoftheequinoxes.html |
|
precession of the equinoxes
precession of the equinoxes in astronomy, the slow retrograde motion of equinoctial points along the ecliptic; the resulting earlier occurrence of equinoxes in each successive sidereal year.
As the earth rotates about its axis it responds to the gravitational attraction of the sun upon its equatorial bulge, so that its axis of rotation describes a circle in the sky, with a period of about 26,000 years. The precession of the equinoxes was discovered by Hipparchus in c.125 bc, when the vernal equinox was in Aries. |
|
|
Cite this article
ELIZABETH KNOWLES. "precession of the equinoxes." The Oxford Dictionary of Phrase and Fable. 2006. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. ELIZABETH KNOWLES. "precession of the equinoxes." The Oxford Dictionary of Phrase and Fable. 2006. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O214-precessionoftheequinoxes.html ELIZABETH KNOWLES. "precession of the equinoxes." The Oxford Dictionary of Phrase and Fable. 2006. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O214-precessionoftheequinoxes.html |
|
precession of the equinoxes
precession of the equinoxes See MILANKOVICH CYCLES.
|
|
|
Cite this article
AILSA ALLABY and MICHAEL ALLABY. "precession of the equinoxes." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "precession of the equinoxes." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O13-precessionoftheequinoxes.html AILSA ALLABY and MICHAEL ALLABY. "precession of the equinoxes." A Dictionary of Earth Sciences. 1999. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-precessionoftheequinoxes.html |
|
|
Search over 100 encyclopedias and dictionaries:
HighBeam™ Research, Inc. © Copyright 2012. All rights reserved.
The Encyclopedia.com advertising network includes:
