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First Point Of AriesThe First Point of Aries, also called the vernal equinox point, is one of the two points on the celestial sphere where the celestial equator intersects the ecliptic. It is defined as the position of the Sun on the celestial sphere at the time of the vernal equinox. It was named after the constellation in which it occurred in ancient times - Aries. However due to precession, the point gradually moves around the ecliptic. It entered the constellation Pisces in about 500 A.D. (or around 100 A.D. if the modern constellation boundaries are used) and will enter Aquarius in about 2600 A.D. The other such point, at the autumnal equinox, is the First Point of Libra. For astronomical purposes, then, to define an accurate reference frame, one must select a particular epoch, which will fix the First Point of Aries as a definite point in the sky. The J2000.0 epoch is in use nowadays for that purpose. In the equatorial coordinate system the First Point of Aries is defined to have a right ascension of zero. The declination is also zero due to the position on the celestial equator. It is important to keep in mind the geocentric nature of this definition, and how things can seem backwards when thinking solar-centrically, as when trying to use orbital elements. For instance, Earth reaches perihelion shortly after the hibernal solstice, three-fourths of a year after vernal equinox. With all the vernal-equinox talk, it's easy to think Earth's longitude of perihelion should be about 270°. But longitude 0 is the far side of the Sun at vernal equinox -- i.e. Earth won't reach longitude 0 until the autumnal equinox, and so the longitude of perihelion is more like 90° (about 103° in fact.) Sidereal clocks agree with conventional clocks at the autumnal equinox instead of the vernal for similar reasons. See also External links
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