Ecliptic
Ecliptic, in astronomy, the apparent great-circle annual path of the sun in the celestial sphere, as seen from the earth. It is so named because eclipses occur only when the moon is on or near this path. The plane of this path, called the plane of the ecliptic, intersects the celestial equator (the projection of the earth's equator on the celestial sphere) at an angle of about 23°27’. This angle is known as the obliquity of the ecliptic and is approximately constant over a period of millions of years, although at present it is decreasing at the rate of 48 seconds of arc in each century and will decrease for several millenniums until it reaches 22°54’, after which it will again increase.
The two points at which the ecliptic intersects the celestial equator are called nodes or equinoxes. The sun is at the vernal equinox about March 21 and at the autumnal equinox about September 23. Halfway on the ecliptic between the equinoxes are the summer and winter solstices. The sun arrives at these points about June 21 and December 22, respectively. The names of the four points correspond to the seasons beginning in the northern hemisphere on these dates. The equinoxes do not occur at the same points of the ecliptic every year, for the plane of the ecliptic and the plane of the equator revolve in opposite directions. The two planes make a complete revolution with respect to each other once every 25,868 years. The movement of the equinoxes along the ecliptic is called the precession of the equinoxes. A correction for precession must be applied to celestial charts to find the true position of the stars at any given time.
The ecliptic is also used in astronomy as the fundamental circle for a system of coordinates called the ecliptic system. Celestial latitude is measured north and south of the ecliptic; celestial longitude is measured east and west of the vernal equinox.
In astrology, the ecliptic is divided into 12 arcs of 30° each, called the signs of the Zodiac. These signs, or “houses of heaven,” are named after the constellations through which the ecliptic passes.
See also Eclipse.
The two points at which the ecliptic intersects the celestial equator are called nodes or equinoxes. The sun is at the vernal equinox about March 21 and at the autumnal equinox about September 23. Halfway on the ecliptic between the equinoxes are the summer and winter solstices. The sun arrives at these points about June 21 and December 22, respectively. The names of the four points correspond to the seasons beginning in the northern hemisphere on these dates. The equinoxes do not occur at the same points of the ecliptic every year, for the plane of the ecliptic and the plane of the equator revolve in opposite directions. The two planes make a complete revolution with respect to each other once every 25,868 years. The movement of the equinoxes along the ecliptic is called the precession of the equinoxes. A correction for precession must be applied to celestial charts to find the true position of the stars at any given time.
The ecliptic is also used in astronomy as the fundamental circle for a system of coordinates called the ecliptic system. Celestial latitude is measured north and south of the ecliptic; celestial longitude is measured east and west of the vernal equinox.
In astrology, the ecliptic is divided into 12 arcs of 30° each, called the signs of the Zodiac. These signs, or “houses of heaven,” are named after the constellations through which the ecliptic passes.
See also Eclipse.
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