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Starting point of measurement

  1. Jul 4, 2010 #1
    Where is the starting point of measurement of the Ecliptic Longitude, the Ecliptic Latitude, Right Ascension, and Declination?
  2. jcsd
  3. Jul 4, 2010 #2
    I am working from memory here, always a dangerous thing, but hopefully google can take it from here.

    Ecliptic Longitude: Is zero at the First Point of Aries, a line drawn on the plane of the ecliptic from the sun thru the earth as of the vernal equinox

    Ecliptic Latitude: Is zero along the plane of the ecliptic, the plane of Earth's orbit

    Right Ascension: Is zero at the ascending node, which for Earth only is also at the first point of Aries, also called the celestial meridian, where the plane of the ecliptic and the celestial plane - the plane perpendicular to the Earth spin axis - intersect at a line. For other planets, this zero point is set relative to Earth's zero point using the Right Ascension of Ascending Node

    Declination: I dunno. Please define.

    The line defining the first point of Aries actually traverses, as it were, around the zodiac in about 25000 years. It was zero and pointing at Aries in 70 BC (I looked that one up) but now it is pointing more towards Pisces.

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