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OBLIQUITY OF THE ECLIPTIC (epsilon)

   DATE =  2010 AD  September 04 = Saturday [Gregorian Calendar]
TIME UT =  00:00:00.000 =  0 d
Delta T = +00:00:00.000 = +0 d
     JD = 2455443.5 = JD for combined date, time and Delta T
      T = 0.10673511293634 = Julian centuries reckoned from J2000.0

Mean Obliquity = 23° 26' 16.452" = 23.437903279°

Nutation in obliquity = +01.822" = +0.000506052° (1980 IAU theory)
True Obliquity = 23° 26' 18.274" = 23.438409331°

The obliquity of the ecliptic is essentially the angle of inclination of the Earth's equator with respect to the plane of the Earth's orbit, which to us, produces the apparent relative tilt of the Earth's polar axis which causes the annual seasons.

This angle is approximately 23.45 degrees and is the apparent maximum/minimum angle of the sun above or below the equator during the year as viewed from Earth.   This value varies slowly as the orbit of the Earth evolves over time.

This is one of the values we need to know when computing the apparent position of a planet as viewed from the Earth as it defines the instantaneous inclination of the Earth's orbit.

http://en.wikipedia.org/wiki/Ecliptic

The algorithm used here is based on data published by J. Laskar
Astronomy and Astrophysics, Vol 157, p68 (1986),
New Formulas for the Precession, Valid Over 10000 years,
Table 8.



Provided by the NASA Astrophysics Data System

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Laskar's Formula For The Mean Obliquity

In this case, the time variable (t) of Table 8 is reckoned in terms
of 10000 Julian years from J2000.0 corresponding to the JD and
may be found from



The value 84381.448 seconds of arc equates to 23° 26' 21.448"
which is the mean obliquity of the ecliptic of J2000.0

Based on the Table 8 data above (bottom NGT column),
The mean obliquity (in arc seconds) of the ecliptic at time (t) is:



See also:
 Nutation in Obliquity