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Apparent longitude is celestial longitude corrected for aberration and nutation as opposed to mean longitude.
Apparent longitude is used in the definition of equinox and solstice. At equinox, the apparent geocentric celestial longitude of the Sun is 0° or 180°. At solstice, it is equal to 90° or 270°. This does not match up to declination exactly zero or declination extreme value because the celestial latitude of the Sun is (less than 1.2 arcseconds but) not zero.
The common claim is that at equinox, the declination of the Sun is exactly zero and at solstice it is at an absolute extreme value. Of course, the formal definition is apparent longitude integer multiple of 90°. But not everybody are able to grasp this.[citation needed]
Sources
edit- United States Naval Observatory. Astronomical Almanac Glossary Chapter.
- Meeus, Jean. Astronomical Algorithms, Second Edition.