Longitude Equivalent of Declination Charts

The longitude equivalent of each chart point is derived from its declination. The derivation is based on converting a declination to its corresponding longitude on line of the ecliptic.

Generally, each declination has two possible longitude equivalents, and in the standard method, the one used is that which falls into the same zodiacal quadrant as the original longitude of the point. The alternative longitude equivalent point is always the antiscia point (i.e. reflection in the 00Cn00/00Cp00 axis) of the standard one.

A special provision applies if a point’s declination is out-of-bounds (i.e. beyond the maximum declination of the ecliptic, which is about 23 degrees and 26 minutes). In such cases, the declination is first adjusted by “mirroring” it about the maximum point. This is effected by subtracting the excess declination from its maximum value. For example, a point at 25N00 has an excess of 1°34’, so its adjusted declination is 23N26 - 1°34’, which equals 21N52.