Astrological longitude and The First Point of Aries

Astrological longitude is a way of measuring a position around the Zodiac. The complete Zodiac is treated as a 360° circle, divided into twelve signs of 30° each. Instead of measuring from an arbitrary point, longitude begins at 0° Aries and increases through the Zodiac.

So the signs correspond to these longitude ranges:

Zodiac sign	Astrological longitude
Aries	0°–29°59′
Taurus	30°–59°59′
Gemini	60°–89°59′
Cancer	90°–119°59′
Leo	120°–149°59′
Virgo	150°–179°59′
Libra	180°–209°59′
Scorpio	210°–239°59′
Sagittarius	240°–269°59′
Capricorn	270°–299°59′
Aquarius	300°–329°59′
Pisces	330°–359°59′

So, for example, 15° Scorpio = 225° astrological longitude because Scorpio begins at 210°.

The First Point of Aries is the 0° reference point from which this longitude is measured. In the tropical Zodiac used in Western astrology, it is defined by the March equinox, when the Sun crosses the celestial equator moving northward. That point is defined astrologically as 0° Aries.

There is an important astronomical distinction. Because of the precession of the equinoxes, the First Point of Aries is no longer physically located against the background stars of the constellation Aries. It has moved into the astronomical constellation Pisces. The tropical Zodiac does not move with the constellations because 0° Aries remains tied to the March equinox.

This also gives a very clean way of explaining planetary positions on Zodiac Astrology Horoscopes. If our ephemeris says The Sun is at 29° Scorpio on November 24th, its astrological longitude is about 239°. When it enters Sagittarius, it crosses 240° longitude, which is 0° Sagittarius.

So in the simplest terms: astrological longitude tells you how far around the Zodiac something is from the First Point of Aries, and the First Point of Aries is 0°, the starting point of the tropical Zodiac.