Foundations 05

The Wandering Equinox

The point the whole sky is measured from is sliding out of Pisces. When it reaches Aquarius, and why the zodiac signs no longer match the stars.

Working out when this is best placed from where you are…

Latitude

Longitude

Which night

Date

Hour

Used for this lesson only — your map settings, and every other lesson, stay as they are.

Sources

HYG stellar database v4.2 — star positions
Lieske et al., A&A 58, 1 (1977) — the IAU 1976 precession used for every date here; built for a few thousand years either side of 2000, so the 68 BC date is at the edge of its range
Meeus, Astronomical Algorithms, 2nd ed. (1998), ch. 21 — precession of the equinoxes
Meeus, Mathematical Astronomy Morsels (1997) — the March equinox enters Aquarius in 2597
IAU constellation boundaries (Delporte 1930), VizieR VI/49 — which constellation the equinox is in, in any year

← GUIDES

Two lines cross in this patch of sky south of the Great Square of Pegasus. One is the ecliptic, the Sun's yearly path; the other is the celestial equator, Earth's equator projected onto the sky. The Sun crosses here heading north about 20 March: the March equinox. No star marks it. It is a place, not a thing.

This crossing is the zero of right ascension. Every star's RA is counted east from here, the way longitude is counted from Greenwich — so this is 0h. To find it, drop 15.8° from Algenib, the Square's south-east corner, roughly south: about a fist and a half at arm's length.

The point does not stay put. Earth's axis wobbles like a top — the North Star lesson follows the pole's end of it — and the equator swings with it, so the crossing slides west along the ecliptic by 50.3″ a year: 1° every 72 years, the full circle in about 25,800. The track drawn here runs from 68 BC to 2597.

Run it back. From about 1865 BC the equinox lay in Aries, the Ram of Hamal and Sheratan — why astronomers still call it the First Point of Aries. On the IAU boundaries it crossed into Pisces in 68 BC, 29.1° back along the ecliptic from where it is now.

Forward, the equinox has 8.0° of Pisces left before it reaches Aquarius. At 50.3″ a year that is 8.0 × 3600 ÷ 50.3 ≈ 570 years: the crossing comes in 2597, the year Jean Meeus also found. By the IAU boundaries, that is when the Age of Aquarius begins.

Astrologers do not all wait for 2597. An astrological age is usually counted in equal 30° slices of the ecliptic, not by these boundaries, which Eugène Delporte drew, unevenly, for the IAU in 1930 — so their dates differ, by centuries. Both count the same slow drift; they disagree only about where the lines go.

The sign of Aries starts where the Sun stands at the March equinox and runs 30° east along the ecliptic. The constellation Aries now starts 29.5° along. In 2026 the Sun reached the equinox on 20 March, in Pisces, and entered Aries on 19 April — just as the sign of Aries ended.

Neither side has made a mistake. Western astrology's signs are tied to the seasons — Aries is the month after the March equinox, wherever the stars are — and the constellations are tied to the stars. The two lined up about two thousand years ago and have slid nearly a whole sign apart since. In about 25,800 years they will line up again.

Six thousand years on one page: the March equinox's place along the ecliptic among the stars, year by year, across the constellations' uneven stretches. The signs are tied to the equinox and slide with it; the constellations stay put. 68 BC and 2597 are where it crosses.