Two equinoxes, two names
Every year has two equinoxes: one in March and one in September. At each, the Sun crosses the celestial equator, and day and night are roughly equal everywhere on Earth.
The vernal equinox is whichever of the two marks the start of spring where you live:
| Where you live | Vernal (spring) equinox | Autumnal equinox |
|---|---|---|
| Northern Hemisphere | March (around March 20) | September (around September 22 or 23) |
| Southern Hemisphere | September (around September 22 or 23) | March (around March 20) |
So in London, Toronto or Tokyo the vernal equinox comes in March, while in Sydney, Buenos Aires or Johannesburg it comes in September. Someone in Chile and someone in Canada can both say “the vernal equinox is today” and mean dates six months apart.
To avoid this confusion, many scientists and almanacs use the month-based names instead: the March equinox and the September equinox. Those names work everywhere. Exact times for both are listed on the seasons page.
The astronomical meaning
In astronomy, “vernal equinox” has a second, more technical meaning. It is not only an event but also a point on the sky: the place where the Sun’s yearly path (the ecliptic) crosses the celestial equator going north.
That point serves as the zero for right ascension, the sky’s equivalent of longitude. Star catalogs, telescope software and spacecraft navigation all measure positions from it. In this sense the vernal equinox always means the March crossing, regardless of hemisphere, because the convention was fixed long ago by astronomers in the north.
From Aries to Pisces
The vernal equinox point is traditionally called the first point of Aries, because more than 2,000 years ago it lay in the constellation Aries. It no longer does.
Earth’s axis slowly wobbles like a spinning top, tracing a circle about once every 26,000 years. This motion, called precession, makes the equinox points slide westward along the ecliptic by about one degree every 72 years. Today the vernal equinox point lies in Pisces, and in several centuries it will move into Aquarius. The name “first point of Aries” has stayed in use anyway.
Precession also explains why astrological sun signs, which are tied to the equinoxes, no longer match the constellations the Sun is actually in on those dates.
Calendars built on the vernal equinox
Several calendars and festivals are anchored to the northern vernal equinox:
- Easter: Western and Eastern churches set Easter as the first Sunday after the first full moon on or after the vernal equinox. For this calculation the equinox is fixed by convention at March 21, rather than using the astronomical date. Eastern churches apply the rule with the Julian calendar, which is why their Easter often falls later.
- Nowruz: the Persian New Year, celebrated in Iran, Afghanistan, Central Asia and beyond, begins at the March equinox. The official Iranian calendar starts its year at the equinox.
- Japan: Vernal Equinox Day (Shunbun no Hi) is a national public holiday in March, part of a week of Buddhist observances called Higan.
All of these follow the March equinox, so in the Southern Hemisphere they fall in early autumn.
Why the seasons flip
The flip in naming comes from the tilt of Earth’s axis, about 23.4 degrees. After the March equinox, the Northern Hemisphere tilts increasingly toward the Sun, gets longer days and warms up toward summer. At the same time the Southern Hemisphere tilts away and heads toward winter. Six months later the situation reverses.
Day length changes quickly around either equinox, as you can see on the sunrise and sunset page. The opposite event in each hemisphere is covered in the guide to the autumnal equinox.