A point and an instant
The celestial equator and the ecliptic, the Sun's yearly path, are two great circles tilted by about 23.44°. They cross at two opposite points. The vernal equinox, also called the March equinox or the first point of Aries (symbol ♈ or γ), is the crossing the Sun passes in March, moving from south to north of the equator. The other point is the autumnal equinox, which it passes in September.
The same name is used for the instant when the Sun reaches that point. Astronomers define it precisely: the moment when the Sun's apparent geocentric ecliptic longitude is exactly 0°. At that moment the Sun is overhead at the Earth's equator, and spring begins in the northern hemisphere and autumn in the southern one, which is why "March equinox" is the more neutral name.
Dates of the March equinox
| Year | Date and time (UT) |
|---|---|
| 2025 | 20 March, 09:01 |
| 2026 | 20 March, 14:46 |
| 2027 | 20 March, 20:25 |
| 2028 | 20 March, 02:17 |
Because the calendar year of 365 days is shorter than the tropical year of 365.2422 days, the equinox falls about a quarter of a day later each year, and the leap day brings it back. The Gregorian rule of skipping three leap years every four centuries keeps it between 19 and 21 March.
How Meeus computes the equinox
Astronomical Algorithms gives the instant in two steps. A polynomial first gives the mean instant, in Julian Ephemeris Days, for the years 1000 to 3000, with Y = (year − 2000) / 1000:
JDE₀ = 2451623.80984 + 365242.37404 × Y + 0.05169 × Y² − 0.00411 × Y³ − 0.00057 × Y⁴For 2026, Y = 0.026 and JDE₀ = 2461120.116, that is 20 March 2026 at about 14:47 Terrestrial Time. Meeus then adds 24 periodic terms that account for the perturbations of the Earth's motion, or, for the highest accuracy, iterates on the Sun's apparent longitude computed with VSOP87 until it is exactly 0°. Subtracting ΔT, about 69 seconds in 2026, converts Terrestrial Time to Universal Time and gives the published 14:46 UT.
The origin of celestial coordinates
The vernal equinox is the zero point of two coordinate systems at once. Right ascension is counted eastward from it along the celestial equator, and ecliptic longitude along the ecliptic. When the Sun is at the vernal equinox its right ascension is 0h, its declination 0° and its ecliptic longitude 0°.
It also defines sidereal time: local sidereal time is the hour angle of the vernal equinox, so the equinox crosses your meridian when your sidereal clock reads 0h. Around 22 September this happens at about midnight.
Precession: the equinox moves
The equinox is not fixed among the stars. The Earth's axis precesses like a spinning top, and the vernal equinox slides westward along the ecliptic by about 50.3″ per year, completing a full circuit in about 25,800 years. Hipparchus discovered this "precession of the equinoxes" around 130 BC.
The name first point of Aries dates from that time, when the equinox lay in the constellation Aries. It has since moved into Pisces, and will cross into Aquarius around the year 2600. This is also why the tropical zodiac, tied to the equinox, and the sidereal zodiac, tied to the stars, now differ by about 24°.
Positions must therefore name their equinox. "Equinox of date" uses its position at the moment of observation; J2000.0 uses its position on 1 January 2000, which is the reference of modern catalogues and close to the origin of the International Celestial Reference System.
Equal day and night?
Equinox means "equal night", but on the day of the equinox daylight is slightly longer than 12 hours everywhere. Sunrise and sunset are counted from the Sun's upper edge, not its centre, and atmospheric refraction lifts the Sun by about half a degree at the horizon. The day of equal daylight and darkness, sometimes called the equilux, comes a few days before the March equinox in the northern hemisphere. On the equinox itself the Sun does rise almost exactly due east and set due west, everywhere on Earth.
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Frequently asked questions
When is the vernal equinox in 2027?
On 20 March 2027 at about 20:25 UT. In 2026 it was on 20 March at 14:46 UT, and in 2028 it will be on 20 March at about 02:17 UT.
Why is it called the first point of Aries if it is in Pisces?
The name was fixed more than two thousand years ago, when the equinox lay in Aries. Precession has moved it westward by about 30° since then, into Pisces. The name was kept for the point, not the constellation.
Is the vernal equinox always on 21 March?
No. With the Gregorian calendar it falls between 19 and 21 March, and in the 21st century almost always on 20 March. The fixed date of 21 March is the ecclesiastical equinox used to compute Easter.
