The sky — astronomy

Equinox: Spring and Autumn

An equinox is the moment when the Sun crosses the celestial equator, which happens twice a year, around 20 March and around 22–23 September. On those days the Sun rises almost due east and sets almost due west, and day and night are close in length, but not exactly equal.

The equinox is one of the four key moments in the Sun's yearly journey. The word comes from Latin and means “equal night”, yet the real story is a little more subtle, and more interesting, than that name suggests.

The Sun crosses the celestial equator

The celestial equator is Earth's equator projected out onto the imaginary celestial sphere. Earth's axis is tilted by about 23.4° relative to its orbit, so over the year the Sun appears to drift north and south of that line. The Sun's apparent path among the stars is called the ecliptic, and the two points where it crosses the celestial equator are the equinoxes.

At the moment of equinox, the Sun's declination, its angular distance from the celestial equator, is 0°. Sunlight falls directly overhead at Earth's equator, and the line between day and night passes through both poles. That is why, from almost anywhere on the planet, the Sun rises close to due east and sets close to due west.

An equinox is an instant, not a whole day. Astronomers give it as a precise time in Coordinated Universal Time (UTC), so the calendar date can differ between time zones.

The spring and autumn points

There are two equinoxes each year:

• The March equinox, around 20 March. In the Northern Hemisphere it is the spring (vernal) equinox: the Sun moves from south to north of the celestial equator, and days begin to outlast nights. In the Southern Hemisphere, the same moment marks the start of autumn.
• The September equinox, around 22–23 September. In the north it is the autumn equinox, as the Sun crosses to the south. In the south it marks the start of spring.

So “spring” and “autumn” depend on where you live. Astronomers often prefer the neutral names March equinox and September equinox. You can read more about the whole cycle in The Seasons, and about the other two key moments in Solstice.

The dates shift slightly from year to year because the tropical year is not a whole number of days, and leap years pull the calendar back into step. That is why, in the 21st century, the March equinox almost always falls on 20 March in UTC, and in a few years on 19 or 21 March.

The March equinox point also has a special job: it is the zero point for measuring right ascension, the celestial equivalent of longitude. Historically this point lay in the constellation Aries, so it was called the First Point of Aries. Because of precession, the slow wobble of Earth's axis with a period of roughly 26,000 years, the point has slowly slid westward along the ecliptic and today lies in Pisces.

Why day and night are not exactly equal

A popular belief says that on the equinox day lasts exactly 12 hours and night exactly 12 hours. In reality, day is slightly longer. There are two main reasons:

1. How sunrise and sunset are defined. Sunrise is the moment the upper edge of the Sun's disk appears above the horizon, and sunset is when that edge disappears. The Sun is not a point but a disk about half a degree wide, so daytime begins a little earlier and ends a little later than if we timed its centre.
2. Atmospheric refraction. Air bends sunlight, so near the horizon we see the Sun slightly higher than it geometrically is. We can still see it when it is, strictly speaking, just below the horizon, which adds a few more minutes of daylight.

The result is that at most latitudes, daytime on the equinox is a few minutes longer than night. The day when daylight and darkness come closest to being equal is called the equilux. It does not fall exactly on the equinox: in the northern mid-latitudes it comes a few days before the March equinox and a few days after the September one (the reverse holds in the south), with the exact date depending on latitude.

Near the poles the situation is quite different, since around the equinoxes the Sun first appears or finally sets for the year, and day length changes extremely fast.

The equinox across cultures

Many cultures have linked this moment with renewal or balance.

• Nowruz is the Persian New Year, celebrated on the day of the March equinox. It is observed in Iran and in many other regions of Central and Southwest Asia.
• Chichén Itzá, in Mexico, draws crowds around the equinoxes, when light and shadow on the steps of the pyramid El Castillo create an effect that resembles a serpent descending the staircase.
• Japan observes both equinoxes as public holidays (Shunbun no Hi and Shūbun no Hi), connected with honouring ancestors and appreciating nature.
• The calendar: the March equinox underlies the calculation of the date of Easter in Christian tradition, which uses a fixed calendar date for the equinox.

In the astrological tradition, the March equinox is the starting point of the tropical zodiac, 0° Aries. That is a convention of the tradition, not a claim that the stars shape events. You can learn more in Tropical and Sidereal Zodiac.

In short

An equinox is a precise astronomical instant at which the Sun crosses the celestial equator. Day and night are then close, but not equal, because of the Sun's disk size and the bending of light in the atmosphere. It is a nice invitation to step outside and watch where the Sun rises: you will find it very nearly due east. To learn more about the star at the centre of all this, read about the Sun, and about the planet we watch from, Earth.

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