What is an equinox?

What is an equinox?

An equinox is a precise instant — not a whole day — when the Sun, traced along its apparent yearly path through the sky, crosses the celestial equator: the great circle directly above the Earth's equator. The U.S. Naval Observatory phrases the same idea geometrically: "On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth."¹ The two equinoxes mark the two times each year that this crossing happens: once in March, when the Sun moves from south of the equator to north, and once in September, when it moves the other way.²

The astronomical event has nothing to do with calendar dates by itself; the date attached to each equinox is whatever date in Coordinated Universal Time (UTC) the crossing happens to fall on. The U.S. Naval Observatory publishes the times for any year between 1700 and 2100 to one-minute precision in its Earth's Seasons data service, and the Astronomical Almanac — produced jointly with the United Kingdom's Nautical Almanac Office — tabulates them alongside the year's other principal solar events.³⁶

The word itself comes from the Latin aequinoctium: aequus, "equal", plus nox, "night".² The Romans named the event for its most striking surface feature — that day and night are nearly equal in length on each occasion. As the page explains below, "nearly" carries a real correction; the equinox is not the date of exactly equal day and night, and only roughly the date of one. The astronomical content of the word is the equator-crossing.

When are the equinoxes?

The March equinox falls on or near 20 March; the September equinox on or near 22 or 23 September. The exact instant slips by a few hours from one year to the next, and the date of the equinox in the calendar can shift by a day depending on how the slip lines up with the leap-year cycle. In the 21st century the March equinox can fall on 19, 20, or 21 March in UTC; the September equinox on 21, 22, or 23 September. The earliest March equinox of the century — and the first 19 March equinox since 1796 — falls on 19 March 2096.⁵⁷

The drift mechanism is the small mismatch between the calendar year and the tropical year (the time the Sun takes to return to the same position relative to the equinoxes). The Gregorian calendar runs at exactly 365.2425 days per year on average; the tropical year is about 365.24219 days. Across a regular 365-day year, the equinox therefore falls about 5 hours and 49 minutes later in the day than the year before; across a 366-day leap year, it jumps roughly 18 hours and 11 minutes earlier — a four-step drift pattern that returns close to its starting point every four years.² Over the centuries the residual mismatch is corrected by the Gregorian calendar's century-rule for leap years (years divisible by 100 are not leap years unless also divisible by 400), which is precisely the reason that rule exists.

Locally, the calendar date reported for an equinox can differ from the UTC date by a day in either direction depending on the time zone. An equinox occurring at, say, 02:00 UTC on 20 March falls on 19 March in time zones west of UTC by more than two hours and on 20 March everywhere else; an equinox at 22:00 UTC on 22 September is dated 23 September in time zones east of UTC by two hours or more. The site's equinoxes for the current year page renders the moments in UTC alongside sunrise, sunset, and day length tables for major cities, so that the date and the local effect are visible together.

Why isn't day equal to night at the equinox?

The most common misconception about the equinox is that day and night are exactly equal in length on the date itself. They are not. The U.S. Naval Observatory rebuts the claim directly: "Day and night are not exactly of equal length at the time of the March and September equinoxes. The dates on which day and night are each 12 hours occur a few days before and after the equinoxes."¹ On the equinox, the day everywhere is some minutes longer than the night, by an amount that grows with latitude.

The bias has two physical causes, both built into the standard definitions of sunrise and sunset. First, sunrise is defined as the moment the upper edge of the Sun's disc — not the centre — first appears above the horizon, and sunset as the moment the upper edge disappears beneath it. The Sun's apparent angular radius is about 16 arcminutes, so the disc starts to appear and finishes disappearing roughly a minute or two before and after the centre crosses geometrically. Second, atmospheric refraction lifts the Sun's apparent position by roughly 34 arcminutes when it is near the horizon, so an observer continues to see direct sunlight for some minutes after the geometric Sun has set. Together, the two corrections place the centre of the Sun's disc at about 50 arcminutes (≈0.833°) below the geometric horizon at the moment of sunrise and again at sunset.⁸ Both effects extend the day at the expense of the night, on the equinox as on every other date.

The numerical bias grows with latitude because the same vertical offset projects into a longer interval of clock time when the Sun's path through the sky is shallower. The U.S. Naval Observatory quantifies it: at the equinoxes, day is "about 7 minutes longer than the night at latitudes up to about 25 degrees, increasing to 10 minutes or more at latitude 50 degrees."¹ The actual day-when-day-equals-night — the equilux, a 1980s neologism distinct from equinox — therefore lands a few days before the March equinox and a few days after the September equinox in the Northern Hemisphere, with the dates mirrored in the Southern.⁴ At 40° north, the equiluxes are around 17 March and 26 September; at the equator the days never reach an exact 12-hour balance at all, because the day is biased longer by about seven minutes year-round.¹

Why do equinoxes happen?

Earth's rotational axis is tilted about 23.44° from the perpendicular to its orbital plane. The tilt is fixed in space (over the timescale of human history; the slow precession of the axis is the subject of a later section), so as Earth orbits the Sun once a year, the hemisphere pointing toward the Sun changes through the year. In June the Northern Hemisphere is tilted toward the Sun and the Southern away from it; in December the situation is reversed. In between, the Sun's apparent position in Earth's sky drifts north and south of the equator twice a year — passing through it on each crossing.²

The Sun's apparent path on the sky is called the ecliptic: a great circle inclined to the celestial equator at the same 23.44° angle as Earth's axis. The two circles intersect at exactly two points; the Sun reaches them once each on its yearly journey along the ecliptic. Those two crossings are the two equinoxes — the March equinox when the Sun moves from south of the equator to north, the September equinox when it moves from north to south. The Sun's declination, its angular distance north (positive) or south (negative) of the celestial equator, is zero at each crossing and slides through ±23.44° at the solstices in between.²

Two consequences fall out of the equator-crossing geometry. First, with the Sun on the celestial equator, the terminator — the line dividing the Earth's lit and unlit halves — passes through both poles, and the planet's two hemispheres are equally illuminated. Second, an observer anywhere on Earth sees the Sun rise due east and set due west on the equinox, the point on the horizon below which the Sun's geometric centre is rising or setting being directly east or west wherever you are. The seasonal swing of the rise-and-set azimuth — the bearing on the horizon — passes through east-west at each equinox and reaches its maximum excursion at each solstice.

What's the difference between the vernal and autumnal equinoxes?

Vernal and autumnal are the seasonal names: vernal for the equinox at the start of spring, autumnal for the equinox at the start of autumn. In the Northern Hemisphere, the March equinox is the vernal equinox and the September equinox is the autumnal; in the Southern Hemisphere, the assignment is reversed, with the September equinox starting spring and the March equinox starting autumn.²

The asymmetry is the reason most modern astronomical references prefer the neutral month-based names — March equinox and September equinox — over the seasonal ones. The Wikipedia article on the equinox notes the alternative directional names, "the northward equinox occurs in March when the Sun crosses the equator from south to north, and the southward equinox occurs in September when the Sun crosses the equator from north to south", which name the same events by the apparent direction of the Sun's motion.² Northward and southward are unambiguous on a planet with two hemispheres; vernal and autumnal are not.

The site uses the month-based names for the same reason. March equinox and September equinox are the names attached to the data tables and to the per-year pages; spring and autumn appear only when describing the seasonal effect at a specific latitude.

How are the equinoxes observed culturally?

The two equinoxes anchor a number of cultural and religious observances, some predating their astronomical understanding by centuries. The dates of the older festivals are typically derived from the equinox by a fixed offset or by a moon-sighting rule, rather than set to the equinox itself; the more recently codified ones tend to track the astronomical instant exactly.

The most widely observed equinox-anchored festival is Nowruz, the Iranian and Persian new year, celebrated as the moment of the March equinox by populations across Iran, Afghanistan, Central Asia, the Caucasus, and parts of South Asia and the Balkans.⁹ The Solar Hijri calendar used in Iran and Afghanistan begins each year on the day in which the March equinox falls between two consecutive solar noons, and the Iranian government publishes the Tehran-time instant of Nowruz to the second each year. UNESCO inscribed Nowruz on its List of the Intangible Cultural Heritage of Humanity in 2009; the United Nations recognised it as the International Day of Nowruz the following year.⁹

In Japanese Buddhism, Higan (彼岸, "the other shore") is a seven-day observance straddling each equinox — three days before, the equinox itself, and three days after — given over to ancestral grave-visits and temple services. Both the spring (Shunbun no Hi) and the autumn (Shūbun no Hi) equinoxes are public holidays in Japan, with the dates fixed each year against the astronomically computed equinox time.¹⁰⁷ Korean Chuseok (a major harvest festival), Chinese Chūnfēn and Qiūfēn (the spring and autumn middle-points of the 24-solar-term calendar), Iranian Mehregan (a Zoroastrian autumn festival), and the modern neopagan Ostara and Mabon celebrations all bear similar relationships to one or both equinoxes.⁵⁷

The Christian Easter computation has its own ancient relationship to the March equinox. Since the Council of Nicaea in 325, Easter has been calculated as the first Sunday after the first ecclesiastical full moon on or after the vernal equinox, with the church fixing the equinox in its tables at 21 March rather than tracking the astronomical instant. The discrepancy between the ecclesiastical date and the actual equinox was one of the motivating problems behind the Gregorian calendar reform of 1582, which deleted ten days from October that year to bring the astronomical equinox back to about 21 March (see the page on the leap year).⁵

What is the precession of the equinoxes?

The word equinox has a second, technical meaning in astronomy: the direction in space picked out by the line where the celestial equator and the ecliptic cross. Treated as a direction rather than an instant, the equinox is the conventional zero point of celestial coordinate systems — the vernal point or first point of Aries, against which the right ascensions of all stars, planets, and other celestial objects are measured.¹¹

That direction is not fixed. Earth's rotational axis traces out a slow conical wobble in space — a westward precession driven mainly by the gravitational pull of the Sun and Moon on Earth's equatorial bulge — and the vernal point drifts along the ecliptic in step with it. The full precession cycle takes about 25,800 years; the equinox direction therefore shifts at roughly 50 arcseconds per year, or about 1.4° per century.¹¹ Modern star catalogues are referenced to a fixed standard direction — currently J2000.0, the equinox of 12:00 Terrestrial Time on 1 January 2000 — and corrected for precession when used.

Hipparchus of Nicaea is generally credited with the first recorded discovery of the precession of the equinoxes, having compared his own measurements of the star Spica's position around 129 BC against measurements taken by Timocharis a century and a half earlier and noticed that the gap between the star and the autumnal equinox had widened.¹¹ The same precession is the slow mechanism behind the changing identity of the pole star (Polaris is currently the closest bright star to the celestial north pole; Vega will hold the role around AD 14,000), and behind the gradual mismatch between the astrological zodiac and the actual constellations the Sun passes through.

How does the equinox relate to other sun events?

The two equinoxes and the two solstices are the four cardinal points of the Earth's solar year — the four moments when the Sun's declination passes through zero or reaches a turning point. Between an equinox and the next solstice, day length changes most rapidly near the equinox itself, where the Sun's declination is changing fastest, and tapers off as the Sun approaches the solstice; the page on day length covers the daily-rate-of-change behaviour and the polar-circle edge cases in detail. The site's day length calculator shows the year's pattern as a single chart for any city, with marker lines on each equinox and solstice.

The equinox and solar noon meet at one specific point on Earth: at the equator, at solar noon on either equinox, the Sun stands directly overhead and a vertical pole casts no shadow. Inside the tropics — between 23.44° south and 23.44° north — the Sun stands directly overhead at solar noon on two days a year, the days the Sun's declination crosses the observer's latitude, and the equinoxes are the two days on which those zenith passages coincide for the entire equator. Outside the tropics the Sun never reaches the zenith, and noon altitude on the equinox is exactly 90° minus the observer's latitude.

The equation of time — the offset between time on a sundial and time on a clock — is unrelated to the equinox in cause, but happens to pass through zero a few days after the September equinox each year, a coincidence that briefly makes the timing of sunrise and sunset at any longitude depend only on latitude and the longitude itself, not on a separate time-of-year correction. The page on the equation of time covers the four annual zero-crossings in detail.

Forward references: the companion four-times-yearly event to the equinox is the solstice, the moment the Sun's declination reaches its maximum positive or negative value; the polar phenomena that occur for weeks or months either side of each solstice are midnight sun and polar night. Both will receive their own pages on the site.

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