Twice a year, the equinox marks a moment when day and night are almost exactly equal in length across the globe. This happens in March and September, as the Sun crosses the celestial equator from south to north, then back again. For anyone curious about the mechanics of our planet, the equinox offers a clear illustration of how Earth's axial tilt and orbit create the seasons.

What Exactly Is an Equinox?

The word comes from Latin aequus (equal) and nox (night). An equinox occurs when the Sun appears directly above Earth's equator. At that instant, the terminator — the line separating day from night — runs from pole to pole, slicing the planet into two equal halves. This happens because Earth's axis is tilted at 23.5° relative to its orbital plane, and twice a year that tilt aligns so neither hemisphere leans toward or away from the Sun.

There are two equinoxes: the March (vernal) equinox around 20 March and the September (autumnal) equinox around 22 September. These dates shift slightly each year due to the mismatch between the calendar year and Earth's orbital period of 365.24 days.

Why Day and Night Are Not Exactly Equal

Despite the name, day and night are not perfectly 12 hours each at an equinox. Several factors cause a slight discrepancy:

  • Atmospheric refraction: The Sun's upper edge becomes visible a few minutes before it geometrically rises, and remains visible after it sets, because Earth's atmosphere bends sunlight. This adds about 2–3 minutes of extra daylight.
  • Definition of sunrise: Sunrise is defined as the moment the Sun's upper limb appears over the horizon, not its centre. Similarly, sunset uses the upper limb. This further extends daylight.
  • Solar disc size: The Sun subtends about 0.5° in the sky, so its centre must travel an extra half-degree after the geometric event for sunset to complete.

In practice, the date when day and night are most nearly equal is often a few days before the March equinox or after the September equinox. You can check precise times using our daily sunset times tools.

The Equinox and the Seasons

The equinoxes mark the transition between astronomical seasons. The March equinox begins spring in the Northern Hemisphere and autumn in the Southern Hemisphere. The September equinox does the reverse. Between equinoxes, the Sun's path across the sky shifts northward or southward, changing day length.

During an equinox, the Sun rises due east and sets due west everywhere on Earth (except at the poles). This makes it a useful reference for orienteering and aligns with ancient monuments like Chichen Itza's pyramid, where a serpent shadow appears at sunset on the equinox.

For a deeper look at how daylight varies through the year, see our explanation of day length through the year.

Where the Sun Rises and Sets at Equinox

As the equinox approaches, the sunrise point on the horizon has been migrating toward due east. After the March equinox, it moves north of east for the Northern Hemisphere, reaching its maximum northward extent at the June solstice. This swing reverses after the September equinox.

At the equator, the Sun rises and sets almost vertically, and day length changes very little between seasons. Residents of Nairobi, Kenya experience this directly — you can explore near-constant days at the equator with our data.

In contrast, higher latitudes see dramatic shifts. For example, at 60°N (e.g., southern Alaska), the March equinox brings roughly 12 hours of daylight, but by June the day exceeds 18 hours. The solstices mark the extremes of this cycle.

Observing the Equinox

You do not need special equipment to notice the equinox. On the day itself, stand outside at noon and look at your shadow. Near an equinox, it will be shortest if you are close to the equator, but more importantly, the Sun's altitude at midday equals 90° minus your latitude. For instance, at 40°N, the Sun reaches 50° above the horizon.

To confirm the equal-day claim, measure the time between sunrise and sunset on the equinox date. You will find it is a few minutes longer than 12 hours due to refraction. The true moment of equal day and night occurs when the Sun's centre is exactly on the horizon — a geometric event that is hard to observe directly.

The Equinox in Culture and Science

Many cultures celebrate the equinox as a time of balance. The Persian Nowruz (New Year) begins at the March equinox. The Japanese holiday Higan honours ancestors during both equinox weeks. In astronomy, the equinox serves as a reference point for celestial coordinates: the "first point of Aries" marks the March equinox location in the sky, though precession has since shifted it into Pisces.

Understanding the equinox also aids in planning outdoor activities. Photographers favour the golden hour around sunrise and sunset during equinox weeks because the low Sun angle creates long shadows and warm light. Gardeners use equinox dates to time planting, as soil temperatures and daylight hours become more stable.

The equinox is a simple astronomical event with subtle nuances — refraction, solar disc size, and definitions of sunrise all nudge the experience away from perfect symmetry. Whether you use it to mark the start of spring, plan a trip, or simply understand why day length varies, the equinox remains one of the most accessible astronomical phenomena to observe firsthand.