Solstices & equinoxes

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Related astronomy & calendar pages

FAQ

Frequently asked questions

What's the difference between a solstice and an equinox?

A solstice (June, December) is when Earth's axial tilt is most strongly aligned toward or away from the Sun — the longest or shortest day of the year. An equinox (March, September) is when the Sun crosses the celestial equator and day and night are nearly equal everywhere on Earth. There are two solstices and two equinoxes per year, alternating roughly every three months. Together they mark the four astronomical season boundaries.

How many solstices and equinoxes are there each year?

Four total: the March equinox (≈ March 20), June solstice (≈ June 21), September equinox (≈ September 22 or 23), and December solstice (≈ December 21). Each is a single instant, not a 24-hour day, and the exact UTC moment shifts by about 5 hours 49 minutes from one common year to the next, with leap-year corrections.

Are the four events evenly spaced through the year?

Almost, but not exactly. The March-to-June and September-to-December gaps are slightly longer than the June-to-September and December-to-March gaps. The reason: Earth's orbit is mildly elliptical, and we move faster around the Sun when we're closer to it (perihelion, in early January). So we travel through the post-equinox quadrant containing perihelion in fewer days. The summer half-year in the Northern Hemisphere is about seven days longer than the winter half-year.

What are cross-quarter days?

Cross-quarter days are the four points midway between consecutive solstices and equinoxes — Imbolc (≈ February 1), Beltane (≈ May 1), Lammas / Lughnasadh (≈ August 1), and Samhain (≈ November 1, the origin of Halloween). Many modern holidays evolved from them: Groundhog Day, May Day, Lammas, Halloween. They originated as Celtic agricultural festivals marking the start of seasonal stages — "first stirring of spring," "start of summer," "first harvest," "start of winter."

Why do astronomical and meteorological seasons disagree?

Astronomical seasons are defined by the solstice and equinox instants — astronomical summer in the Northern Hemisphere runs from the June solstice to the September equinox, etc. Meteorological seasons are calendar-month brackets: summer = June, July, August. Meteorologists use the simpler definition because climate statistics, cherry-blossom records, and crop calendars line up better with whole months than with shifting astronomical boundaries.

When is the next astronomical season change?

The next solstice or equinox is shown in the live countdown cards at the top of this page. The most-recent past event is also implied by the current calendar position — for instance, between the September equinox and the December solstice the Northern Hemisphere is in astronomical autumn.

How accurate are the times on this page?

Each event is computed using Meeus's algorithm (Astronomical Algorithms, 2nd ed., Chapter 27) with a small ΔT correction for time-scale conversion. Accuracy is about 30 seconds for years 1000–3000, matching the published USNO and IMCCE almanacs to within a minute. All times are UTC; convert to your local zone with our world clock or time-zone converter.