Why the seasons happen
Earth's axis is tilted about 23.4 degrees from the plane of its orbit. As we travel around the Sun, first one hemisphere and then the other leans toward it, receiving more direct light and longer days. That lean, not distance, creates the seasons. In fact Earth is closest to the Sun in early January, in the middle of northern winter.
The equinoxes and solstices are the four points where this tilt shows itself most clearly. Because they are defined by the Sun's position and not by the calendar, they wander between roughly the 19th and the 23rd of their month, nudged by the leap-year cycle.
The four moments
Each has its own page with the hemisphere-by-hemisphere meaning.
- March equinox. Around 20 March the Sun crosses the celestial equator heading north: spring begins in the north and autumn in the south.
- June solstice. Around 21 June the Sun stands farthest north: the longest day for the northern hemisphere, the shortest for the southern.
- September equinox. Around 22 or 23 September the Sun crosses back to the south: autumn in the north, spring in the south.
- December solstice. Around 21 December the Sun stands farthest south: the shortest day in the north, the longest in the south.
Astronomical and meteorological seasons
Astronomers start each season at the equinox or solstice. Meteorologists prefer tidy months, so they begin spring on 1 March, summer on 1 June, autumn on 1 September and winter on 1 December in the northern hemisphere, and shift each by half a year in the south. Neither is wrong: one follows the Sun, the other follows the weather statistics.
The same flip applies to the dates themselves. If you live in Australia, Brazil, Argentina, Chile or South Africa, read the June solstice as the start of winter and the December solstice as the start of summer.
Celebrations around the world
People have marked these days for as long as they have watched the sky. Persian communities welcome Nowruz at the March equinox, and Japan keeps Higan around both equinoxes. Sweden and Finland celebrate Midsummer, and Stonehenge draws crowds at the solstice sunrise. In Cusco the Inti Raymi festival honours the Sun at the southern winter solstice, while East Asia keeps Dongzhi in December.
In astrology the March equinox is the doorway to the tropical zodiac, the moment the Sun enters 0 degrees of Aries. That is a tradition, a symbolic new year for the signs. To follow the rhythm of the Moon alongside the Sun, visit the Moon phases hub, or see when the sky darkens at the eclipses hub.
Questions people ask
Are day and night exactly equal at an equinox?
Almost, but not quite. Atmospheric refraction and the way sunrise is defined make the truly equal day, the equilux, fall a few days away from the equinox.
Why does the date change from year to year?
A calendar year is not a whole number of days, so the exact moment slips later each year and snaps back at a leap year. That keeps each event within about a four-day window.
Is it summer everywhere on the June solstice?
No. It starts astronomical summer in the northern hemisphere and winter in the southern hemisphere, because the seasons are opposite on either side of the equator.