What happens at the March equinox
Earth’s axis is tilted by about 23.4 degrees relative to its orbit. As Earth travels around the Sun, that tilt makes the Sun appear to drift north and south in the sky over the year. Astronomers describe this by projecting Earth’s equator onto the sky as the celestial equator.
At the March equinox, the Sun sits exactly on the celestial equator while moving from south to north. At that instant, the Sun is directly overhead at noon for someone standing on Earth’s equator, and the line between day and night (the terminator) runs through both poles.
The word equinox comes from Latin for “equal night”, because on this date day and night are roughly the same length everywhere.
When the March equinox falls
The equinox is a precise instant, not a full day. In Coordinated Universal Time it usually lands on March 20, but it can fall on March 19 or 21 depending on the year and your time zone.
The date wanders slightly because a calendar year of 365 days is about six hours shorter than the time Earth takes to return to the same point in its seasons. Each year the equinox drifts roughly six hours later, and then a leap day pulls it back. For the exact time in your area, see the seasons page.
Is day equal to night?
Not quite. On the March equinox, most places get slightly more than 12 hours of daylight. Two effects add a few minutes:
- Sunrise and sunset are measured at the Sun’s top edge, not its center, so the day starts a little earlier and ends a little later.
- Atmospheric refraction bends sunlight over the horizon, so the Sun appears above it while it is geometrically still just below.
The day when daylight and darkness really are closest to 12 hours each is sometimes called the equilux. In the Northern Hemisphere it comes a few days before the March equinox; in the Southern Hemisphere a few days after. You can see your own daylight hours on the sunrise and sunset page.
Day length changes fastest now
Around the equinoxes, day length changes more quickly than at any other time of year. At mid-latitudes, the days can grow or shrink by roughly two to four minutes per day, and faster still at high latitudes. Near the solstices, by contrast, the change slows almost to a halt.
That is why late March can feel like a sudden jump toward brighter evenings in Europe and North America, while people in Australia, New Zealand, South Africa and South America notice their evenings drawing in just as quickly.
What it means in each hemisphere
| Northern Hemisphere | Southern Hemisphere | |
|---|---|---|
| Astronomical season that begins | Spring | Autumn |
| Days after the equinox | Longer than nights | Shorter than nights |
| Common name for the event | Vernal or spring equinox | Autumnal or autumn equinox |
Because the names flip between hemispheres, many astronomers prefer to say “March equinox” for a name that works everywhere. The hemisphere-specific names are covered in the guide to the vernal equinox.
At the poles
The March equinox is a turning point for the polar regions. At the North Pole, the Sun rises around this date after months of darkness and then stays above the horizon until the September equinox. At the South Pole, the Sun sets for the long polar night. Refraction shifts these moments by a few days, but the pattern holds.
Seasons in the calendar
The astronomical seasons are not the only way to divide the year. Weather services usually count spring in the Northern Hemisphere from March 1 so that each season covers whole months. The guide to meteorological vs astronomical seasons explains the difference, and the September equinox is the mirror image of this event, six months later.