Earth's tilt, the solstices and the equinoxes, in 3D.
How to use the seasons simulator
The simulator opens on the next solstice or equinox from today, with Earth seen side-on and the Sun beside it, so you can see which half of the planet leans toward the light. The buttons on the card step to the following solstices and equinoxes in turn. Press the amber play button to run time forward and watch day and night sweep around Earth, drag to look from another side, and use the planet list to visit Mars, Saturn or Uranus and compare their tilt. Events lists every equinox and solstice coming up from the date on the clock.
Why Earth has seasons
Earth's axis is tilted about 23.4° from the perpendicular to its orbit, and through the year it keeps pointing in the same direction in space, close to the North Star. So as Earth goes around the Sun, first one hemisphere and then the other leans toward it.
The hemisphere leaning toward the Sun gets sunlight from higher in the sky, which concentrates the same light on a smaller patch of ground, and it gets longer days to collect it. That hemisphere has summer; the other, with low sunlight and short days, has winter. Near the equator the Sun stays high all year and the seasons are mild; toward the poles they become extreme, until inside the Arctic and Antarctic circles the Sun does not set at all around midsummer, or rise at all around midwinter.
Solstices and equinoxes, 2026 to 2028
At a solstice one hemisphere leans most toward the Sun: the June solstice is the longest day in the north and the shortest in the south, and the December solstice the reverse. At an equinox the tilt is sideways to the Sun, which shines straight down on the equator, and day and night are close to twelve hours each everywhere.
| March equinox | June solstice | September equinox | December solstice | |
|---|---|---|---|---|
| 2026 | March 20, 14:46 | June 21, 08:24 | September 23, 00:05 | December 21, 20:50 |
| 2027 | March 20, 20:25 | June 21, 14:11 | September 23, 06:02 | December 22, 02:42 |
| 2028 | March 20, 02:17 | June 20, 20:02 | September 22, 11:45 | December 21, 08:19 |
Times are in UTC, from the U.S. Naval Observatory. The simulator computes the same moments itself, to within about a minute of these.
It isn't the distance
A common guess is that summer comes when Earth is closest to the Sun. It doesn't: Earth's orbit is nearly a circle, and Earth is closest in early January, on January 3 in both 2026 and 2027, at about 147.1 million km, and farthest in early July, at about 152.1 million km. That makes sunlight about 7% stronger in January than in July, a small effect beside the tilt, and it comes in the northern winter. If distance caused the seasons, both hemispheres would have summer at the same time.
Seasons on other planets
The same rule holds across the solar system: the tilt sets the seasons, and the length of the year sets how long they last. Open each planet from the list and look at its axis.
| Planet | Tilt | Year | Seasons |
|---|---|---|---|
| Mars | 25.2° | 687 days | Like Earth's, about twice as long |
| Jupiter | 3.1° | 11.9 years | Almost none |
| Saturn | 26.7° | 29.4 years | About seven years each; the rings tilt toward and away from the Sun |
| Uranus | 97.8° | 84 years | About 21 years each; each pole has about 42 years of daylight, then 42 of night |
Venus, tilted about 177°, spins almost upside down, so its axis is nearly straight up and down and it has almost no seasons either.
Accuracy
The equinoxes and solstices come from Astronomy Engine, the open-source library that places the planets in the simulator, and they agree with the U.S. Naval Observatory's times to within about a minute. Each planet's axis is turned by the International Astronomical Union's rotation model, so the tilts you see are the real ones, and the tilts and year lengths above come from NASA's planetary fact sheets. More on the limits is in accuracy and sources. To see the Moon's shadow instead of the Sun's light, step through the eclipses in the eclipse simulator.
Sources
- U.S. Naval Observatory, Astronomical Applications Department, Earth's Seasons and Apsides for 2026, 2027 and 2028 (aa.usno.navy.mil), read October 12, 2026: equinox, solstice, perihelion and aphelion times.
- NASA Goddard Space Flight Center, Earth Fact Sheet (nssdc.gsfc.nasa.gov), read October 12, 2026: perihelion 147.095 and aphelion 152.100 million km, obliquity 23.44°; planetary fact sheets for the other planets' tilts and years.
- D. Cross, Astronomy Engine: the season searches behind the simulator.
Questions
Why does Earth have seasons?
Because Earth's axis is tilted about 23.4° to its orbit and keeps pointing the same way in space all year. In June the northern hemisphere leans toward the Sun, so the Sun climbs higher and days are longer there: northern summer. In December the southern hemisphere leans toward the Sun, and the seasons swap.
Is Earth closer to the Sun in summer?
No. Earth is closest to the Sun in early January (perihelion, about 147.1 million km) and farthest in early July (aphelion, about 152.1 million km). The difference gives about 7% more sunlight in January, far less than the effect of the tilt, which is why January is winter in the northern hemisphere.
When are the solstices and equinoxes in 2027?
In 2027 the March equinox is on March 20 at 20:25 UTC, the June solstice on June 21 at 14:11, the September equinox on September 23 at 06:02 and the December solstice on December 22 at 02:42 UTC, according to the U.S. Naval Observatory. The simulator steps to each of them.
Why are the seasons opposite in the southern hemisphere?
The two hemispheres lean toward the Sun at opposite times of year. When the North Pole tilts toward the Sun in June, the South Pole tilts away, so Australia, South Africa and Argentina have winter while Europe and North America have summer.
Do other planets have seasons?
Any planet with a tilted axis does. Mars, tilted about 25°, has seasons much like Earth's but nearly twice as long. Jupiter, tilted only about 3°, has almost none. Uranus is tipped about 98°, so each of its seasons lasts about 21 years, and each pole has about 42 years of daylight followed by 42 of night.