Solar System Simulator · Sky · seasons
Why do we have seasons?
Because Earth is tilted. Its axis leans 23.4° and always points the same way in space, so for half the year the north leans toward the Sun and for the other half the south does. The distance to the Sun has almost nothing to do with it.
It's not the distance
Earth is closest to the Sun in early January, 147.1 million km away, and farthest in early July, 152.1 million km. That 3% change gives about 7% more sunlight in January than in July, and January is midwinter in the north.
If distance made the seasons, the whole planet would have summer at once. Instead, when it's winter in New York, it's summer in Sydney.
It's the tilt
Earth spins on an axis tilted 23.4°, and the axis keeps pointing the same way in space, at Polaris, all year. As Earth goes round the Sun, that fixed tilt leans the northern hemisphere toward the Sun in June and away from it in December. The southern hemisphere gets the opposite.
Leaning toward the Sun does two things.
The Sun climbs higher
At 40° north (New York, Madrid, Beijing) the noon Sun stands 73.4° above the horizon at the June solstice and only 26.6° at the December one. Low sunlight arrives at a slant and spreads out: a beam one metre square covers 1.04 m² of ground in June but 2.24 m² in December, so each square metre gets less than half the energy.
The days get longer
At 40° north the Sun is up for 15 h 1 min at the June solstice and 9 h 19 min at the December one. Summer gets more hours of stronger sunlight; winter gets fewer hours of weaker sunlight.
Try any date
October 22148.9 million km from the Sun
40° north
Autumn
10 h 53 mindaylight
38.8°noon Sun height
40° south
Spring
13 h 25 mindaylight
61.2°noon Sun height
Solstices and equinoxes
At a solstice the tilt points most directly toward or away from the Sun: the longest day in one hemisphere and the shortest in the other. At an equinox neither hemisphere leans toward the Sun, and day and night are close to 12 hours everywhere.
| Date | North | South |
|---|---|---|
| Dec 21, 2026December solstice | Shortest day | Longest day |
| Jan 3, 2027Closest to the Sun | 147.1 million km | |
| Mar 20, 2027March equinox | Spring begins | Autumn begins |
| Jun 21, 2027June solstice | Longest day | Shortest day |
| Jul 5, 2027Farthest from the Sun | 152.1 million km | |
| Sep 23, 2027September equinox | Autumn begins | Spring begins |
Worked out on this page from Earth's orbit. Dates in UTC.
What the simulator does
- It opens on today's sky, with every planet where it really is, and time runs on from there. Any date from 1800 to 2200 works.
- Tap Earth. Its card gives the distance from the Sun, and the closest and farthest points of its orbit: 0.983 and 1.017 AU.
- Jump to the day Earth is closest to the Sun and the day it is farthest. One falls in northern winter, the other in northern summer.
Questions people ask
Is Earth closer to the Sun in summer?
Not in northern summer. Earth is closest to the Sun in early January, about 147.1 million km away, and farthest in early July, about 152.1 million km. That 3% change matters far less than the tilt.
Why are the seasons reversed in the southern hemisphere?
Earth's axis leans the same way all year, so when the northern hemisphere leans toward the Sun, the southern hemisphere leans away. December brings long days and a high Sun to Australia, Argentina and South Africa, and June brings their winter.
What is a solstice?
The moment the Sun reaches its farthest north or south, around 21 June and 21 December. It brings the longest day of the year to one hemisphere and the shortest to the other.
Are day and night equal at the equinox?
Nearly. At 40° north the Sun is up for about 12 h 9 min on the equinox: sunrise and sunset are timed at the Sun's top edge, and the air bends sunlight over the horizon, which adds a few minutes.
Keep going
Every number on this page is worked out from Earth's real orbit and tilt: the solstices and equinoxes by Meeus's method, closest and farthest from the VSOP87 orbit series, and day lengths by the standard sunrise equation.