Why are the days getting shorter?
Earth spins on an axis tilted 23.4°. Between June and December the Northern Hemisphere leans away from the sun, so the sun rises later, sets earlier and rides lower in the sky, until the shortest day around December 21.
A lower sun: weaker heat, longer shadows, more glare at commute time.
Less daylight shifts your body clock and mood.
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It's the tilt, not the distance
Earth is actually closest to the sun in early January, in the middle of northern winter. What matters is the tilt. As Earth orbits, the axis keeps pointing the same way in space, so for half the year your hemisphere leans toward the sun and for half it leans away. Leaning toward means the sun climbs higher and stays up longer; leaning away means short days and a low sun whose light spreads thin across the ground.
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How fast the days change
Day length changes fastest around the equinoxes in March and September and barely at all near the solstices. In San Diego the days shrink by about two minutes a day in late September and October; at 40°N, the latitude of Denver or Philadelphia, it's closer to three. The total swing grows with latitude: about 14 hours 20 minutes of daylight in San Diego in June versus about 9 hours 55 minutes in December, while Anchorage swings from over 19 hours to under 6.
Why it gets colder after the shortest day
The coldest weeks usually come in January, after the days have started growing. Land and especially oceans keep giving off stored heat for weeks, so temperatures lag the sunlight, just as the hottest part of a summer day comes in mid-afternoon rather than at noon.
Questions people ask
- When is the shortest day of the year?
- The December solstice, around December 21 in the Northern Hemisphere. In the Southern Hemisphere it's their longest day.
- Why doesn't the earliest sunset fall on the shortest day?
- Because Earth's orbit is slightly oval and its axis is tilted, solar noon drifts later through December. The earliest sunsets come in early December and the latest sunrises in early January.
- Do days change length at the equator?
- Barely. Near the equator day and night stay close to 12 hours all year; the swing grows the farther you go toward the poles.