What Causes the Seasons on Earth?

Ask people why summer is hot and winter is cold, and many will say Earth is closer to the Sun in summer. It is a sensible guess. It is also wrong. NASA calls this idea out directly, and the real reason is the way Earth leans as it travels around the Sun.

The distance idea, tested

Earth's orbit is not a perfect circle. It is slightly stretched, so Earth's distance from the Sun changes during the year. NASA Space Place gives the numbers:

• Closest point (perihelion): 91,400,000 miles from the Sun
• Farthest point (aphelion): 94,500,000 miles from the Sun

That is a difference of about 3.1 million miles, which sounds big but is only about 3 percent of the roughly 93 million mile average distance the National Weather Service cites.

Here is the part that settles it. Earth is closest to the Sun in early January and farthest in early July. If distance drove the seasons, the Northern Hemisphere would have its summer in January. It has winter instead. The National Weather Service states plainly that the distance difference is not significant in terms of climate and is not the reason we have seasons.

There is a second clue: when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. Both halves of the planet are at the same distance from the Sun at the same moment, so distance cannot explain opposite seasons.

The real cause: a tilted axis

Earth spins around an imaginary line through its center called its axis, which is why we have day and night. NASA explains that this axis does not stand straight up compared with Earth's path around the Sun. It leans. The National Weather Service gives the size of that lean: 23.5 degrees.

The key detail is that the tilt keeps pointing the same direction in space all year. So as Earth travels around the Sun:

• Around June, the North Pole is tilted toward the Sun. The Northern Hemisphere gets the Sun's most direct rays, and it is summer there.
• Around December, the South Pole is tilted toward the Sun. Now the Southern Hemisphere gets the direct rays, and the Northern Hemisphere has winter.

Why direct rays mean warmer weather

When sunlight hits the ground straight on, its energy is concentrated on a small area. When it arrives at a low, slanting angle, the same energy spreads across a larger area, so each patch of ground gets less. You can see this with a flashlight: point it straight at a table and you get a small, bright circle; tilt it and the light spreads into a dimmer oval.

The tilted hemisphere also gets longer days, so the ground has more hours each day to absorb sunlight and fewer hours of darkness to cool off.

Solstices and equinoxes

The National Weather Service describes four turning points:

• Summer solstice: the moment the tilt toward the Sun is at its maximum. In the Northern Hemisphere, the Sun is directly over the Tropic of Cancer at 23.5 degrees north latitude. North of that line, it is the longest day of the year.
• Winter solstice: the shortest day and longest night. For the Northern Hemisphere, the Sun is directly over the Tropic of Capricorn at 23.5 degrees south.
• Equinoxes (spring and fall): the two times when the axis tilts neither toward nor away from the Sun, giving nearly equal daylight and darkness everywhere. At the equator, the Sun is directly overhead at noon.

Notice that the tropics sit at 23.5 degrees, the same number as the tilt. That is not a coincidence. The tropics mark how far north and south the Sun can be directly overhead.

Even on an equinox, day and night are not exactly equal. The National Weather Service explains that the atmosphere bends sunlight, making the Sun visible a little before it rises and after it sets. Day length on the equinox runs about 12 hours and 6.5 minutes at the equator, 12 hours and 8 minutes at 30 degrees latitude, and 12 hours and 16 minutes at 60 degrees.

Why the hottest days come after the longest day

If the longest day is in late June, why is July usually hotter? The National Weather Service calls this a lag. Ground and water take time to heat up, so temperatures keep climbing for weeks after the solstice, the same way the warmest part of a day usually comes a few hours after noon.

Its Cleveland office gives a local example: the warmest daily temperatures arrive nearly 3 weeks after the solstice, in mid-July, and July averages 3.3 degrees Fahrenheit warmer than June.

Where did the tilt come from?

NASA Space Place describes the leading idea: long ago, a large object, called Theia, is thought to have struck the young Earth and knocked it off-kilter. Most scientists think debris from that collision eventually formed the Moon.

Key takeaways

• Earth's seasons are caused by its 23.5 degree axial tilt, not by its changing distance from the Sun.
• Earth is actually closest to the Sun in January, during Northern Hemisphere winter.
• The hemisphere tilted toward the Sun gets more direct sunlight and longer days, so it has summer.
• Solstices mark the maximum tilt toward or away from the Sun; equinoxes mark the two times the tilt points neither way.
• The warmest weather lags the longest day by weeks because land and water take time to heat up.