Topics Earth, weather and climate

Why do we have seasons?

We have seasons because Earth's axis is tilted, about 23 and a half degrees, relative to the plane of its orbit around the Sun. As the planet travels around the Sun, that tilt keeps pointing in the same direction in space. So for part of the year the Northern Hemisphere leans toward the Sun, and for part of the year it leans away. The Southern Hemisphere does the opposite, which is why July is summer in Australia and winter in Canada.

What makes this interesting is how many people believe the wrong answer. Earth is actually slightly closer to the Sun in early January than in early July, so distance cannot be the cause. The tilt does two things at once: it changes how directly sunlight strikes the ground, and it changes how long the days last. Both add up to more or less heat arriving at a given place.

An episode would walk through the tilt, the angle of the light, day length, the solstices and equinoxes, and why the warmest days come after the longest ones. It would also cover what seasons look like near the equator and the poles. The hosts on bre are AI, so they can get things wrong, and you can press Talk to ask a question mid-episode.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. The wrong answer almost everyone givesMany people assume summer means Earth is closer to the Sun. We look at why that fails: the planet is nearest the Sun in early January, in the middle of Northern winter.
  2. A tilted planet on a long loopEarth's axis leans about 23 and a half degrees and keeps pointing the same way as it orbits for a year. That fixed lean is the whole engine behind the seasons.
  3. Why the angle of sunlight mattersHigh-angle light concentrates energy on a small patch of ground, while low-angle light spreads the same energy over a wider patch and passes through more air.
  4. Long days, short daysThe tilt also decides how many hours a place spends in sunlight. More daylight means more time to warm up and less time to cool down overnight.
  5. Solstices, equinoxes and the lagThe solstices are the extremes of the tilt and the equinoxes sit in between. The hottest weather usually trails the longest day because land and oceans take time to warm.
  6. Seasons at the equator and the polesNear the equator the day length barely changes, so the seasons are mild or tied to rainfall. Near the poles the Sun can stay up or down for weeks.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Here's a quick test. Is it hot in July because Earth is closer to the Sun? Most people say yes, and I'd have said yes too.

  2. CalAI host

    I did say yes. For years. Confidently, at dinner parties.

  3. breAI host

    Okay, here's the part nobody tells you. Earth is actually closest to the Sun in early January. That's the middle of winter for the Northern Hemisphere.

  4. CalAI host

    Wait, so we're nearest the furnace when it's snowing? That feels like it breaks the whole idea. If distance isn't doing it, something else has to be doing a lot of work.

  5. breAI host

    It is. The planet is tilted, and that tilt changes how the sunlight lands on you. Picture a flashlight pointed straight down at a table, then tipped sideways.

  6. CalAI host

    The straight beam makes a small bright circle, and the tipped one smears out into a dim oval. Same flashlight, same energy, very different punch per square inch of table.

  7. breAI host

    Exactly that. And the tilt does a second thing too, which is change how long the Sun stays up. We'll get to both.

Questions people also ask

Is Earth closer to the Sun in summer?
No. Earth is closest to the Sun in early January and farthest in early July, and the difference is small. The seasons come from the tilt of Earth's axis, which changes sunlight angle and day length. Distance plays only a minor role.
Why are seasons opposite in the Northern and Southern Hemispheres?
When the Northern Hemisphere tilts toward the Sun, the Southern Hemisphere tilts away. The north gets more direct light and longer days while the south gets weaker light and shorter days. Half a year later the situation reverses.
What are the solstices and equinoxes?
Solstices are the two moments each year when one hemisphere is tilted most toward or away from the Sun, giving the longest and shortest days. Equinoxes fall in between, when day and night are nearly equal in length across the planet.
Why is the hottest part of summer after the longest day?
The longest day brings the most sunlight, but land, air and especially oceans take time to heat up. Warmth keeps building while incoming energy exceeds outgoing energy, so peak temperatures typically arrive weeks after the solstice.

Related topics

More: all 300 topics, earth, weather and climate, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.