Earth's Tilt, Revolution and the Seasons
It is 95 °F in Houston on July 4, and the sun does not set until after 8 p.m. In December, the same city can feel chilly, and it is dark by 5:30 p.m. Many people think summer is hot because Earth moves closer to the Sun. Is that true? By the end of this page, you will be able to explain what really causes the seasons.
Two motions of Earth
Earth is always moving in two ways at the same time.
Rotation is Earth spinning on its axis, an imaginary line through the North and South Poles. One rotation takes about 24 hours. Rotation gives us day and night.
Revolution is Earth traveling in its path (orbit) around the Sun. One revolution takes about 365 days, or one year.
Earth takes about 365 days to travel once around the Sun. Name this motion of Earth around the Sun. Then name the motion that causes day and night, and say how long it…
Earth takes about 365 days to travel once around the Sun. Name this motion of Earth around the Sun. Then name the motion that causes day and night, and say how long it takes.
Earth's tilt
Earth's axis is not straight up and down compared with its path around the Sun. It is tilted about $23.5^\circ$.
The most important fact is this: the tilt always points the same direction in space (toward the North Star) as Earth goes around the Sun. The tilt does not wobble back and forth during the year. It only looks different because Earth is in a different place in its orbit.
Because the axis tilts one way, at some points in the orbit the Northern Hemisphere leans toward the Sun, and at the opposite point it leans away.
Earth's axis is tilted about $23.5^\circ$ from straight up and down compared with its path around the Sun. In one or two sentences, explain what this tilt means and say…
Earth's axis is tilted about $23.5^\circ$ from straight up and down compared with its path around the Sun. In one or two sentences, explain what this tilt means and say whether the tilt direction stays the same during the whole year.
Direct sunlight and length of daylight
The tilt changes two things in the hemisphere that is leaning toward the Sun.
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Directness of sunlight. The Sun is higher in the sky, so its rays hit the ground more straight on. The same amount of light is concentrated on a small area, which heats it more.
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Length of daylight. Days are longer, so the ground has more hours to warm up.
You can test the first idea with a flashlight. Shine it straight down on paper: you get a small, bright circle. Shine it at a slant from the same distance: the light spreads into a larger, dimmer oval.
In the model, the flashlight is the Sun, the paper is Earth's surface, and the slant stands for how low the Sun is in the sky (or how Earth's tilt turns a region away from the Sun).
Jayden shines a flashlight straight down onto a sheet of paper, and then shines it at a slant from the same distance. The straight beam makes a small, bright circle. The…
Jayden shines a flashlight straight down onto a sheet of paper, and then shines it at a slant from the same distance. The straight beam makes a small, bright circle. The slanted beam makes a larger, dimmer oval. Explain how this model shows why the Sun's rays heat Earth more in summer than in winter. Say what the flashlight, the paper and the slant represent.
The table shows approximate hours of daylight in Houston, Texas, on four dates. | Date | Daylight (hours) | |---|---| | March 20 | 12.0 | | June 21 | 13.9 | | September…
The table shows approximate hours of daylight in Houston, Texas, on four dates.
| Date | Daylight (hours) |
|---|---|
| March 20 | 12.0 |
| June 21 | 13.9 |
| September 22 | 12.0 |
| December 21 | 10.4 |
- a) Which date is closest to the start of summer and which to the start of winter? b) By how many hours does the longest day differ from the shortest day? c) Explain what position of Earth's tilt relative to the Sun causes the longest day.
The four seasons
Follow Earth around its orbit. The seasons below are for the Northern Hemisphere. The Southern Hemisphere has the opposite season at the same time.
| Date (approx.) | Northern Hemisphere | Southern Hemisphere | What is happening |
|---|---|---|---|
| March 20 | Spring begins | Fall begins | Neither tilted toward the Sun: equal day and night |
| June 21 | Summer begins | Winter begins | North tilted toward the Sun: most direct light, longest day |
| September 22 | Fall begins | Spring begins | Neither hemisphere tilted toward the Sun: equal day and night |
| December 21 | Winter begins | Summer begins | North tilted away: least direct light, shortest day |
So when Emma in Denver sees the long, sunny day on June 21, summer is starting in Colorado, and winter is starting in Australia.
Emma lives in Denver, Colorado, in the Northern Hemisphere. On June 21 her family notices that the day is very long and the Sun is high in the sky. Which season is…
Emma lives in Denver, Colorado, in the Northern Hemisphere. On June 21 her family notices that the day is very long and the Sun is high in the sky. Which season is starting in Denver? Which season is starting at the same time in Australia, in the Southern Hemisphere?
A classmate says, "It is hot in summer because Earth is closer to the Sun." Explain why this idea is incorrect. In your answer, use the tilt of Earth's axis and the…
A classmate says, "It is hot in summer because Earth is closer to the Sun." Explain why this idea is incorrect. In your answer, use the tilt of Earth's axis and the directness of sunlight. Include what happens to the length of daylight in the Northern Hemisphere in summer.
Draw or describe a model showing Earth at four points in its orbit around the Sun: positions A, B, C and D, each about three months apart. At position A the Northern…
Draw or describe a model showing Earth at four points in its orbit around the Sun: positions A, B, C and D, each about three months apart. At position A the Northern Hemisphere is tilted toward the Sun. For each position, state which hemisphere receives more direct sunlight and name the season in the Northern Hemisphere. Then explain why the seasons would NOT happen in the same way if Earth's axis had no tilt at all, even though Earth still revolved around the Sun.
Earth is closest to the Sun in early January, at about 147 million km, and farthest in early July, at about 152 million km. Mateo claims this proves distance causes the…
Earth is closest to the Sun in early January, at about 147 million km, and farthest in early July, at about 152 million km. Mateo claims this proves distance causes the seasons. Use these facts and what you know about seasons in both hemispheres to write an argument (claim, evidence, reasoning) that evaluates Mateo's claim. Explain what would be different about July weather in Chicago and in Buenos Aires, Argentina, if distance were the cause.
Back to the start: Houston is hot in July because the Northern Hemisphere leans toward the Sun. Sunlight is more direct and days are long. In December the tilt leans away, so the Sun is lower and days are short. Distance is not the cause.