I learned this today. The Earth wouldn’t really have seasons if it wasn’t tilted on its axis, and that is thanks to the moon.
The Earth is tilted at an angle of 23.5°. If the Earth was completely straight, we would have mild temperature changes as the Earth moved closer and farther away from the sun, but nothing like the seasons we have.
Venus is a good example. Venus has a tilt of 2.7° and its orbit around the sun is nearly completely circular. This means it has no seasons. There is no difference in the angle of its axis throughout the year so its surface temperature of 460℃ doesn’t change.
Earth’s seasons are not caused because we move closer and farther away from the sun. The Earth’s axis always points in the same direction as we travel around the sun, so our seasons are caused because the planet’s tilted angle means one side leans towards the sun and then leans away from the sun at different times through the year. For example, in December, the southern hemisphere is angled towards the sun and the northern hemisphere is angled away. In July, the opposite happens. This is also why the length of daylight changes with the seasons.
So, why is Earth tilted? A number of the planets in the solar system are tilted. Uranus tilts the most. It tilts 98°, which means it is basically sideways. All of these tilts have been caused by impacts as the planets were forming.
In the early days of the solar system, 4.5 billion years ago, the inner solar system probably had about 100 moon to Mars sized planets. These planets kept colliding with each other and slowly amalgamated and grew larger.
Scientists think that the Earth suffered ten giant collisions in the first 100 million years of the solar system. Each one changed the tilt of the Earth a little. The last collision was the one that formed the moon. A planet the size of Mars collided with the early Earth. The collision melted the two planets together and flung a huge amount of debris out into space. This debris made a ring around the Earth, that slowly condensed into the moon. And this is the impact that gave the Earth its 23.5° tilt. If we didn’t have our moon the Earth’s tilt would change, but the gravitational pull of the moon keeps it at 23.5°.
So, the Earth tilts by 23.5° because of the impacts from many early planets and finally, the Mars-sized planet that became the moon. Thanks to this tilt, we have our seasons because a different side of the Earth leans towards and away from the sun throughout the year. And this is what I learned today.
Sources
https://www.wmdt.com/2014/12/all-about-the-tilt-earths-tilt/
https://spaceplace.nasa.gov/seasons/en/
https://www.bbc.co.uk/bitesize/articles/zhtvscw
https://en.wikipedia.org/wiki/Giant-impact_hypothesis
https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System
https://www.universetoday.com/36887/does-venus-have-seasons/
