This Friday, at 6.50 in the morning, we will have the winter solstice.
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But why do we have seasons in the first place? Well, we have the Earth's tilt to thank for that.
The solstice on Friday is the point of maximal tilt of the Earth, such that the southern hemisphere is tilted away from the Sun.
This means we receive less light, having shorter daylight hours, and less heat, so colder weather.
In the northern hemisphere, the Earth is titled the maximum amount towards the Sun.
This means it receives more light, so longer days, and more heat and therefore warmer weather.
As of today, the Earth is tilted 23.44 degrees. However, the Earth's tilt does wobble a bit so it can range from 22.1 to 24.5 degrees.
What caused the Earth's tilt?
We think the Earth's tilt is the same reason we have the Moon - a giant collision 4.5 billion years ago.
Called the Giant-impact hypothesis, a baby planet the size of Mars, called Theia, shared a similar orbit as Earth.
Around 50 - 100 million years after the planets started to form, this collision occurred.
In doing so, a bunch of rock and debris was ejected from the Earth, along with the breaking up, in part, of Theia.
This rock eventually coalesced together into the Moon.
The composition of the Moon and how similar it is to Earth, its unique orbit around the Moon where one side always faces the Earth, and the Earth's tilt are just some of the reasons causing us to think this occurred.
![The reason planet Earth has seasons is because of its tilt - likely gained after a collision. Picture Shutterstock The reason planet Earth has seasons is because of its tilt - likely gained after a collision. Picture Shutterstock](/images/transform/v1/crop/frm/XBxJDq6WLub2UphQ8wEq23/e45bef8d-540e-4f63-b888-72c4cc844ecf.jpg/r0_373_7000_4309_w1200_h678_fmax.jpg)
A tilted planet is not how we would have started, we would have essentially had no tilt - spinning straight up and down.
Our solar system is a giant disc, and all the planets lie on this disc and go around the Sun on it.
The tilt is the angle between how a planet spins with respect to how we go around the Sun.
If we spin straight up and down with respect to how we go around the Sun, we have no tilt.
If we spin in the same direction that we go around the Sun, our tilt is 90 degrees, like we are on our side.
When the planets formed, they would have formed proportionate to this disc, spinning, and orbiting in the same way, so no tilt, or a very tiny one.
Planets like Mercury and Jupiter have no tilt. Venus doesn't really either, but it spins the wrong way which is a bit confusing.
Mars, Saturn, Uranus, and Neptune all have tilts.
SUNDAY SPACE:
Mars's tilt is about 25 degrees, making it very similar to the Earth.
We think it may have been by hit a protoplanet or a large moon.
Uranus has a tilt of almost 98 degrees, meaning it is like a ball on its side rolling around.
Instead of a collision, we think its dwarf planets have migrated and as they move away, pulled the planet over.
These tilts make the seasons on each planet.
While the seasons have nothing to do with us being closer or further from the Sun, we have an ancient, giant collision to thank for it - and for the Moon.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU.