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Why Are There Seasons

Earth’s Tilt

9 Dec , 2009



The Earth’s tilt is approximately 23.5°. Okay, the exact number is 23.439281°. And it’s because of this tilt that we have the amazing seasons here on Earth.

So what exactly is the Earth’s tilt? Imagine there’s a line going directly through each object in the Solar System, passing from the north pole to the south pole and out into space in both directions. This is an object’s axis of rotation. The Earth has one, the Sun has one and every object in the Solar System has one.

The most important axis of rotation is the Sun’s; it’s the biggest, brightest object in the Solar System, so we’d better pay it some respect. Now if you compare the line that makes the Sun’s axis of rotation to the line that makes the Earth’s axis of rotation, you’ll get an angle. This is the Earth’s axial tilt away from the Sun’s axis of rotation. And that angle is 23.5°.

The Earth north pole is always pointed towards the same star in the sky – Polaris. And so as it orbits around the Sun, the various hemispheres of the Earth get more or less sunlight. When the Sun is on the same side as Polaris, the northern hemisphere receives more sunlight – this is summer for the north, and winter for the southern hemisphere. And then 6 months later, the Sun and Polaris are on opposite sides of the Earth. The north pole is tilted away from the Sun, and so it receives less sunlight. That’s when we have winter in the northern hemisphere.

If the Earth didn’t have a tilted axis, there wouldn’t be any seasons. Regions in the north and south would always be cold, while the equator would always be hot. And that just wouldn’t be any fun.

We’ve written many articles about the axial tilt of planets for Universe Today. Here’s an article about the tilt of Mars, and here’s an article about the tilt of Saturn.

If you’d like more information on the Earth’s tilt, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about Earth. Listen here, Episode 51: Earth.

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