Why Are There Seasons
The angle of the Sun and the Earth's seasons. Image credit: NASA

Astronomy, Guide to Space

Tilt of the Earth

10 Mar , 2009 by

The Earth’s axis is tilted 23.5 degrees away from the plane of the ecliptic. It’s because of this tilt that the Earth experiences seasons as it orbits around the Sun.

Imagine the Sun is at the center of a spinning record. All of the planets orbit the Sun, like grooves in a record, including the Earth. And imagine that each planet is a spinning top, with their north and south poles always pointed in the exact same angle as they revolve around the Sun. Measure the angle that the Earth’s poles make with the orbit that the Earth makes around the Sun, and you get this number of 23.5 degrees.

During one point in the Earth’s orbit, the north pole of the Earth is pointed towards the Sun. This is summer for the northern hemisphere. And then 6 months later, when the Earth is at the opposite side of its orbit, the north pole is pointed away from the Sun; that’s winter in the north, and summer in the south.

The Earth’s tilt varies over a 42,000 year period, ranging from 22.1 degrees to 24.5 degrees. The direction that the Earth’s axis is pointed also changes over a long cycle, lasting almost 26,000 years. Right now, the Earth’s north pole is pointed towards the star Polaris. Over the course of this cycle, the seasons for summer and winter reverse (at 13,000 years), and then return to their original positions (after another 13,000 years).

All of the planets in the Solar System have some amount of axial tilt. Mars is very similar to the Earth at 25.2 degrees, while Uranus is tilted right over at 97.8 degrees.

We have written many articles about the Solar System for Universe Today. Here’s an article about the axial tilt of Mars, and the tilt of Uranus.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

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Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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