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To better understand this, first we have to think of the Sun. Imagine there’s a line passing through the north pole and south poles of the Sun. This is the Sun’s axis of rotation. Then imagine a disk coming out from the Sun’s equator in all directions. This disk is called the plane of the ecliptic, and it’s where all of the planets in the System are located. Astronomers then measure the axis of rotation passing through each of the planets. The axial tilt of each planet is measured by the angle it makes compared to the Sun’s axis of rotation.
And so, in the case of our planet, the Earth’s axis measures 23.5° away from the Sun’s axis of rotation. Because the Earth axis is tilted, we experience the seasons on Earth. During summer in the northern hemisphere, the Earth’s axis is tilted so that the north pole of the Earth is tilted towards the Sun. Regions in the northern hemisphere experience more sunlight and so they’re warmer than the southern hemisphere. Then in autumn, both the northern and southern hemispheres receive equal sunlight. And in winter for the northern hemisphere, the north pole is tilted away from the Sun, and so it’s colder in the north and warmer in the south.
We’ve written many articles about the Earth and the seasons for Universe Today. Here’s an article about the Earth’s seasons, and here’s an article about how the Earth might have flipped over in the past.
If you’d like a more precise number for the Earth’s axis, here you go: 23.439281°
The angle of the Earth’s tilt is stable over long periods of time, but the Earth slightly wobbles on its axis like a top – this is called precession. The date of the seasons is slowly changing over a 25,800 year cycle, so that the times for summer and winter will eventually switch, and will become summer in the northern hemisphere during December and winter during June.
NASA: What’s a Solstice?