Science Interviews


Tue, 19th Jan 2016

Why do we have seasons?

Professor David Rothery, The Open University

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Before we get to embedded in sleepy subjects it might be useful to know why weSeasons have seasons at all? Connie Orbach checked in with professor of planetary geosciences David Rothery to find out.

David - We have seasons because the Earthís axis is not exactly upright relative to its orbit - it's tilted at 23Ĺ degrees.  In northern hemisphere winter, the Earthís north pole is tilted away from the Sun by 23Ĺ degrees, so the Sun is low in the sky and it isnít seen at all from near the North Pole.  In northern hemisphere summer, the Earth has gone halfway round the sun but the axis is still pointing the same direction in space so the North Pole is tilted at 23Ĺ degrees toward the Sun.  So the Sun never sets there and at spring and autumn, halfway in between those two points, youíve got days of equal length because the actual tilt is neither towards nor away from the Sun.

Connie - So, let me get this right.  The tilt's staying in the same direction but, as itís orbiting the Sun, that tilt is, in some places, directed away from the Sun and in other places directed towards it.

David - And itís a mirror image between north and south hemispheres.  When weíve got summer in the northern hemisphere, itís winter in the southern hemisphere.

Connie - And so, does that tilt ever change?

David - The tilt does change -  we think over about 41,000 years it changes slowly between about 22 and about 24Ĺ degrees.  Itís a very slow, very slight change in the tilt, which means that over geologic time, seasons have been pretty much the same.

Connie - Do other planets have this tilt of their poles or is it just us?

David -   No - weíre not the only planet.  Mercury has none; its axis is exactly 90 degrees to its orbit  but Mars has a tilt thatís 22.5 degrees at the moment - it has seasons and we think Marsí axis wobbles rather more extremely than the Earthís, and itís possible that Marsí axis wobbles by more than Earthís because Earthís axial tilt is stabilized with the presence of a large moon.


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