Why do we have seasons?

19 January 2016

Interview with

Professor David Rothery, The Open University

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 Earth's axis is not exactly upright relative to its orbit - it's tilted at 23½ degrees.  In northern hemisphere winter, the Earth's north pole is tilted away from the Sun by 23½ degrees, so the Sun is low in the sky and it isn't 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 23½ degrees toward the Sun.  So the Sun never sets there and at spring and autumn, halfway in between those two points, you've 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 it's orbiting the Sun, that tilt is, in some places, directed away from the Sun and in other places directed towards it.

David - And it's a mirror image between north and south hemispheres.  When we've got summer in the northern hemisphere, it's 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 24½ degrees.  It's 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 - we're not the only planet.  Mercury has none; its axis is exactly 90 degrees to its orbit  but Mars has a tilt that's 22.5 degrees at the moment - it has seasons and we think Mars' axis wobbles rather more extremely than the Earth's, and it's possible that Mars' axis wobbles by more than Earth's because Earth's axial tilt is stabilized with the presence of a large moon.

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