Thursday 30 January 2014

The difference between Obliquity and Precession

Earth (with axis shown) orbiting the Sun
I don't know about you but I was confused by week 2's video description of Obliquity (Axial tilt) and Precession.

The video's description made them sound the same with only the period of time different.

So, after much research, here's the actual difference:

Obliquity is about the angle of tilt of the Earth's axis relative to it's orbit round the Sun. This tilt gives us our seasons e.g. In June to August, the northern hemisphere leans towards the Sun giving us Summer. In December to February, the southern hemisphere tilts towards the Sun giving them their Summer.

If you ignore all other effects, this tilt changes from 21.5 to 24.5 degrees over a 41000 years period. The more the angle, the more pronounced the seasons

Precession is a wobble the axis gets from the pull of the Moon and the Sun. If you ignore all other effects, this wobble makes the Earth's axis trace out a circle over a 23000 year period. On it's own it would also give us our seasons.

So yes, Obliquity and Precession are very similar but subtly different.


  1. This is a perfect explanation of the difference between the two. Thank you very much! I have been confused over and over again about this every time it comes along.

  2. But surely precession affects obliquity (the wobble must change the tilt relative to the orbit), so the trace of the axis of the wobble must not be circular and in fact goes in a 41,000 year cycle?