What Does Obliquity Mean?

Obliquity is an astronomical term describing the angle of tilt of the Earth's axis of rotation. In technical jargon, it is the angle between the plane of the Earth's equator and the plane of the Earth's orbit around the Sun.

The obliquity is approximately 23°27' but it is not fixed. Instead, it varies slowly because both the Earth's axis of rotation and its orbital motion are affected by the gravitational attractions of the Moon and planets. Our logo below shows what the obliquity looks like. The horizontal is the Earth's orbit, whilst the "pie wedge" shows the tilt of the equator. The line running through the globe is the axis of rotation, through the North and South Poles.

Obliquity diagram

Over very long timescales (tens of thousands of years), the obliquity can be as small as 22° and as large as 24.5°.

No Obliquity, No Seasons

If the obliquity was equal to zero, the Sun would rise at 6 a.m. and set at 6 p.m. every day of the year, everywhere in the world. There would be no long summer days and long winter nights. The Sun wouldn't be high in the sky in summer and low in winter. It would take the same path across the sky every day of the year.

Every day would be the same as every other. There would be no seasons. The whole world's weather would be completely different. Life would probably have evolved in a totally different way.

The Obliquity and the Seasons

In reality, the obliquity causes the Sun to move north and south of the equator during the course of a year. Between late March and late September, it's north of the equator, giving summer in the northern hemisphere and winter in the south. Between late September and late March, the Sun is south of the equator, so that the southern hemisphere has summer whilst the northern hemisphere has winter.

This is how Milton expressed it in Paradise Lost:

Some say, he bid the angels turn askance
The poles of earth twice ten degrees or more
From the sun's axle; they with labour push'd
Oblique the centric globe: some say, the sun
Was bid turn reins from th'equinoctial road
Like distant breadth to Taurus with the seven
Atlantic Sisters, and the Spartan Twins,
Up to the Tropic Crab; thence down amain
By Leo, and the Virgin, and the Scales
As deep as Capricorn, to bring in change
Of seasons to each clime.

[About Obliquity]