Spinning Through Space

Earth is speeding around the Sun at over 107,000 kilometres an hour and swings closer to the Sun over Christmas, but you’d never know it! Its axial tilt causes the seasons to come and go and the long-term axial wobbles, and changes in its orbit, result in Earth’s 100,000 year Ice Age cycles! You can enjoy reading and learning all about it here!

Interesting Facts About The Earth’s Orbit, Seasons & Cycles!

For the majority of human history (until a few hundred years ago) Earth was believed to be the centre of the universe and everything orbited around it! In fact, Earth is really only the 3rd planet from the Sun which everything in the solar system ultimately orbits around.

  • Earth orbits at an average distance of 149.6 million kilometres from the Sun, in an area of space scientists often refer to as ‘the habitable zone’. This means that Earth orbits near the middle of a zone which receives just enough energy from the Sun to keep surface water in a liquid state. Like the story of Goldilocks and the Three Bears, the Planet Venus is too close to the Sun (so is too hot) and Jupiter is too far (so is too cold), but Earth is just right!
  • No orbit is a perfect circle and Earth’s orbit around the Sun is no different. On January 4th each year, the Earth is slightly closer to the Sun (known as perihelion) with a distance of 147.1 million km (91.4 million miles). Six months later during the first week of July, Earth is further away from the Sun at a distance of 152.1 million km (94.5 million miles).
  • The slight difference in distance from the Sun means the southern hemisphere receives 6.9% more sunlight energy during its summer than summers in the northern hemisphere when the Earth is further from the Sun!
  • Despite what you may have heard, Earth’s seasons aren’t caused by Earth’s distance from the Sun! The seasons are caused by the tilt of Earths rotational axis - that imaginary line through each of the poles which Earth rotates around!
  • Currently the Earth is rotating at an angle of 23.4° relative to its orbit of the Sun.
  • This ‘axial tilt’ causes one hemisphere to be pointed more towards the Sun at certain times of the year. This means that for half a year one hemisphere experiences longer days, than the other, so is warmer because of the extra sunlight! This results in the seasons alternating between summer and winter every year as the Earth orbits the Sun.
  • If the Earth rotated straight up and down, as it orbited the Sun, we’d have no seasons!

Long Term Earth Cycles

Earth’s orbital characteristics aren’t fixed over long periods of time (Learn about orbits). Over thousands of years Earth’s orbital shape, axial tilt and the direction the axis is pointing changes. This leads to slight variations in the amount of energy parts of the Earth’s surface receives.

A very clever scientist named Milutin Milankovitch calculated the way these cycles change over time and how they are related to variations in Earth’s climate. These three changes in Earth’s orbital characteristics (known as the Milankovitch Cycles) cause the formation of the long-term 100,000 year Ice Ages cycles!

  1. The shape of Earth’s orbit changes from almost circular to mildly elliptical (oval shaped) over the course of ~100,000 years.
  2. The angle of Earth’s axial tilt (known as obliquity) varies between 22.1°- 24.5° over 41,000 years.
  3. The direction that the Earth’s rotational axis points in space (known as precession) wanders and completes a full circle every 25,700 years.

These changes to Earth’s orbit and rotational axis are caused by the gravitational influences of the Sun, Moon, Jupiter, Saturn and their tidal forces. But don’t worry we’re currently in a ‘warm’ period of the climate cycle so another ice age isn’t expected for over 50,000 years!

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Earth Orbit - The Earth at the start of the 4 (astronomical) seasons as seen from the north.