Thursday 4 July 2013

The Earth's aphelion.

On Friday 5 July 2013, at 2.44 pm GMT, the Earth will reach its aphelion; it will be at the furthest point from the Sun in its orbit. The Earth circles the Sun every 365.26 days at an average distance of 149 598 261 km (1 AU), but its orbit is not completely circular, at its closest it is only 147 098 290 km, or 0.983 AU, from the Sun (this is called the perihelion and occurs on or near 3 January) and at its furthest it is 152 098 232 km, or 1.17 AU away.

Very simple diagram of the Earth at its aphelion (not to scale). Swinburne University of Technology.

This is counter intuitive to inhabitants of the Earth's Northern Hemisphere, who often assume that the Earth is closest to the Sun in midsummer, when in fact it is at its furthest away. This is because the tilt of the Earth plays a far greater role in our seasons than the distance from the Sun, and the Northern Hemisphere has just passed its Summer Solstice, i.e. the point at which the North Pole was pointing as close to the Sun as it ever gets, so that the Northern Hemisphere is currently getting much more sunlight than the Southern.

In fact the Earth could potentially move quite a bit in its orbit and still maintain an equitable climate, possibly even if it was as far out as Mars (1.5 AU), though presumably this would be somewhat cooler. Mars is a frozen wasteland largely because it is small and airless. The Earth, being larger, is able to sustain a thicker gaseous atmosphere, leading to a greenhouse effect that keeps the planet warm. Probes on the Red Planet have found abundant geological indicators of running water on the surface, suggesting that ancient Mars had a thicker atmosphere which could support liquid water, but this has now gone, the low gravity of the planet having let it escape molecule by molecule.

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