Wednesday 4 July 2012

The Earth reaches its aphelion.

On Thursday 5 July 2012, at 4.00 am GMT, the Earth will be at its furthest point from the Sun. 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.

Simple diagram of the Earth's orbit.

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.

Simple diagram showing how the Northern Hemisphere gets more Sunlight during the Northern Summer. US National Weather Service.

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.

The comparative sizes of Earth and Mars. The smaller size of the Red Planet has prevented it retaining a thick atmosphere. Federation of American Scientists.

In fact the atmosphere has been the most important factor in the Earth's climate since long before the origin of multi-cellular organisms. This has lead to well documented swings in the Earth's climate over recent geological time, with the climate swinging between hothouse phases such as those in the Carboniferous and Jurassic and icebox phases such as the Ordovician or Pleistocene. These climate changes have been recorded in the geological record by the rise and fall of the sea, the spread and retreat of glaciers, and changes in the Earth's flora and fauna (for example it is possible to tell a cold forest from a tropical one even in fossil form), and these changes matched to geochemical indicators which record the levels of Carbon Dioxide (CO₂) in the atmosphere. This has constantly shown a connection between the atmosphere and the climate; when CO₂ levels rise, so does the temperature.  There are of course other factors that effect the Earth's climate, most notably the position of the continents. When the poles are free of land the climate tends to be warmer, and when the oceans can circulate without major currents nearing the poles, this also raises the temperature. However the role of CO₂ is clearly very important, hence the concern of climatologists and environmentalists at rising CO₂ levels in today's atmosphere.

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