Wednesday, 1 January 2014

The Earth's Perihelion.

On 4 January 2014 at 12.00 noon, GMT, the Earth will reach its perihelion; the closest point on its orbit to the Sun. The Earth orbits the Sun at an average distance of 149 598 261 km, but its orbit is not completely circular, it has a slight eccentricity which takes it from a perihelion of about 147 098 290 km in January to an aphelion of 152 098 232 km in July. 

Simplified diagram of the Earth's orbit. NASA.

This means that the Earth is at its closest to the Sun in the middle of the Northern Hemisphere's winter, counterintuitive to most of the planet's population. This is, however, purely coincidental; the Earth's season's are not caused by its distance from the Sun, which only varies by 3.3%, but rather by the tilt of the planet. The Earth is currently tilted at an angle of 25.5° to its plane of orbit (this varies on a timescale of tens of thousands of years, but remains fixed from the point of view of any human observer), causing the Sun to appear to rise higher and lower in the skies of each hemisphere as the year goes by. 

In the northern winter the Southern Hemisphere is tilted towards the Sun, so that the days are longer there (and in around the Southern Solstice in December, permanently above the horizon at the South Pole). In addition the Sun being directly overhead means that the energy from the Sun has to pass through less of the atmosphere before it reaches the surface of the Earth, so that less energy is lost to the atmosphere, causing the surface to warm.

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