THE SUN AT PERIHELION

On January 4 at 16:17 UTC the Earth passed into perihelion. This beautiful and very detailed image of the solar disc was taken at that very moment from a Sydney observatory in Australia using a Halpha filter.

Perihelion is the point in the orbit of an object at which the distance from the Sun is minimal. The Earth at perihelion is about 147 million km from the Sun, just 2.5 million km less than the average distance. 

This is because the Earth moves on an orbit that is almost circular and therefore the distance from the Sun never changes much.

In the case of objects that move on very eccentric orbits, such as comets, the difference between perihelion and aphelion (the point of maximum distance from the Sun) is considerable. For instance, Halley's comet has a perihelion at only 0.58 AU of the Sun, while its aphelion is above 35 AU.

You might think that the seasons depend on the distance from the Earth from the Sun: when the Earth is near it is summer, when it is far it is winter. This is obviously wrong: the Earth goes to perihelion when we in the northern hemisphere are in winter. 

As we have also seen, the difference between perihelion and average distance is very small and is not able to influence temperatures on our planet. 

The seasons depend rather on the inclination of the Earth's axis (inclined relative to the orbital plane) and hence on the inclination of the sun's rays. When the sun’s rays are very tilted (in winter) the temperatures are lower, while in summer the temperatures are higher because the inclination of the sun’s rays is lower.

Credit: Peter Ward (Barden Ridge Observatory).

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