Are the December solstice and the January perihelion related?
Winter solstice sunrise in Shanghai, China. Photo: Laurence Shan
The December solstice occurs just a few days before Earth reaches its perihelion (the point in its orbit in which it’s closest to the sun), which happens in January. Is this a coincidence or are these two events related?
The answer is yes – it is a coincidence. Earth’s perihelion – our closest point in orbit to the sun – occurs in early January every year. Earth is about 147 million kilometers from the sun at this time of year, in contrast to about 152 million kilometers in July.
So in this century, perihelion always comes just after the December solstice – when Earth’s northern hemisphere is most tilted away from the sun – bringing the longest night of the year for the northern hemisphere. These two astronomical events are separated by about two weeks for us.
But they were closer a few centuries ago – and in fact must have happened on the same day for a few years around 1300 AD. What’s more, these events are now drifting apart – due mostly to the slow precession of Earth’s axis – the same reason the identity of the pole star changes throughout the millennia . Earth’s perihelion – or closest point to the sun – will happen at the same time as the March equinox in about 6000 AD.
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