Why everyone’s talking about... The perihelion

IT MIGHT not seem it, given the freezing temperatures, but the Earth is currently at the nearest point it gets to the Sun all year. How come?

The Earth’s orbit is not a perfect circle, but slightly elliptical, and this weekend – at 1.51pm yesterday, to be precise – we were at the closest point to the Sun. It’s called the perihelion, from the Greek ‘ peri’, meaning ‘close’, and ‘Helios’, meaning ‘Sun’. ( Another t erm useful for word games, perijove, is used for anything orbiting Jupiter.) This weekend, we’re 91.4 million miles from the Sun, compared with 94.5 million at the furthest point, the aphelion, which will next occur at 11.27pm on July 5.

OK, but how come it’s so bleedin’ cold?

Only for us in the Northern hemisphere! We get seasons because the Earth tilts on its axis by 23.5 degrees, and at this time of year the North tips away from the Sun. That has a much bigger effect on how solar rays hit us, and for how long each day, than being three per cent closer to the source.

Who first noticed the perihelion?

The term was coined in the 17th Century by German astronomer Johannes Kepler, who devised laws to describe the motion of planets – from which we can also work out that Earth is now travelling at its fastest around the Sun: 18.8 miles per second compared with 18.2 at aphelion. A few decades later, Isaac Newton proved Kepler’s l aws could be explained by a gravitational force.

Clever stuff!

Except that the equations don’t quite work for every planet. The moment of perihelion shifts forward very slightly for each lap of the Sun. For the Earth, it will take about 23,000 years to get back to where it is now, a variation that helps account for ultra-long-term climate changes. But for Mercury, the closest planet to the Sun, this shift is happening faster than Newton’s theories predicted. Scientists initially blamed space ‘ dust’ near the planet. But when Einstein came up with his General Theory of Relativity, it explained the discrepancy, showing how the Sun’s huge mass warps space-time.

How about a more technical explanation, with algebraic equations and some mathematical terms?

I’d love to, but, as you can see, we’ve run out of space…