STEVE OWENS from Glasgow Science Centre, explains Perihelion, and why we can look forward to a long summer in 2021.

2020 was a long year. Or at least it certainly felt that way. Looked at from a cosmic perspective though, last year was the same as any other. 

The Earth spun and wheeled around the sun at a leisurely 19 miles every second. This year will be the same as last year, astronomically speaking: our planet’s orbit undisturbed by the goings-on on its surface.

While January 1 marked the new year in our calendar, January 2 is a more astronomically significant date. On January 2, 2021 Earth reached perihelion, its closest approach to the sun.

At first glance this might seem confusing – we’re in the depths of winter after all. But remember that our southern hemisphere cousins are in the middle of their summer. In fact, the Earth’s distance from the sun isn’t related to how hot or cold we are, or to our seasons.

The seasons occur because our planet’s axis of rotation is tilted slightly. We are listing at an angle of 23.5 degrees from the upright. This tilt means that sometimes our part of the planet is tilted towards the sun, giving us summer, and sometimes away from the sun, in winter. You’ll be glad to know that we’re heading out of winter now, to the sunlit uplands of a new spring.

On top of our axial tilt, our orbit around the sun is a bit wonky too. It’s not a perfect circle; it’s an ellipse, which is the name we give a slightly squashed circle. This means that sometimes we’re a bit closer to the sun in our orbit, and sometimes a little further away. 

The fact that planetary orbits are not circular was discovered by the German astronomer Johannes Kepler, a contemporary of Galileo. The fact that the planets orbited the sun at all was a relatively new concept in the early 17th century. The Polish mathematician

Nicolaus Copernicus had shown a few decades earlier that, yes, in fact, the Earth did orbit the sun, but most astronomers following that discovery had assumed that planetary orbits were circular.

“Not so!”, said Kepler. After careful study of the path of Mars in our night sky, Kepler realised that planetary orbits were elliptical. This fact is now known as Kepler’s First Law of Planetary Motion.

The degree to which our orbit isn’t circular – called our orbital eccentricity – is rather small, only a few percent. If you drew it out, you’d be hard-pushed to say it wasn’t a circle. But over the scale of the solar system, that few percent difference amounts to millions of miles. 

When the Earth is closest to the sun – which we call perihelion, and which happened at 1350 on January 2, 2021 – it is three million miles closer to the sun than when it’s at its furthest approach, known as aphelion.

Even still, this extra three million miles doesn’t amount to much in the grand scheme of things; it probably passed you by unnoticed.  

Is there any ray of hope we can take in these astronomically dark times, in the depths of winter? Of course there is, and it comes from Kepler’s Second Law.

Kepler realised that, as the planets swung round the sun, their speeds varied at different parts of their orbits. Or as he put it, a line joining a planet and the sun sweeps out equal areas in equal times. This means that, when the Earth is closest to the sun, it moves faster and further in its orbit each day. When it is farthest from the sun, it moves slower.

This means that for us in the northern hemisphere our winter is a few days shorter than our summer.

The winter we’re currently in will last 89 days in total, from December 21, last year until March 20, 2021. Summer will last a glorious 93 days, from June 21 to September 22, 2021: four whole days longer! It’s not much, I grant you, but it is something. Let’s look forward to those few extra days of summer, and thank perihelion for the fact that we’re currently racing through winter as fast as our little planet can carry us.