The Earth’s orbit around the Sun isn’t circular but rather it’s elliptic. Just to make things interesting, the Sun doesn’t sit at the centre of that ellipse, it sits slightly off centre, therefore during its orbit the Earth gets further away from and then closer to the Sun through the course of a year. Today, Thursday 4th January, the Earth is at perihelion, that is to say the Earth is at its closest approach to the Sun in its yearly orbit.
Does that sound a bit unintuitive? We’re closer to the sun than at any other time of the year and it’s winter. Well, yes, winter in the Northern Hemisphere but in the Southern Hemisphere it’s summer.
This is all due to the fact that as the Earth spins around the sun it is tilted at a jaunty twenty-three and a half degrees (e&oe) from a line perpendicular to the plane of its orbit and it just so happens that when the Earth is closest to the Sun, the Northern Hemisphere is tilted away from the Sun and therefore receives less incident solar radiation, that’s sunlight to you and me, than it does in the summer when although the Earth if further away from the Sun, the Northern Hemisphere is tilted towards the Sun and therefore receives more incide…, you know, sunlight.
It is for this reason that summer in the Southern Hemisphere is by and large warmer than summer in the Northern Hemisphere because when it’s summer in the Southern Hemisphere, that is to say, the Southern Hemisphere is tilted towards the sun, the Earth is also closer to the sun.
So here we are, closer to our star that we are going to be for another year. At 16:17 UTC on 4 January 2023 the earth will be (or was, if you are reading this later) 147,098,925 km from the Sun. Halfway through our annual circumnavigation of the Sun, when the Earth is at aphelion on 6th July 2023, we will be 152,093,251 km from the sun, the difference being a little over 4,900,000 km, I’ll not bother with the trifling 90 thousand odd kilometres, after the first four million or so it doesn’t really make a lot of difference.
I wrote in my previous post about the seemingly haphazard way that we reckon the new year, 10 or so days after the winter solstice. The winter solstice being of course the day with the longest night and least amount of daylight. Why not have the year start on that day? Why not have the year start about 14 days later on the day of perihelion?
Because, everything is constantly changing, in about 10,000 years’ time perihelion will occur at around the time of the June solstice when the Northern Hemisphere will be tilted towards the Sun. The Earth’s rotational speed, the angle of Earth’s axial tilt, the time it takes the Earth to go around the Sun, all these things are changing. At present a year is approximately 365.2425 days, that’s 365 days, 6 hours, 9 minutes and a handful of seconds, which is why we have a leap year with an extra leap day every four years, to get us back on track.
We have leap seconds far more frequently, but they don’t get the plaudits that their longer cousins the leap days do, they just happen in the hearts of the atomic clocks that regulate all the other clocks. And no, atomic clocks don’t run off of mini nuclear reactors, they measure the resonant frequency of atoms to provide a baseline for reckoning time.
In the distant past the Earth’s period of rotation on its axis was shorter than it is today and in the far-flung future it will be longer, the Earth is gradually slowing down, I say gradually, it’s estimated to be about 1.8 microseconds per century, meaning that on average over a period of 100 years, a day on Earth is 0.00018 seconds longer. Yes, that sounds gradual to me.
Anyway, as we start another trip around the old home star, Sol, Il Sole, Sól, Со́нце, Le soleil, 太陽, Sólin, השמש, La suno, Słońce, An ghrian, Helios, the Sun, however you care to name it, may it treat you, as I said in the previous post, with grace and favour. Hold on to your hats, here we go again!