Science Questions

Why do mornings get shorter and afternoons longer during the Winter solstice?

Tue, 4th Oct 2011

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Question

Stephen Fleming, via Facebook asked:

At the winter solstice, why does the sunrise get later for several days – the mornings get darker whilst in the evenings, sunset gets later? Why aren’t they in sync?

Answer

Dominic -  That's a really tricky question.  What it comes down to is how we define the length of a 24-hour day.  The traditional way that you would do that is you would look at the sky and you would look at when the sun is highest in the sky and you will call that noon – the middle of the day - and then the next day you would go out and you look up when the sun was highest and that would be the next noon.  The other way that you can define length of the day is to have an atomic clock which is measuring the progress of the laws of physics.  It’s giving you a very precise packet of time of a fixed length. 

Now it turns out, those two definitions of length of day are not exactly the same.  The length of time it takes for the sun to come around from one noon to the next varies over the course of the year, and the reason is because that period is not just the time it takes for the Earth to rotate on its axis, but also the time it takes for the Earth to catch up with the fact the sun has moved a little way across the sky.  The way in which the sun moves across the sky changes at different times of year because, for example, the Earth’s orbit is elliptical and so the Earth is moving at different speeds at different times of year.

Chris -  I did hear one person put this as a bit like slipping a gear because you've got an ellipse, not a circle.  As the sun goes along the flat side of each ellipse, it’s got to sort of slip a gear a bit.  The Earth has got to slip some gears as it turns in order to make it work, if you see what I mean.  It’s not just a straightforward circle.

Dominic -  That's not a bad way of thinking about it.  Now what that means is that noon doesn’t always happen at exactly 12 o’clock on every day of the year.  For example now, at the beginning of October, noon is happening about 10 minutes before 12 o’clock via a watch and it’s getting earlier.  In a couple of week’s time, it will be at quarter to noon, and then it will turn around and it will start getting later as the durations between noons get longer in December towards the winter solstice.  So what's happening at the winter solstice is that sunset turns around in the middle of December, but sunrise turns around about a week later because the solar day – the time between noons – is drifting through the mean day that we call civil time.

Dave -  So the length of day is changing and the time of noon is changing, so the two add up to this effect?

Dominic -  Yes,  that's exactly right.

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