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The Evenings are Drawing Out December 14, 2009

Posted by mwidlake in Perceptions.
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When do the evenings start drawing out? This year, 2015, it is December 13th. On that day sunset will be 15:51 and about 30 seconds in London. On the 14th it will be a few seconds later . By the 16th of December, the sun will resolutely stay in the sky until 15:52 {and a few seconds}. The days will be drawing out at last. {the exact times will be different for other cities and countries but the DATE will be the same – see the links at the bottom of this page to check for your town and country}

English Sunset by Angie Tianshi

But it is not the shortest day of the year {I should say daytime really, all days are the same length based on the time it takes for the earth to spin once in relation to the galactic centre.}

What is the shortest day, I hear you all cry?

This year, 2015,

The shortest day is December 22nd

The date with the shortest period of daylight {in the Northern hemisphere} is the the 21st/22nd December, depending on how long ago the last leap-year was. And everyone knows that the the shortest day will also be the day where the sun sets earliest, it makes sense.

Except it does not quite work like that.

We probably all remember from our school days that the earth goes around the sun at an angle from the “vertical”, if vertical is taken as at 90 degrees to the circle the planet takes as it spins around the sun. Think of it like an old man sitting in a rocking chair. He is rocked back in his chair, head pointing back away from the sun in the Northern Hemisphere’s winter and feet pointing slightly towards the sun. Come Midsummers day, around June 24th he has rocked forward, head pointing towards the sun for the Northern Hemisphere summer. One rocking motion takes a year. That rocking motion gives us all summer and winter.

The other complication is that the earth does not go around the sun in a perfect circle. You might remember from diagrams of comets at school that they circle the sun in a big “egg” shape, sweeping in towards the sun, swinging around close to it and then looping out in the solar system before coming back around and in. Well, Earth and all the planets do the same slightly. We are actually closest to the sun during the Northern Hemisphere winter – the day we are closest will be Jan 2nd 2016. Or maybe Jan 3rd… there is not a lot in it. It is interesting that most people who live in the Northern Hemisphere just assume we are furthest from the sun’s warmth in our winter. Well, we are closest. We are furthest around the good ‘ol 4th July.

A result of being closer to the sun is that we are move through our orbit around the sun slightly faster – when in orbit, the closer you are to what you are orbiting the quicker you move. Our day length is made up of the time it takes the earth to spin  and also about 1/365th of a day – as we in effect spin one extra time in relation to the sun by going around the sun each year. That is why I said the earth spins at the same rate every day when compared to a more “fixed” point of reference like the galactic centre. It is about 23 hours, 56 minutes and 4 seconds. The extra 3 minutes and 5-ish seconds is taken going around the sun a bit and that time varies depending how fast we are orbiting.

Thus at closest approach (our winter here in the UK) solar noon {sun at highest point} to solar noon is less than 24 hours and in our summer it is slightly more than 24 hours. But we do not change our clocks, that would be too complicated, we just let them stick to 24 hours. And, as a result, the clock day and the astronomical day are slightly different.

If you think of a day as a white bar on a black string of 24 hours, that white bar gets longer and shorter as we go through summer and winter. But the bar also moves slowly left and right along the string as the year progresses.

The two don’t quite match as, not only is the earth like an old man rocking in his chair, he is also slumped slightly to one side – he is not sitting up straight.

So there you go. Here in the UK this year (2015)

  • The nights start drawing out on the 13th December.
  • The shortest period of daylight is the 22nd December.
  • We are actually closest to the sun, despite it being our winter, on the 3rd January 2016.

And just some interesting things to remember:

  • A day as measured from noon to noon (sun at it’s highest point) varies by a few minutes over the year but on average is 24 hours.
  • Part of that day length is not the earth spinning but the earth going around the sun – about 4 minutes.
  • Summer and Winter is not due to closeness to the sun but the tilt of the Earth’s axis in relation to the orbit around the sun.
  • Earth is slumped to one side in it’s orbit
  • Even though this is all simple “clockwork” Newtonian mechanics and mathematicians can work it out very accurately, it is not as simple as it seems.

If you take into account the tiny changes made by the moon and other planets (Mars does have a slight influence on our orbit) it gets even more complex, but those differences are tiny tiny tiny in any given year. Over a million years they make a difference though.


The below tables will help you look up sunset, sunrise, day length and all those things.  I include one for the UK and one for Australia. The jolly nice site the links go to allows you to change the location to wherever you are in the world (well, the nearest Capital).

Table of sunrise/sunset times for London

Table of surise/sunset for Sydney, Australia

What has this to do with Oracle, Database Performance and my day job? Nothing much, except to highlight that the obvious is not always correct, just as it is with Databases, IT and in fact science in general.

I’ll finish with a sunset picture from Auz. Ahhhh.

Outback sunset from ospoz.wordpress.com


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