Why aren't the UK sunrise and sunset times in my diary not symmetrical?

Question

I would expect the times given to be based on the Greenwich Mean, and to mirror each side of midday, but they don’t.

For example, Sat 6th Jan 2007: sunrise 8:05, sunset 16:08. This means sunrise is 3 hours 55 mins before midday, but sunset is 4 hours and 8 mins after midday.

They don’t peak at the same time either. The latest sunrise is around Sat 29th this year, but the earliest sunset is around 11th December. Why is this?

Answer 1

Good question. Yes, if you were on the Greenwich line, then it seems reasonable that sunset and sunrise GMT should be symmetric.

Except the earth's orbit is not a nice constant-speed circle around the sun, but rather an ellipse that speeds up a little when we are nearer the sun, and slows down when we are farther. But the earth's 24-hour rotation rate is constant, so the sun spends part of the year getting ahead of midday, and part of the year falling behind midday.

A further complication is earth's tilt, causing the sun to get ahead a few seconds per day during solstices when its latitude is turning around, and fall behind a few seconds per day during equinoxes when a larger component of its motion is southward or northward. Those seconds add up to plus and minus several minutes over the year.

You've discovered the "equation of time" and an associated figure called the analemma. References below have more details.

Answer 2

They could not conceivably be a mirror. They could only be symmetric about midday at one point on Earth and on one specific day.

The reason is that the Earth moves around the sun. This leads to a difference between the sidereal day (the day between times the Earth is in the same absolute orientation) and the solar day (the day between times that the sun is directyl overhead). The latter is about 4 minutes longer than the former.

Answer 3

For your first question, the times do mirror a lot better, if you do not use midday, but the actual time of transit of the sun, when it is highest in the sky. A transit of the sun only occurs at midday at 4 times of the year. All other times can be obtained from 'The Equation of Time', which you should Google to find out what it all means. I could go into it a bit more, but right now, I need sleep.

For your second question, try these :

http://aa.usno.navy.mil/faq/docs/equinoxes.html

http://www.nmm.ac.uk/server/show/conWebDoc.348

Answer 4

The Earth's axis – the imaginary line that goes through the Earth and around which the Earth spins — is tilted. It's tilted about 23.5 degrees relative to our plane of orbit (the ecliptic) around the Sun. As we orbit our Sun, our axis always points to the same fixed location in space. Our northern axis points almost directly toward Polaris, the North Star.The “fixed” tilt means that, during our orbit around our Sun each year, different parts of Earth receive sunlight for different lengths of time. It also means that the angle at which sunlight strikes different parts of Earth's surface changes through the year. Sunlight striking the surface at an angle is “spread” across a wider area compared to sunlight striking perpendicular to Earth's surface. Areas that receive more scattered sunlight receive less energy from our Sun. All of these factors combine to give Earth its annual cycle of seasons.