It’s only a matter of time

Today, December 18th 2012, is the day when the nights start to get lighter. Yesterday, the Sun set at 15:48, but today and tomorrow, the Sun will set at 15:49. Only a minute, but to some people it marks a turning point, showing that Spring is just around the corner.

Revolving earth at winter solstice on the northern hemisphere. (Photo credit: Wikipe

There is always some confusion surrounding the shortest day and the winter solstice. The winter solstice is the time of the year when the Sun at noon is at its lowest altitude above the horizon. In other words, the noon day Sun is at its lowest point in the sky. This usually occurs on December 21st or 22nd, depending on when the Sun rises. This year, the solstice is on Friday December 21st at 11:12.

The significance of the winter solstice is that this is the point at which the daylight hours start to increase, but this doesn’t mean that the Sun will rise earlier and set later. The Sun continues to rise later until January 3rd 2013, when the turning point is reach. At this point, the Sun both rises earlier and sets later.

Rising and setting times of the Sun in Nottingham

So, why does the Sun continue to rise later? The answer lies in ‘the equation of time‘.

The equation of time is defined as the difference between apparent solar time and mean solar time. Apparent solar time is measured by the current position of the sun, whereas the mean solar time is measured by a clock set so that over the year its differences from apparent solar time average to zero. Apparent time can be ahead (fast) by as much as 16 minutes 33 seconds (around November 3rd), or behind (slow) by as much as 14 minutes 6 seconds (around February 12th).

Put simply, a day is not exactly 24 hours long, 23 hours, 56 minutes and 4 seconds to be precise, so Earth time gets out of step with the orbiting of the Sun. As a result of Earth’s elliptical orbit and the Earth’s axial tilt, apparent solar days are shorter in March (26–27) and September (12–13) than they are in June (18–19) or December (20–21).

So, when people refer to the ‘shortest day’, they actually mean the day with the shortest daylight hours. If you really wanted to complicate things, you could say that the Sunday after the last Saturday in March is the shortest day, as this is when the clocks go back, meaning that the day has only 23 hours in it. But let’s leave that discussion for another time.

Related articles

• Equation of time (en.wikipedia.org)
• Everything you need to know: Winter solstice 2012 (earthsky.org)
• NASA Captures Solstice Solar Portrait (news.discovery.com)