Dec 212014
 

At 22:03 tonight (December 21st 2014), the Sun will shine directly over the Tropic of Capricorn in the Southern Hemisphere, which is the southern-most position the Sun reaches before reversing its direction and heading back towards the Northern Hemisphere. This is called the Winter Solstice, or Summer Solstice if you’re reading this in the Southern Hemisphere. It is also the day with the shortest sunlight hours, or longest if you’re in the south.

Solstice comes from the Latin solstitium, meaning the Sun stands still. 

In the Northern Hemisphere, astronomers and scientists use the December Solstice as the start of the winter season, which ends on the March Equinox. For meteorologists, however, winter began three weeks ago on December 1st.

English: Illumination of Earth by Sun on the d...

Illumination of Earth by Sun on the day of winter solstice on northern hemisphere. A view to eastern-hemisphere showing noon in Central European time zone (ignoring DST) on the day of winter solstice (on northern hemisphere – this is summer solstice on southern hemisphere). (Photo credit: Wikipedia)

It would seem logical that the solstice would mean earlier sunrises and later sunsets, but this is not the case. The Sun’s latest rising time is 08:17 on January 2nd 2015 at which point mornings will start to get lighter. The earliest sunset was actually on December 18th 2014 when the Sun set at 15:48. Since this date, the nights have started to get lighter, albeit at a very slow rate initially. It won’t be until January 2nd 2015 that the Sun will set after 16:00.

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).

Revolving earth at winter solstice on the nort...

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

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).

During the Northern Hemisphere winter the Earth actually makes its closest approach to the Sun. 

According to timeanddate.com,

Seasons have little to do with the Earth’s distance to the Sun, but with how it spins around its own axis. As the earth revolves around the sun, it also rotates around its axis, which is tilted at an angle of 23.5 degrees.

The direction of the tilt of the earth does not change as the Earth moves around the Sun – the two hemispheres point towards the same position in space at all times. What changes as the Earth orbits around the Sun is the position of the hemispheres in relation to the Sun – the Northern Hemisphere faces away from the Sun during the December Solstice, while the Southern Hemisphere tilts towards the Sun. The opposite happens around the June Solstice, when the Southern Hemisphere faces away from the Sun during the December Solstice, while the Northern Hemisphere tilts towards the Sun. This is why people in the Northern Hemisphere experience winter around December Solstice and summer during the June Solstice.

In fact, the Earth is on its Perihelion – the point on the Earth’s orbit closest to the Sun – a few weeks after the December Solstice.

English: Tropic of Capricorn sign close to the...

Tropic of Capricorn sign close to the … well, Tropic of Capricorn in Namibia. (Photo credit: Wikipedia)

It all sounds very complicated, but put simply, what this means is that the darkest nights will soon be behind us and we can look forward to the Sun’s rays warming the earth again – that is if you live in the Northern Hemisphere.

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