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The short and long of it - December Solstice

Mark Thompson Astronomy Mark Thompson Astronomy | 04:00 UK time, Thursday, 22 December 2011

sunset, UK

Image credit Stu.bloggs/Flickr

Distance travelled ~ 913'772'800 km

Today sees the shortest day in the northern hemisphere and for southern hemisphere dwellers, the longest day! A peculiar idea to get your head round perhaps but it's all actually pretty straightforward. The cause of the changing length of the 'day' (and by this I mean the hours of sunlight, not a 24 hour period) is the same thing that causes the changing of the seasons and we can look to the orbit of the Earth around the Sun for the answer.

Surprisingly perhaps, its not the Earth's distance from the Sun which brings the colder months to the northern hemisphere in fact, the Earth is closer to the Sun during December than it is in July. We need to look at the axis of the Earth's rotation to understand why we experience seasons and changing quantities of sunlight.

The Earth spins once on its axis every day, to be precise once in 23hours, 56minutes and 4seconds which is why stars seem to rise about 4 minutes earlier every day. The axis that the Earth spins around is tilted with respect to its orbital plane around the Sun by just over 23 degrees. During a northern winter, the northern hemisphere is pointing away from the Sun and during the northern summer it's pointing toward it. The opposite is true for the southern hemisphere. This all means that during the northern hemisphere winter, there are less hours when the Sun is above the horizon, indeed around the north pole, there are 6 months of it while the southern pole is basking in 6 months of sunlight.

The exact amount of sunlight we receive at any place on Earth is determined by its latitude. The further away from the equator, the less light is experienced during winter. Even small differences in latitude make a big difference to number of daylight hours for example, London will today experience just 7 hours 49 minutes of sunlight whereas the north of Scotland will experience 6 hours 35 minutes, over an hour of difference. What this means astronomically is that the Sun reaches its southernmost point in the sky today which we call the Winter Solstice, more precisely at 05:30 GMT (or UT - Universal Time). This corresponds to the Sun being overhead at local noon at the Tropic of Capricorn in the Southern Hemisphere.

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