Winter solstice 2015: What is the astronomy behind the shortest day of the year?

The winter solstice will take place on 22 December in the northern hemisphere, marking the longest night and shortest day of the year in terms of daylight hours. From this date until the summer solstice on 20 June 2016, the days will get progressively longer.

What is a solstice?

The winter solstice is an astronomical event caused by Earth's tilt on its axis and orbit around the sun. Earth doesn't orbit upright but is instead tilted on its axis, meaning Earth's Northern and Southern Hemispheres trade places in receiving the sun's light and warmth most directly. The tilt of the Earth is what causes winter and summer. The winter solstice occurs when Earth is leaning furthest from the sun – it is positioned so the sun stays below the North Pole horizon.

In the southern hemisphere, the opposite is true and people experience the longest day of the year during the December solstice. The summer solstice usually marks the midpoint of summer and is the longest day of the year.

When is sunrise and sunset?

Sunrise is at 08.03 and sunset will be at 15.53 on 22 December in London, giving Brits 7.49 hours of daylight. The day is 8 hours and 49 minutes shorter than the June solstice – or the summer solstice – which marks the midpoint of summer and is the longest period of sunlight in the year. In most locations north of the equator, the shortest day of the year is around this date.

Why isn't the earliest sunset of 2015 the winter solstice?

Although the 22 December is the shortest day of the year in terms of daylight, it does not have the latest sunrise and earliest sunset of the year, because of a discrepancy between modern-day timekeeping methods and how we measure time using the Sun.

Astronomers call the difference the equation of time, which put simply, is the difference between time that is measured using a sundial – the true or apparent solar time – and time that is measured using a watch or a clock – the mean solar time.