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December solstice brings year’s longest days

This curved line is called an analemma.  It shows sun's declination - its annular distance from the celestial equator - and difference (in minutes) between time as measured by the clock and time as measured by the sun.  Click here to read more about this image.

Tonight for December 18, 2015

The December solstice will come on December 22, at 4:48 Universal Time. At this instant in the central US, it’ll be December 21, at 10:48 p.m. CST. For us in the Northern Hemisphere, the December solstice ushers in our shortest period of daylight and longest period of darkness for the year. And Yet – if we consider the length of the day in another light – the longest days of the year come each year in December for the entire globe.

When we say the longest days of the year come each year around the December solstice for the entire globe, we’re talking about day not as a period of daylight – but as the interval from one solar noon – or midday – to the next.

In December, a day – one rotation of Earth relative to the noonday sun – is about one-half minute longer than the average 24 hours.

Keep in mind that the clocks on our walls don’t measure the true length of a solar day. To measure the time from one solar noon to the next, you need a sundial. A sundial will tell you the precise moment of local solar noon – when the sun reaches its highest point for the day.

The featured photo at the top of this post shows an analemma. It shows sun’s declination – its angular distance from the celestial equator – and difference (in minutes) between time as measured by the clock and time as measured by the sun. Click here to read more about this image.

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So it’s December now, and that means one rotation of the Earth relative to the sun – what we call a solar day – is about one-half minute longer than the average 24 hours, for the entire globe.

Days are always longer – as measured from one solar noon to the next – than 24 hours around the solstices (and less than 24 hours around the the equinoxes).

The days are at their longest now – for the whole globe – because we’re closer to the sun on the December solstice than we are at the June solstice. Earth’s perihelion – closest point to the sun – always comes in early January. When we’re closest to the sun, our planet is moving a little faster than average in its orbit. That means our planet is traveling through space a little farther than average each day. The result is that Earth has to rotate a little farther on its axis for the sun to return to its noontime position. Hence the longer solar day.

Half a minute longer doesn’t sound like much, but the difference adds up. The longer days cause noontime to come earlier by the clock before the December solstice than at the December solstice. Moreover, the longer days cause noontime to come later by the clock after the December solstice than at the December solstice. Because the clock and sun are most out of sync right now, some befuddling phenomena cause people to scratch their heads.

In the Northern Hemisphere, the year’s earliest sunset precedes the December winter solstice, and the year’s latest sunrise comes after the December winter solstice.

In the Southern Hemisphere, the year’s earliest sunrise precedes the December summer solstice, and the year’s latest sunset comes after the December summer solstice.

Although the solstice brings the shortest/longest period of daylight, the earliest sunset/sunrise always comes before the solstice, and latest sunrise/sunset always comes afterwards.

The fact that we’re closest to the sun in early January also means that Northern Hemisphere winter (Southern Hemisphere summer) is the shortest of the four seasons. At the same time … ‘tis the season of bountifully long solar days.

Sundial outside of Alder Planetarium in Chicago Illinois. A sundial lets you measure the time from one solar noon – – when the sun reaches its highest point for the day – to the next. Image via Ben+Sam

Bottom line: As measured from one solar noon to the next, December has the longest days – the longest day/night cycle – for the whole Earth.

Read more: Earliest sunsets (or sunrises) always happen before the winter (or summer) solstice

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