Happy solstice, everyone. The December solstice will come tomorrow, on December 21, at 23:03 Universal Time. At this instant, it’ll be at 5:03 p.m. for the Central U.S. At this celebrated moment, it’ll be sunset in North and South America, sunrise in far-eastern Asia, midnight in Africa and Europe and noontime over the Pacific Ocean. (See worldwide map below.) We in the Northern Hemisphere will have our shortest day and longest night of the year. And yet – if you 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 in December 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.
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 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 day.
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 days for the entire globe.
By the way, half a minute longer doesn’t sound like much, but the difference adds up.
You may know that – although the solstice brings the shortest (or longest) period of daylight – the earliest or latest sunsets and sunrises don’t come precisely on the solstice. Mid-northern latitudes had their earliest sunsets about two weeks ago, and, at that time, solar noon came seven minutes earlier by the clock that it will on the December solstice.
Two weeks from now, when mid-northern latitudes wake up to their latest sunrises, solar noon will come about seven minutes later by the clock than on the December solstice.
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.