solstice explained

Or, newscasters don’t want you to know this one weird trick about the seasons…

four side by side pictures of the same tree seen in spring, summer, fall, and winter — image by Kathy Collins, science.howstuffworks.com ()

Link opens in new window — image by Kathy Collins, science.howstuffworks.com ()

December 21, 2021, 10:59am ET

Hey Astro fans,

Ever heard the term “land of the midnight sun”? Or its sibling term, “land of the noontime dark”? OK that second one is not actually a term; I confess I just made it up. But it should be, because today we are smack in the middle of 6 months of the Sun never rising at the North Pole. Hopefully Santa doesn’t suffer from seasonal affective disorder. Or if he does, hopefully he will spend equal time in the Southern Hemisphere to balance it out, because conversely we are exactly in the middle of 6 months of the Sun never setting at the South Pole. It’s the South’s turn to be “land of the midnight sun”. And that really is a term. For any animal lovers, take heart that the majestic Emperor penguins — endemic to Antarctica — make good use of the light; it’s their chick-rearing and voracious feeding season.

Sol + stice ( for “Sun” + “standing”, as in “still”, or “stationary”) means that the Sun’s gradually changing path in the sky reaches its lowest* path of the year for Northern hemisphere residents.* It also rises and sets farthest south of east and west respectively. For the past six months the Sun’s path has been getting slightly lower* and rising/setting farther south each day. Now that downward* progression stops and reverses. Hence the word for “stationary” on this day.

3-D view of the Sun's path through the sky during different seasons, showing the Sun rising in the East and setting in the West, but being generally lower in the sky in winter — The sun rises in the east and sets in the west. This is true in both hemispheres. In the Northern hemisphere, the sun’s path is always angled toward the southern sky, year round. Image by Pearson Education, www.ifa.hawaii.edu ()

Note that stationary doesn’t mean the Sun doesn’t move at all today; of course the Sun does rise and set today as always because the Earth is rotating. If you feel like time has stopped you are probably just reading the news too much. Today merely begins 6 months of a gradual upward* progression of the Sun’s path and rising/setting slightly to the north of east and west each day.

*For the Southern hemisphere, swap highest for lowest, higher for lower, and upward for downward in the paragraph above.

Because of this changing path of the Sun, on this day we have the fewest consecutive hours of daylight in the Northern hemisphere. And the farther north you are, the fewer you have. If you’re anywhere in the Arctic circle, you have zero hours of sun. In the Southern hemisphere, it’s the reverse: the December solstice brings the most consecutive hours of daylight, and the farther south, the more you have.

an animated diagram showing how the sun's varying path in the sky is similar reversed in the southern hemisphere — Southern hemisphere view. The sun’s path is always angled toward the northern sky, year round. Dec. 21 is their summer solstice. Notice that the cardinal directions are shown from opposite perspectives in the two diagrams: North is on the left in the first, and on the right in the second. GIF by Nick Lomb, maas.museum ()

But we all agree that a full “solar” day is 24 hours, so hours of daylight + hours of dark must add up to 24 hours everywhere on Earth any day of the year. So it’s a little misleading to say that today is the “shortest day of the year”, because a full day is always 24 hours. But “shortest day” is how it is usually phrased in the news for efficiency. It’s more precise to say that it’s the “shortest length of daylight” of the year. And weakest strength of sunlight, for that matter. Just sayin’. If you’re going to skip wearing sunscreen any day of the year, today is your best choice. Anyway if you hear someone say the “shortest day” thing, you needn’t correct them. If you like, you can just smile smugly and feel smart that you appreciate the subtle distinction between shortest “day” and “length of daylight”.

Speaking of smug, here’s a Fun Fact: technically it’s 23 hours and 56 minutes for a “sidereal” day, if you’re referencing the stars returning to the same point tomorrow instead of the Sun. This takes into account the Earth moving 1/365th of the way around in its orbit each day. It doesn’t change the length of a solar day being 24 hours, but it is responsible for any given constellation or star rising 4 minutes earlier each day throughout the year. This phenomenon gives us “summer” and “winter” constellations — which constellations are up at night in various seasons. In the other seasons, they’re up during the day. For example it’s why we Northern hemisphere residents can see Orion up at night in the winter but not in the summer, and we can see Cygnus up at night in the summer but not winter.

For more solstice details, see https://www.timeanddate.com/calendar/december-solstice.html ( Link opens in new window )

For more about summer vs. winter constellations, see https://www.space.com/10821-night-sky-changing-seasons.html ( Link opens in new window )

p.p.s. For More Fun. Why would anyone call the day of least daylight of the year the first day of winter? Shouldn’t it be the middle day of winter? Judging by the amount of sunlight, yes. However traditionally the seasons are marked by the extremes of temperature. And the coldest part of winter (and also hottest part of summer) typically comes a month or so after the solstice. This lag is because it takes a long time for the land and water to change temperature due to increasing and decreasing length/strength of sunlight. This is especially true of water, because water has a high “specific heat” and thus heat capacity (i.e. thermal inertia). Ever notice how long it takes an unheated swimming pool to warm up in summer? That’s why. This thermal inertia is the same reason the temperature doesn’t vary as quickly in coastal regions, but can vary wildly hundreds of miles inland. So the implication for seasons, anywhere on Earth, is that the change in temperature has a few weeks or even months of lag following the change in sunlight. If you want to be pedantic (who doesn’t) you can call today “midwinter’s day”. Shakespeare would have done, amirite?