Greetings, Earthlings trapped in darkness. Summer Ash, astrophysicist to the (radio) stars, has generously lent us her smarts. Ash is hanging out at a little college in Cambridge, England, where they know from the winter solstice.
Ash shares this lesson from her blog, Newtonianism for the Ladies.
— Special to the BPP from Summer Ash:
The winter solstice comes tonight. The shortest day and the longest night of the year. I, for one, am ready for daylight to make a comeback.
Did you know that the word solstice come from the Latin for sun (sol) and standing still (sistere)? The solstice is when the sun stands still.
Well...that isn't exactly what happens, but I can see how the ancients might have seen it that way.
The sun actually moves a lot in our sky. In addition to tracing out a path across the sky from sunrise to sunset, the height of the sun at midday also changes throughout the year. The winter solstice, in the general sense, marks the day when the sun is the lowest in the sky during midday. However, in astronomical terms, the winter solstice is the exact moment when Earth's axis is tilting farthest away from the sun.
From Astronomy 101, you may remember that our axis is tilted 23.5 degrees from vertical with respect to the plane of the Solar System. You may also be familiar with the North Star, right?
Well the North Star got it's name for a reason — it's where our axis appears to point if you were to extend a line straight out into space from the north pole. And that line always points there, regardless of the season. This is because while the Earth rotates on its axis and orbits around the sun, the tilt of our axis doesn't change. So for the winter solstice, the Earth is at the exact spot in its orbit where the our axis tilts the farthest away from the sun and for the summer solstice, the axis is tilting towards the sun.
So from now until June 21, the sun will only rise to a higher and higher point during its daily travels. However, right now it's killing me by only getting up to a little over 16.5 degrees above the horizon. Here in Cambridge, I am at 52 degrees latitude. For comparison, back in the states, New York City is around 40 degrees latitude and will see the sun reach 25.75 degrees at its highest. Those 9-ish degrees in latitude may not seem like much, but they translate to 1.5 less hours of daylight for me versus you New Yorkers out there. Given England's penchant for grey skies, I want all the sun I can get. I may have to overcompensate and move the equator after my tenure here so I can benefit from the sun wandering back and forth across the zenith. So while the sun never exactly stands still in our sky, on the solstice it appears to stop at the high or low extreme of it's midday elevation. I can see where the ancients were going with that.
Actually, the illustration above helps to correct a common misconception. The sun does not shine directly overhead every day at noon on the equator — it only does so on the vernal and autumnal equinoxes. The equinoxes are the halfway points between the solstices in the spring and fall where the hours of day and night are equal. Don't forget that the equinoxes and solstices are switched below the equator in the Southern Hemisphere!
Another myth that I would like to dispel is that the Earth is closest to the sun on the summer solstice and consequently farthest from the sun this time of year. To that I say: first, props for remembering that Earth's orbit is elliptical — I'm glad you are paying attention, but second, that is unfortunately an incorrect conclusion. Turns out the Earth's distance from the Sun makes less of a difference than the tilt of its axis. When the Earth is at perihelion (its closest point to the sun, peri for close, helio for sun) the heat from the sun's radiation doesn't differ drastically enough as compared to at apehelion (its farthest point). However, when the northern hemisphere is tilted 23.5 degrees away from the sun, less radiation strikes the atmosphere in the north and therefore causes the cold temperatures we know as winter. The opposite being true in the summer.
Lastly, I feel I should warn you about a dangerous trend regarding the winter solstice. It precesses, along with Earth's axis. In fact, one day in the future, Christmas will be in July and Independence Day will be in December...so to speak. My point being that the solstices will be opposite to how they are now, with the shortest day being June 21 and the longest day being Dec. 21. That will make for quite the change in holiday traditions! Of course, I'm scaring you for no reason, because this won't happen for tens of thousands of years. Breathe easy and go make a snowman.
(N.B. For you those of you who prefer an exact science, the solstice actually occurs on Dec. 22 at approximately 06:09 UT, also known as Greenwich Mean Time. That's a.m. here in England. East Coasters are five hours behind at 1:09 a.m. — West Coasters are 8 hours behind at 10:09 pm, back on the 21st.)