Narrow daylight: what does the winter solstice look like?
29 December 2006
For the last few years I have lived on the East side of an island with a mountainous interior, and missed one of the most vital pairs of events in the world. At about 6:30 on a morning not long ago, I watched the Sun inch above the horizon to illuminate the summit of Mauna Kea, followed by a gentle avalanche of sunlight down the slopes and into full daylight. Then, beginning at about 5:45 that night, after a two-hour drive and reunion with family members, we all relaxed on a marble lanai cantilevered over tide pools as the Sun completed its trek and faded out. Long after the upper limb of the Sun had vanished, the clouds unmasked their true delicate hues, usually overwhelmed by the direct glare of the Sun.
Seeing sunrise and sunset in the same day may not be such a big deal, except that it is one of the classic examples of events that we take for granted. In fact, sunrise is taken so much for granted that sunrise is an oft-cited definition of the Bayesian inference. Right up there with the phases of the Moon, the incremental changes in the daily motion of the Sun continue their dance, waiting for us to notice some day.
Yesterday was one of those—the winter solstice.
The usual description as the shortest day and longest night of the year is true as far as it goes, but doesn’t get at the nuts and bolts of it. It also displays a certain chauvanism of hemisphere, as our winter solstice is the summer (and longest day of the year) for those in the south. At any rate, in December the Earth is near its closest spot to the Sun on its orbit. The actual closest point (perihelion) comes in January, so the cold weather and short days have a cause other than distance from the fire.
While we can explore the cause of this phenomenon below, the appearance is probably the best place to start. The December solstice is the shortest day of the year for northern hemisphere observers, but it is neither the earliest sunrise nor the latest sunset. Those occur on two different dates that vary according to your latitude, but often in November and January. What does happen on the winter solstice, though, is that the Sun rises at its farthest North point on the horizon, no matter where you are. While the Sun does rise in the East and sets in the West, it doesn’t always fall exactly on those cardinal directions.
Careful observation of the rising and setting of the Sun measured against fixed points like hills or stones makes for extremely accurate calendars, and such sites are found, often in pairs, all over Europe. E.C. Krupp, among others, has some excellent pictures and narratives about megalithic observations in his books. Once you have a good set of observations, building places like Stonehenge, Newgrange, and many South and Central American sites to match the alingments of those risings and settings can be achieved with relative ease. Once you have determined the bearings (direction on the horizon, not necessarily related to a compass) to these seasonal sunrises and sunsets, you’re pretty much set for a given location, since they will recur every year.
Nowadays, with satellite pictures of our ‘Blue Marble,’ we are accustomed to celestial events so esoteric they are almost abstract, rather than observational. The measurement of the Sun’s position relative to the stars is the way to determine the ‘day’ of the solstice, although it is like the full moon—something that we ascribe to a whole day that only persists for a moment. In this case, the moment is when the Sun reaches the farthest North spot in the sky, which not all observers will be able to see (e.g. it may be during nighttime for some). Just to make it trickier, when measuring the Sun’s position against the stars, it’s pretty hard to see both at the same time, hence using the bearings for rising or setting.
The steadily tilted axis and repetitive yearly revolution of the Earth bring the Sun directly overhead to different spots on Earth at different times of the year, then the daily rotation of the Earth turns those spots into rings of latitude all the way around. The latitude where the Sun is overhead on the December solstice is called the Tropic of Capricorn, and the June counterpart is the Tropic of Cancer. Split the difference between the two and you get the Equator, which plays into the Equinoxes, to be discussed in a later post.
That cause of all of this seasonal variation is the tilt of the Earth’s axis that ubiquitous 23 and change degrees off of the perpendicular, measured from the flat plane of its orbit. If the Earth’s axis at the North Pole is the stick of a caramel apple, the stick points at the same spot in space no matter where the Earth is around the Sun. So if the Sun is a clove-studded orange (for our holiday flavor) and both it and the caramel apple sit on the table, in December the caramel apple stick points diagonally away from the orange. In constrast, on the June solstice the stick points diagonally at the orange, across the top of it.
And, of course, celebration of this time of year predates any historical records and any contemporary religions, most of which have subsumed the celebration into their calendars. See Guy Ottewell’s Astronomical Companion for more on that. Happy winter solstice—I’m going to the beach.