Nature Guide Journal
Although we’ve already had the start of “winter weather” on the Oregon Coast, the official first day of winter is the 21st of December this year—and the coldest days for most of North America will come a few weeks from now.
So why is the 21st the “First Day of Winter”?
Ultimately, it’s the tilt of Earth’s axis that gives us the seasons, a tilt that’s about 23.5 degrees from “straight up and down.” The tilt stays pretty steady as we travel around the sun in our year, so at one end of our orbit the North Pole tilts toward the sun (our summer) and at the other end of our orbit the North Pole tilts away from the sun (our winter).
Our shortest day of the year occurs when the Earth’s orbit lines us up so the tilt is pointing the North Pole furthest from the sun and the sun appears lowest in the sky to us: Winter Solstice. (Summer Solstice is the longest day of the year, when we’re tilted to get the most sunlight; Spring Equinox and Fall Equinox are the half-way points in the orbit.) From our Earth-bound view, the sun travels a lower—and shorter—arc through the winter sky, giving us shorter/cooler days; the sun travels a higher—and longer—arc through the summer sky, giving us longer/warmer days.
North of the Arctic Circle it’s dark all “day” long on the Winter Solstice, and the North Pole hasn’t seen a complete sunrise for a couple of months. At Oregon’s latitude, about half-way between equator and pole, we get about nine hours between sunrise and sunset by the end of December. (By late June, we’ll get about 15 hours between sunrise and sunset.)
The shorter daylengths are the defining characteristic of “winter” since less sunshine means cooler temperatures.
If it’s the day with the least sunlight, why isn’t the Winter Solstice the coldest day of the year? There’s a lag because it takes longer for the oceans to heat—and cool—than the land, and most of our planet’s surface is water.
Since winter and summer are terms relative to hemisphere (our “winter” comes as the same time as Australia’s “summer,” for example), it’s more globally correct to call the solstices “Northern Solstice” (our summer) and “Southern Solstice” (our winter).
Daylength isn’t the only variation in our relationship with the sun through the year. The Earth’s orbit around the sun isn’t circular, it’s elliptical. Our planet is actually 3.11 million miles nearer the sun in early January (about the Northern Solstice) than it is in early July (about the Southern Solstice)—we’re a little nearer the sun during our winter than during our summer. That nearness means sunshine is about 7% brighter in December than in June.
That extra boost of sunshine comes when it’s summer in the southern hemisphere. However, the southern hemisphere has a higher proportion of water to land than the northern hemisphere has; since water absorbs heat less than land does, the southern hemisphere’s greater water surface moderates the heating of a slightly nearer sun.
Our days have been growing increasingly shorter since the Northern Solstice on June 20. This year, the Southern Solstice is 11:12am UT, 21 December—2:44am PST—the official start of our winter.
Our clocks don’t line up exactly with the planet’s actions. By New Years, the sun will rise 3 minutes later than on the solstice, but will set 7 minutes later, gaining us a whopping 4-minutes longer daylength.
On December 22, dawn starts a wee bit earlier and dusk starts a wee bit later as we start inching back into longer days. Welcome back, sun!
(published in The World, Coos Bay, OR, 2016-12-22)
© M. Giles