Animation showing the Earth – with tipped axis – revolving around the sun. Seasons are shown for the northern hemisphere.
It happens every mid-winter. I wake up earlier and earlier, unconsciously responding to the daylight that spills beneath the window shade as the pace of the season quickens.
We’ve been putting seconds and minutes in our sunny-day piggy bank every since the winter solstice last Dec. 21. Those deposits are now accumulating rapidly as February gives way to March. Where I live, days were as short as 8 hours 32 minutes in late December. Today that time has swelled to 10 hours 24 minutes.
As Earth revolves around the sun, its 23.5 degree-angled axis points toward, perpendicular to and away from the sun over the year to make the seasons. Credit: Tao’olunga with additions by B. King
While a half hour of extra light may not be enough to notice, 1 hour and 52 minutes is a revelation. Many of us now drive home in bright twilight at the end of a work day. This has beneficial effects like seeing more sunsets and full moon rises. We also feel more connected with the world because we can see it. Humans weren’t born to live as troglobites in dark caves. We crave sunlight as much as clear,dark nights.
I like the extra daylight for hiking and skiing. Shorter nights also mean less time for the Earth to loose heat and the temperature to dip below zero. If you’d like to see how your day/night account is coming along, check out the UNSO’s Duration of Daylight/Darkness Table.
All things warm and fuzzy (and cold and spiky) come our way because of Earth’s axial tilt. The axis remains fixed at an inclination of 23.5 degrees, but as the planet revolves about the sun during the year, the northern hemisphere tilts toward the sun in summer and away in winter. These are the extremes. In between, we have the spring and fall equinoxes, when both hemispheres are “face on” to the sun and receive equal amounts of daylight and night.
There are mini-seasons too. Mid-February is as good a time as any to call by that name. We’re moving away from winter toward spring with night on the run and daylight gaining the upper hand. A month from today, on the verge of the spring equinox, daylight will have increased an additional 1 1/2 hours to 12 hours. For a moment day and night will balance. The next moment day surpasses night and won’t relinquish its lead until after the fall equinox.
The sun’s always high in the sky at low tropical latitudes, so the seasons don’t vary much. This diagram shows the sun’s position around noon on the winter and summer solstices and equinoxes. Stellarium
Daylight length depends upon your latitude. If you took a tropical vacation this winter, you probably noticed that the sun rose around 6 a.m. and set around 6 p.m. Closer to the equator, the sun’s path is steeply inclined to the horizon every day of the year with little change in sunrise and sunset times. The sun’s always high in the sky there at the noon hour, bringing with it those consistently warmer temperatures we’re willing to pay big bucks for.
A mid-winter sun shines through an icicle formation on Lake Superior. Credit: Bob King
At mid and high latitudes, the yearly variation in sun’s position in the sky puts it high in the sky during summer and low in the sky during winter. Low means less time above the horizon, shorter daylight hours and cold temperatures.
To better understand this, consider that on the first day of spring and fall on the equator, the sun rises due east, passes directly overhead and sets due west. On the first day of summer, the sun at noon passes 23.5 degrees ( a little more than two fists held at arm’s length) north of the overhead point, while on the winter solstice it’s 23.5 degrees south of overhead. No matter the season, the sun will always shine down from a high altitude at noon.
This view shows the sun from a mid-northern latitude city like Minnepolis, Minn. Notice how the sun’s yearly elevation spread take it much closer to the horizon (wintertime) and also quite high (summertime). The scale of this map is different from the one above because it doesn’t need to include as much sky near the overhead point. Stellarium
In Minneapolis, halfway between the equator and north pole at latitude 45 degrees north, the sun is 45 degrees high at noon on the first day of spring and fall or halfway between the overhead point and southern horizon. Come the first day of summer, it’s way up there at 68.5 degrees and roasts the back of your neck, but on the winter solstice it peaks out at just 21.5 degrees high. Better protect that neck with a scarf.
The full range of the sun’s yearly motion – 23.5 degrees north to 23.5 degrees south of the celestial equator – is the same no matter where you are on Earth, but if you live far from the equator, the sun’s altitude reaches greater extremes, making the seasons more pronounced.