Winter is coming! Wait, it’s been here since November, right? For many of us it certainly feels that way. I’ve been shoveling snow and chipping ice for the past six weeks. Ready or not the season’s official start begins at 10:19 p.m. (Central Time) Saturday, December 21.
On the first day of summer, the sun reaches the peak of its yearly circuit in the sky and then immediately begins sliding south, arriving at its lowest point six months later on the winter solstice. As we know from personal experience, the low sun rises late, climbs to a measly 20°-25° altitude and sets early. That’s why the days are so short and the nights so long. In winter the sun spends far more time below the frosty horizon than above. The northern hemisphere responds accordingly. The temperature drops, rain turns to snow and ice and darkness rules. Winter brings 15-16 hours of darkness (depending on your latitude) and only 8-9 hours of daylight.
On Saturday, if you live in the northern hemisphere you’ll experience the shortest day and longest night of the year. Long ago people lit big fires and held huge parties around this time to stave off the winter blues and encourage the sun to get back to work. Indeed many of our current year-end festivities like Christmas and the yule log have their origins in ancient pagan celebrations that celebrate this solar low point.
But hidden within the longest night of the year are the seeds of summer. When you’ve hit rock bottom the only way to go is up. And that’s exactly what the sun does after the winter solstice. It begins to climb — ever so slowly — back toward its summertime perch, taking the next six months to get there.
This yearly up and down movement of the sun changes the day length which in turn creates the seasons. What makes the sun go up and down? Two things — the Earth’s tilted axis and its yearly orbit around the sun. The tilt remains a steady 23.5° year-in and year-out no matter the season. But on the first day of summer, the northern hemisphere tips toward the sun as if bowing, causing the sun to appear high in the sky. In winter, the planet has moved to the opposite side of its orbit. Now the northern hemisphere tips away from the sun, which makes it dip lower in the sky.
If the Earth rotated straight up and down, in other words with zero tilt, the sun would follow the same path across the sky every day of the year. On the equator it would always rise due east, pass directly overhead at noon and set due west. At mid-northern latitudes it would likewise rise due east and set due west but would climb only about halfway up in the southern sky at noon. At the poles the sun would circle around the horizon neither rising nor setting.
Earth’s tilt combined with its orbital motion makes the sun appear to bounce up and down very slowly during a year’s time.
You would think that on the winter solstice the sun would set earliest and rise latest, but that’s not what happens. The earliest sunset happened on Dec. 8 and the latest sunrise occurs during the first week of January. If you look at the diagrams above and below, you’ll see that the sun is moving mostly to the east (to the left) and only a little bit to the south before the solstice. After the solstice, it continues to move mostly east and only a little bit north.
At this time of year the Earth is close to the sun (closest on Jan. 5) which makes the sun appear to move a little faster to the east. Before the solstice, this speedier eastern movement outpaces the southward slide and delays sunset, causing the sun to set later and lengthen the day. Remember that when something moves to the east it pulls away from the western horizon, delaying sunset. That same eastward motion also delays the time of sunrise, making the sun rise later for a time until its northward motion compensates for the eastern movement, and the sun starts rising earlier. This happens around Jan. 10.
How to celebrate all these celestial motions? You can build a fire or toast with a bottle of champagne, but rhythmic and repeated celestial movements suggest another activity. I say grab your partner and go dancing! When you both lean back to face one another while twirling about the dance floor, you’ll feel Earth’s tilted atlas and its yearly dance around the sun in your heart and bones.