solstice and seasonal cycles

The Days are Getting Longer: Winter Solstice

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It may have been hard to tell, with this seemingly interminable series of cloudy days, but we have finally passed the longest night. Now our days are getting longer and our solar panels more effective again … here’s why!

SO, THE EARTH GOES AROUND THE SUN.

Solstice gets its name from the Latin word solstitium, meaning the sun stands still. When the sun reaches its furtherest most point on the Earth, it appears to stand still on the Tropic of Capricorn/Cancer before heading back the other direction. The heliocentric cosmography (the idea that we go around the sun, rather than the sun circling us) has been the the going theory since Nicholaus Coeprnicus published his De Revolutionibus Orbium Coelestium (and then died) in 1543. He wasn’t totally on the mark with his concept of perfect concentric circular orbits for earth and the other planets but he’d made a great leap forward in physics. A few years later, in 1609, Johannes Kepler refined the theory by describing the orbits of the planets as elliptical and pointing out that the planets traveled at different speeds depending on their proximity to the sun. This was later explained by Newton’s Universal Law of Gravitation but at the time, Kepler was just going on the observed evidence of day length and sun position.

There’s still some uncertainty about this simple fact.  A recent survey found that 1/4 of Americans think that the sun goes around the Earth. Check a text book, folks.

IT SPINS ON IT’S ANGLED AXIS.

Night and day are the result of a given spot on the earth being in sight of the sun … or not. Our seasons come from the tilted axis.

As the earth spins on its axis, the Chicago sun comes into our view in the east, passes overhead and then sets in the west. If we were at the equator, each day and night would always be about 12 hours long. If the axis of the earth were perpendicular to our orbit of the sun we’d have straight angle sun on the equator and a murky dusk of angled light at the poles all the time.

Luckily for us, we’re spinning around an axis tilted at 23.5 degrees from our orbit, which means that sometimes the Northern Hemisphere is tilted toward to the sun (summer), sometimes away (winter) and sometimes straight on (the equinoxes). That tilt gives us not only a little welcome variation and cyclical nature to the year but moderate our temperatures and basically predicate the entire system of life on earth.

IT FOLLOWS AN ELLIPTICAL PATH.

So even though we have a long (and presumably) cold winter coming ahead of us, things are looking up. The days are going to start getting longer. In fact, the sun has been setting later in the day since way back on December 11th, thanks again to the Earth’s elliptical path of travel and the fact that the whole idea of time is completely made up by man. But we’ll save that for another time. The change will be slow at first (only a few extra seconds on each successive day) and then more dramatic toward the equinox when each day increases or decreases by about two and a half minutes. Hopefully just knowing that they days are getting longer, intellectually, will bring some good cheer.

If you’ve ever needed more than 24 hours in the day, your wish is (a little bit) granted.

If so, this factoid will bring even more. Although we have the shortest period of sunlight at the winter solstice, we actually have the longest day (period from one solar noon time to another). Per our the Solstice Greeting of Chicago’s own Adler Planeterium: “we’re closer to the sun on the December solstice than the June solstice, so the Earth at present moves a little faster than average in its orbit – which means we travel a little farther than average each day. As a result, Earth has to turn maximally on its axis each day for the sun to return to its noontime position.” So there you have it. A few more moments in your day to prepare for the New Year. Use them well!

 

night sky background for image from wikimedia.