Word of the Week: The Tropics

Hey, Sky Fans! Thanks for stopping by. As of a couple of hours ago (10:44 UTC , 5:44 A.M. U.S. Eastern, 2:44 A.M. U.S. Pacific) it’s winter if you’re in the northern hemisphere, and summer if you’re south of the equator. I was… umm… let’s go with lucky enough to be up awake at the moment of the solstice thanks to a noisy moving truck on my street. The skies here are overcast with a frosted lightbulb in the grey, and a layer of icy snow on the ground. Make no mistake; it’s winter.

Here in the north, it’s the shortest day of the year. Where I am, the Sun rose at 7:14 A.M. and will vanish during this afternoon’s Voltron and Heathcliff reruns, at about 4:30. That’s only around 9 hours of sunlight. It’s a nice mid-latitude first day of winter.  If you’re farther north, you’ll get even less daylight. People farther to the south will have a longer shortest day of the year.

As you probably know, the solstice is the exact moment when the Sun reaches its northernmost  or southernmost point in its seasonal travels. So, from the ground, it’s the moment when the Sun reaches its highest or lowest noontime point in the sky. For a day or two after the solstice, the Sun’s northern or southern movement in the sky appears to stop for a couple of days before it heads in the other direction. Solstice comes from Latin, and means something like “sun stopped.” My Latin isn’t what it used to be.

Rather than talk about the solstice itself, let’s talk about a different part of the science seasons: the tropics. Did you know the word tropic is an astronomical term?

It’s like this. The Earth’s axis it tilted by about 23.5 degrees relative to the plane of its orbit around the Sun. As we make our way around the block, the most direct sunlight moves back and forth from its southernmost point, 23.5 degrees south latitude (called the Tropic of Capricorn), at the December solstice, to its northernmost point, 23.5 degrees north latitude (the Tropic of Cancer), at the June solstice. The Tropic of Cancer and the Tropic of Capricorn mark the limits of the Sun’s trip across the tropics.

If you ever wondered what the tropics were, there you have it. The tropics are the band around the middle of the Earth that, over the course of a year, contain all of the rawest, most direct power of the Sun. This planet is fully operational!

Tropic: The band around the earth from 23.5 deg. north latitude to 23.5 deg. south latitude
Tropic: The band around the earth from 23.5 deg. north latitude to 23.5 deg. south latitude

The direct light of the Sun never leaves that band. This is why if you’re in the north, you need to look to the south to see the Sun, and conversely, if you’re in the southern hemisphere, the Sun is in your northern skies. The further you are from the tropics, the further to the south (or north) the Sun is. This means the further from the tropics you are at your local winter solstice, the lower in the the sky the Sun appears. The lower in the sky, the shorter the path across the sky, and the shorter the time from sunrise to sunset. That’s the reason the Sun never rises very high, if at all, in winter when you’re close to the poles.

This has another exciting wrinkle, other than the palm trees and the papayas. Think about  this from the view of someone on the ground in the tropics. Since the direct light of the Sun is always in the tropics, in the tropics, the Sun is directly overhead at some point each year. This spot is called the subsolar point. At that moment, all shadows vanish. There’s never a time outside the tropics when this happens. Outside the tropics, you always have a shadow during the day… unless it’s cloudy.

So, while you’re dreaming of your tropical vacation, your feet in the sand, and exciting fish in the sea, you can also dream of being at the subsolar point, and tricking all your friends into trying to find their shadow. Bwuhahaah!!

Keep in mind that while winter is just starting, and warmer weather might seem like it’s a long way off, after today, the days are going to start getting longer. In fact, by New Year’s Eve, around here we’ll have picked up 15 minutes more daylight on our way to the summer solstice. I happen to like winter. I love the starkness, and the sound of snow crunching under my boots. I also love the rhythm of it all, the sunny summer nights, and the starry winter afternoons. It’s a good reminder to think about time, and to appreciate how things are now.

Thanks for stopping by, and clear skies, everyone!


8 thoughts on “Word of the Week: The Tropics

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