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Almanaque

Alexandre Cherman

Equinox

I’m writing in the Summer, an article that will be published in the Fall. Tha change in seasons will happen (or maybe I should write has happened!) at 8h35PM, local time, on Friday, March 20, 2015. And that is why I’m allowing myself to hit the pause button on our path towards our civil calendar to talk about the seasons.

I talked about this before (link) so I’ll try some new approach. To be honest, last time I talked about that it was more a small rant, a minor complaint with the Planetary Society for being biased to the North, in a planet that clearly has two hemispheres!

Well, this post starts with a rant as well. This time, my target is the Weather Channel. I just saw a video they posted on their Facebook page where a “specialist” (the quotation marks are necessary, believe me!) explains that during the equinox, Earth’s axis will be perpendicular to the Sun! No, it won’t.

(I’m not even dwelling on the geometrical fact that I don’t quite understand what direction is “perpendicular to the Sun”. By the art presented on screen, I take they wanted to say “perpendicular to the Earth’s orbbit’s plane”. Which is just as wrong!)

Earth’s axis is tilted in relation to it’s orbit. Always! The direction of the axis is constant for many years. (It actually changes, but it takes almost 26,000 years to complete a turn, so we can ignore this motion for now.) Let us picture something that has a fixed tilted, both in angle and direction… the Leaning Tower of Pisa, for example!

Let’s imagine the Tower spinning around the Sun… On our first image (below), we can clearly see that the top of the Tower will get more heat and sunlight; on the second picture (click on the button to see), is the bottom part of the Tower that will be privileged.

Tower spinning around the Sun
Click on the button below to switch between images.

That is exactly what happens with Earth! There are some times of the year that one hemisphere will get more heat and sunlight; there are two days (the solstices) that will mark the maximum difference between sunlight and heat received in both hemispheres (in one of the solstices, the Southern Hemisphere will get the best of it; on the next solstice, roughly six months later, it will be the Northern Hemisphere getting more heat and sunlight).

And, of course, if there is a maximum and a minimum, there must be an “in between”. This point is known as an equinox (the word comes from Latin, “equal night”, meaning that the night will have the same length of the day). During the equinoxes, both hemispheres get the same amount of light and heat, even though Earth’s axis remains tilted.

And that is what is just about to happen (or, maybe, it just happened) on March, 20! ■