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Standing Stones at Avebury at sunrise on the Vernal (Spring) Equinox, designated a UNESCO World Heritage Site, the purpose of the ancient obelisks remains an enigma

Photo by: James Osmond

James Osmond

Everybody is Equal in the Equinox

Here comes the sun! At least, if you live in the northern hemisphere of the Earth.

March 17, 2020

Our home planet is tilted at an angle of 23.5 degrees. Tilted against what, you might ask. Good question. Our planet also orbits the sun, and if you imagine the path of our orbit tracing out a line on a giant piece of paper, our north pole isn’t pointing straight up from that piece of paper, but at an angle of 23.5 degrees.

So our north pole points to a particular direction in the sky (very close to the position of the star Polaris, hence the name). This tilt gives us the seasons. For half the Earth’s orbit, the northern hemisphere is tilted towards the sun, giving the topside an abundance of sunlight, with days longer than the nights (i.e., “summer”). For the other half of the year, the southern half of the Earth gets doused in sunlight (i.e. “also summer but for the southern hemisphere”).

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West pine Drive equinox sunrise, Higgins Lake, Michigan

Photo by: Wiltser

Wiltser

In our orbit, there are two and exactly two places where neither hemisphere is tilted away or towards the sun, times when the length of the day is equal to the length of the night: the equinoxes.

The spring (or vernal, if you like fancy words) equinox is approaching soon, hitting us on March 19th, heralding the onset of spring and (eventually) summer for the northern hemisphere, and the beginning of fall and (eventually) winter for folks living south of the equator.

And here’s a fun fact you can use to impress your friends on March 19th. Even though the equinox is touted as the day when the length of the day is equal to the length of night, it’s not, technically correct.

The problem comes down to the definition of “day”. At the equinox, on March 19th, if you were to live in the middle of the ocean (so you would have perfectly flat horizons with nothing to block the view) then it would take twelve hours for the exact center of the sun to travel from one horizon, crawl above you overhead, and dip into the sea on the opposite side.

But that’s not a “day”. A “day” starts at the moment the topmost edge of the sun peeks above the horizon, and ends when the entire disk of our star is completely below the horizon. That extra distance adds up to a few minutes.

Plus, there’s one more effect. Our atmosphere has different layers, with air of different temperatures sitting above each other. These differences bend the path of light, like a lens, and is something we call refraction. This means that even though the sun isn’t actually above the horizon, it can look like it is because the air between us and the sun have bent the path of its light.

All told, on the day of the equinox, depending on where you are on the Earth your day can be up to a few minutes longer than the night.

Paul M. Sutter

Paul M. Sutter is an astrophysicist at Stony Brook University and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.

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