The science behind the Equinox

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The science behind the Equinox
The science behind the Equinox

September 22nd is the Autumnal Equinox for 2020, the fall season’s start for the northern hemisphere.

Standing here on Earth’s surface, nothing may seem particularly out of the ordinary. When you look at it from afar, though, our entire world is slightly off-kilter. As the Earth traces its elliptical path around the Sun every year, the axis it rotates around each day is tilted with respect to that path, by roughly 23.4 degrees.

While we don’t feel the tilt itself, we see its effects, specifically in the changing seasons throughout the year. The planet does not ‘wobbling’ back and forth by 23.4 degrees during the year, though. Earth’s tilt remains roughly the same all year long, with the planet’s axis always pointing out of the North Pole towards a star named Polaris. It’s this consistent tilt, along with our world’s motion around the Sun, that are the reasons for our seasons.

WATCH BELOW: This animation shows Earth’s tilt throughout the year, which gives rise to our changing seasons.

Click here to view the video

As the video above shows, as Earth travels around the Sun, the angle of the planet’s axis stays the same. Each hemisphere ends up pointed most towards the Sun during its summer solstice and pointed furthest away from the Sun at its winter solstice. The panels on the right show how this affects the angle of the sunlight falling on Earth. The bottom right panel shows the angle at the point where the stick figure is standing – at roughly 44 degrees North latitude.

There are two brief moments during the year when both hemispheres are at precisely the same angle to the Sun. These are the equinoxes – Vernal in the spring and Autumnal in the fall. Those two brief moments don’t always happen at the same time and day every year, though, due to the way we tick off our minutes, days and years.

Visit our Complete Guide to Fall 2020 for an in depth look at the Fall Forecast, tips to plan for it and a sneak peek at the winter ahead


Click here to view the video


A few urban myths surround the equinoxes, that seem to make their rounds no matter how many times they’re debunked.

1. Balancing act

Whenever the equinox is approaching, spring or fall, word spreads that this is the day to perform a “fantastical feat” of egg balancing. Despite any implications that you can only accomplish this feat at the exact moment of the equinox, that’s just not true. You can balance an egg on its narrow end at any time of the day, on any day of the year. The feat is not even easier on the equinox.

The reason for this? Firstly, the planet’s tilt does not affect how gravity works. Earth’s gravitational force, on us and everything else on the planet’s surface, always points towards the centre of the planet.

Secondly, Earth’s gravity completely overwhelms any other gravitational forces experienced here on the planet’s surface. Even the Sun’s gravity, which is the next strongest gravitational force on us here, is 1,600 times weaker than the ‘pull’ we feel from the Earth. The moon, the other planets and even the entire universe exert forces on us. Even so, the Earth’s influence over us is so strong, these other forces are entirely negligible.

There are only three factors that matter in balancing eggs: the stability of the surface you are using, the ‘bumpiness’ of the eggs being balanced, and the steadiness of your hands.

2. Those crazy days (and nights)

There are two days during the year when both night and day are roughly 12 hours long.

Despite the word equinox coming from the Latin words for ‘equal’ and ‘night’, those two days do not fall on the equinoxes. Depending on what latitude you’re at, the date when day and night are roughly equal falls between 3 and 21 days before the spring equinox, and from 3 to 21 days after the fall equinox. The closer you are to the equator, the bigger the gap is. Anyone at the equator actually never sees equal day and night.

That may seem strange, but in fact, we do this to ourselves by the way we track sunrise and sunset.

‘Sunrise’ is officially defined as the exact moment when the edge of the Sun crests the eastern horizon. ‘Sunset’, on the other hand, is the exact moment when the Sun completely disappears below the western horizon.

Instead, if we defined them as being when the Sun was precisely centred on the horizon (eastern for sunrise and western for sunset), the dates of the equinox and having equal day and night would line up better.

Since we technically add several minutes to the length of our day, though, the dates will never line up.

3. Shadows stick to us

Peter Pan may have lost his shadow, but it’s very difficult for us to get away from ours.

There have been some claims that you can stand on the equator during the equinox and not cast a shadow. While that might seem credible at first glance, as long as it’s a clear sunny day, you will still cast a shadow.

Even on the equator, with the equinox happening exactly at local noon, you would still look down to see the shadow of your head, shoulders and torso falling across and between your feet.

4. Leaves change colour in the fall

While not specifically about the equinox, this colourful trend does tend to occur after the equinox ushers in fall. It’s an interesting one, in that it is only half-true.

The leaves of deciduous trees do, indeed, change from green to various shades of yellow, orange and red during the fall. That much is right.

However, the green doesn’t technically turn into those other hues. Leaves appear green due to a chemical known as chlorophyll, which absorbs sunlight and converts carbon dioxide and water into sugars and starch. As sunlight becomes less intense towards the end of summer and the beginning of fall, trees shut down this process in preparation for winter. The chlorophyll then breaks down, and the green colour fades.

At that point, the leaves’ natural colours, which were really there all along, finally show through.

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