The science behind the Equinox

Summer's final curtain call. Here's a guide to the science behind this phenomenon, and one of its most colourful effects.

September 22nd is the Autumnal Equinox for 2021, 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.

The planet does not 'wobble' back and forth by 23.4 degrees during the year, though. Earth's tilt remains 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.


While we don't feel the tilt itself, we see its effects. Perhaps the most directly noticeable effect, which can be tracked on a day-to-day basis, is how high the Sun reaches in our sky throughout the year. It's this change in angle that is ultimately responsible for the changing of the seasons.

As the video below 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.

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

There are two brief moments during the year when both hemispheres are at precisely the same angle to the Sun. These are the equinoxes.

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Those two brief moments don't always happen at the same time and day every year, though. Due to the fact that our orbit around the Sun doesn't always take exactly the same amount of time, year-to-year, and also due to the way we tick off our calendar, the timing of the equinoxes changes.

In the northern hemisphere, the vernal equinox typically occurs on the 20th of March, and the autumnal equinox usually happens on the 22nd of September. These dates shift, though, moving forward and backward on the calendar by a day or so, on a cycle that lasts for more than a century. Sometimes the March equinox will flip back and forth between the 20th and 21st, or the 19th and the 20th, on a year to year basis, while the September equinox consistently occurs on the 22nd. Other times, the March equinox will fall on the 20th, year after year for decades, while the September equinox drifts back and forth between the 21st and 22nd, or the 22nd and 23rd.

Right now, in 2021, we are in the part of this cycle were we will mostly see the equinoxes happen on March 20 and September 22. Every four years, though, they'll shift. For example, in 2023, the equinoxes will be on March 20 and September 23, and in 2024, they'll occur on March 19 and September 22.

Visit our Complete Guide to Fall 2021 for an in depth look at the Fall Forecast, tips to plan for it and much more!



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.


These two eggs were balanced on there ends on March 24, 2015, four days after the equinox. Credit: Scott Sutherland

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.

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Secondly, Earth's gravity completely overwhelms any other gravitational forces experienced here on the planet's surface. The Moon's gravity is the next strongest influence, followed by the Sun, and they are the reasons for our ocean tides. However, the tides only occur due to the cumulative effect of these objects pulling on immense bodies of liquid. For something as small as an egg, their effect is so weak that it can be considered 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.


A view of the equinox from space, courtesy the GOES geostationary weather satellite. Credit: NOAA

That may seem unbelievable, but in fact, we do this to ourselves. It's due to how we track sunrise and sunset.

'Sunrise' is officially defined as the exact moment when the edge of the Sun crests the eastern horizon. 'Sunset' is the exact moment when the Sun completely disappears below the western horizon.

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

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

From time to time, claims circulate that say if you stand on the equator during the equinox, you will not cast a shadow. While that might seem universally credible at first glance, it's only true some of the time.

See, the equinox can happen at any time of the day. In 2021, for example, the autumnal equinox occurs at 3:21 p.m. EDT. Unless you have some particularly dark clouds overhead at the time, you're very likely to cast a shadow at that time, regardless of your location and the position of the Sun in the sky. In 2022, though, it will be at 9:03 p.m. EDT, when you are far less likely to cast a shadow.

Now, what if you pick your location perfectly, so that you are at the exact place on Earth where the equinox happens precisely at noon? This year, that would be somewhere in the middle of the equatorial Pacific Ocean. Well, at that time, with the Sun exactly overhead, you'd still see your shadow cast on and between your feet.

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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.