Sunday marks the spring equinox

The reason we have equinoxes is because we do not orbit the Sun very upright. The Earth tilts about its axis about 23.5 degrees, which gives a hemisphere more light and energy from the sun at different times of the year.

But no matter where you live, the equinoxes are unique because daylight and darkness are almost the same everywhere on earth. Sunday is one of only two days of the year – the other being the autumn equinox in September – when both hemispheres experience about 12 hours of daylight and darkness. Most places on Earth, apart from the polar regions, also see the sun rise to the east and set west along the horizon.

Same day and night – not quite

A common misconception about equinoxes is that everyone sees exactly 12 hours of daylight. Although “Equinox” comes from the Latin words “aequus” (equal) and “nox” (night), both hemispheres actually see slightly more than 12 hours of daylight on the equinox.

Washington, DC, for example, sees 12 hours 9 minutes of daylight on Sunday, with sunrise at 7:11 a.m. and sunset at 7:20 p.m. Meanwhile, Equilux – meaning “equal light” in Latin – marks the date when sunrise and sunset come apart for exactly 12 hours. In most of the lower 48s, the equilux occurs a few days before the spring equinox, usually around 16 or 17 March.

Why are there more than 12 hours of daylight on the Equinox?

The reason why we see more than 12 hours of daylight on the Equinox happens for two reasons. One is how we define the terms “sunrise” and “sunset” to measure the length of a day. The other is something called “atmospheric refraction.”

Anyone who has photographed an impressive sunrise or sunset has surely noticed that the sun looks like a huge, lumbering disk when it is near the horizon.

“Sunrise is defined as the moment when the upper limb of the sun rises over the horizon, while sunset occurs when the last rays of sunlight disappear below the horizon,” explains Matthew Cappucci of the Capital Weather Gang in a 2020 article. taking the first-up, last-down approach to defining the length of the day, instead of tracking when a single point on the sun is above the horizon, our day is a few minutes longer than 12 hours. “

The second reason why we see more than 12 hours of daylight is because the Earth’s atmosphere can refract or bend the sun’s light, allowing us to see the sun even when it is technically below the horizon. The amount of refraction depends on the atmospheric pressure and the temperature.

These two factors – as we measure the length of the day and atmospheric refraction – add about 10 minutes of daylight depending on the equinox where you live. At the North and South Poles, the atmosphere “stretches” the distance between the center of the sun and its outer edges, so that the sun takes longer to rise and set. Daylight at the equinox thus varies, from 12 hours 6 minutes at the equator to about 12 hours 20 minutes in the polar regions of the Earth.

Rising daylight, changing sunrise and sunset

In the northern hemisphere, the spring equinox marks the time of year when we receive the greatest daylight. Also in the next 3 months one notices the sun rising and rising a bit further north, because it takes a longer and steeper way through the sky.

As the days get longer since the winter solstice, we experience the largest increase in daylight around the spring equinox.

The rate of increase depends on where you live: Washington gets 2 minutes 32 seconds of daylight closer to the equinox, while more northern cities like Boston, Chicago and Seattle climb closer to three minutes of daylight per day. As you get closer to the equator, the increase is less dramatic: Miami and Houston, for example, win less than two minutes a day.

A warm spring in the cards?

As the days get longer and the sun rises higher in the sky, temperatures will continue their inevitable rise towards summer. The average high temperature in DC is now 58 degrees, but will reach 70 degrees by April 19th.

For most of the country, this spring may be on the warm side. The National Oceanic and Atmospheric Administration recently released its Spring 2022 outlook, forecasting warmer-than-average temperatures over a large area of ​​the United States.