Spring returns with a not-so-equal vernal equinox of 2021

Spring will officially arrive on Saturday morning (March 20) with the occurrence of the vernal equinox.

That occurs when the sun will be positioned directly over the Earth's equator at 09:37 Universal Time; 5:37 a.m. Eastern Daylight Time or 2:37 a.m. Pacific Daylight Time. At that particular moment, the sun will appear to shine directly overhead from a point 5 miles (8 kilometers) south of Meru, in Kenya; a city of approximately 241,000 residents. 

Until the occurrence of the summer solstice on June 20, the sun will appear to migrate northward and the length of daylight in the Northern Hemisphere will continue to increase. As the altitude of the midday sun gets progressively higher, the arc that it takes across the sky will also increase. On the date of the equinox, the sun will rise due east and will set due west. But during the coming days and weeks, as the sun's direct rays are concentrated more and more on the Northern Hemisphere, its rising and setting positions will become increasingly oriented more to the north of due east and north of due west.  

Related: The 2020 vernal equinox was the earliest spring in 124 years

Your clenched fist held at arm's length measures roughly 10 degrees in width. On the first day of summer, as seen from mid-northern latitudes, the sun will be rising not due east, but 33 degrees (or a bit more than "three fists") to the north (left) of due east. And a bit more than 15 hours later, it will be setting not due west, but 33 degrees to the north (right) of due west. 

Our seasons take place because as our planet revolves around the sun, its axis is tilted at a 23.5-degree angle. This tilt causes different latitudes on Earth to receive varying amounts of heat and light from the sun throughout the course of the year. For the Northern Hemisphere, the June solstice marks the start of the summer season and occurs when the direct rays of the sun shine down on that part of the globe 23.5 degrees north of the equator — the so-called Tropic of Cancer. The December solstice marks the beginning of winter, when the direct rays of the sun are shining down on that part of the globe 23.5 degrees south of the equator — called the Tropic of Capricorn. 

The March and September equinoxes occur when both the Northern and Southern Hemispheres equally face the sun and all parts of the world have the sun above the horizon for exactly 12 hours, and below the horizon for exactly 12 hours.  

Equal days and equal nights: The equinox.       

Well … that's not exactly true.

Inequal day and night

A complication revolving around the vernal equinox concerns the length of day versus night.  Since grammar school we all have been taught that on the first days of spring and autumn, that day and night are equal to exactly 12 hours all over the world. Yet, if you check the calculations made by the U.S. Naval Observatory or the sunrise/sunset tables in any reputable almanac, you will find that this is not so. In fact, on the days of the spring and fall equinox the length of time that the sun is above the horizon is actually longer than the time it spends out of sight below the horizon by several minutes.

Every year around this time, almost like clockwork, I will get an email from someone who was studying the weather page of his or her newspaper, looking at the section listing the times of sunrise and sunset and noticing that something appears to be wrong. The difference in the number of hours separating sunrise and sunset on the day of equinox are not equal at all.

Check out New York City. As the table below shows, days and nights are equal not on the equinox, but actually, a few days earlier, on Saint Patrick's Day (March 17):

One factor is that the moments of sunrise and sunset are considered when the top of the sun, and not its center, is on the horizon. This alone would make the time of sunrise and sunset a little more than 12 hours apart on these days. The sun's apparent diameter is roughly equal to half a degree.

It's an illusion 

But the main reason that this happens can be attributed to our atmosphere; it acts like a lens and refracts (bends) its light above the edge of the horizon. In their calculations of sunrise and sunset times, the U.S. Naval Observatory routinely uses 34 minutes of arc for the angle of refraction and 16 minutes of arc for the semi diameter of the Sun's disc. In other words, the geometric center of the sun is actually 0.83º below a flat and unobstructed horizon at the moment of sunrise. 

Or, put in another way, when you watch the sun either coming up above the horizon at sunrise or going down below the horizon at sunset, you are actually looking at an illusion – the sun is not really there, but actually is below the horizon.

As a result, we actually end up seeing the sun for a few minutes before its disk actually rises and for a few minutes after it has actually set.  Thus, thanks to atmospheric refraction, the length of daylight on any given day is increased by approximately six or seven minutes.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.