Vernal equinox 2024 brings spring to the Northern Hemisphere today

This evening, we will have a change of the seasons: The occurrence of the vernal equinox, marking the official start of spring. In fact, it will be a rather auspicious occurrence. The earliest that the equinox has occurred nationwide in 128 years. More on that in a moment. 

At the equinox, the Earth will have reached the point in its orbit where its axis is at right angles to a line from the sun. Thus, the sun will then be directly over a specific point on the Earth's equator moving northward. In the sky, it's where the ecliptic and celestial equator cross each other.

In addition, the sun will appear to rise exactly east and set exactly west. Daylight and darkness are said to be properly proportioned and the sun's rays span the poles. The sun is above the horizon half the day and below for half – but that statement neglects the effect of Earth's atmosphere, which bends the rays of sunlight (called refraction) around the Earth's curvature when the sun lies close to the horizon. But, because of this bending of the sun's rays, the disk of the sun is always seen slightly higher above the horizon than it really is. 

Related: What is an equinox?

In fact, when you see the sun appearing to sit on the horizon, what you are looking at is an optical illusion; the sun at that moment is actually below the horizon. So, we get several extra minutes of daylight at the start of the day and several extra minutes more at the end. 

The supposed equality of day and night gives us the Latin name "equinox," which really means "equal night."  But in reality, thanks to our atmosphere, the day is longer than the night at the equinox.  At the latitude of New York, for instance, day and night are roughly equal a few days before the equinox, on St. Patrick's Day, March 17.

Sun overhead near New Guinea

Astronomers can calculate the moment of the vernal equinox right down to the nearest second. This year it will occur on Tuesday, March 19 at 11:06:20 p.m. EDT. At that moment, the sun will appear directly overhead over the equatorial Pacific Ocean, to the northeast of West Papua, New Guinea, and about 63 miles (101 km) south of Mapia Atoll, historically known as the Freewill Islands or San David. In the days that follow, the direct rays of the sun continue to migrate to the north of the equator and the length of daylight in the Northern Hemisphere will correspondingly appear to increase.

Why so early?

As was previously noted, this will be the earliest that the vernal equinox will occur across the contiguous United States in 128 years.  There are two specific reasons for this:

1) The quadrennial intervention of leap year often causes a slight variation of the date. 

2) Daylight saving time (DST)

When a leap year set us back a day

First, the fact that 2024 is a leap year, is not the reason for the early arrival of this year’s equinox.  Rather, it is the leap year that we observed in the year 2000.

Let’s look at the dates and times of the equinoxes leading up to 2000. Note that each year the occurrence of the equinox happens roughly 6 hours (or one quarter of a day) later in the calendar: 

•  1996 March 20 3:03 a.m. EST 
•  1997 March 20 8:54 a.m. EST 
•  1998 March 20 2:54 p.m. EST 
•  1999 March 20 8:46 p.m. EST 
•  2000 March 20 2:35 a.m. EST* 

Now in 46 B.C., Julius Caesar's consulting astronomer, Sosigenes, knew from Egyptian experience that the solar year was about 365.25 days in length. So, to account for that residual quarter of a day, an extra day – leap day – was added to the calendar every four years. Unfortunately, the new Julian calendar was 11 minutes and 14 seconds longer than the actual solar year. By the year 1582 – thanks to the overcompensation of observing too many leap years – the calendar had fallen out of step with the solar year by a total of 10 days.  

It was then that Pope Gregory XIII stepped in and, with the advice of his own astronomer, Christopher Clavius (1538-1612), produced our current "Gregorian" calendar.  First, to catch things up, ten days were omitted after Oct. 4, 1582, making the next day Oct. 15.  Next, to more closely match the length of the solar year, century-years, which in the old Julian calendar would have been leap years, were not. The exceptions were those century years equally divisible by 400. That's why 1700, 1800 and 1900 were not leap years.     

But 2000 was a century year, evenly divisible by 400, so it was observed as a leap year.  Had we skipped the leap year in 2000 (as was the case in 1900), then the equinox would have begun on March 21 at 2:35 a.m. EST.  

Hence the reason we have placed an asterisk (*) next to that date. 

So, thanks to February having an extra day in 2000, the date of the equinox slipped back a day to March 20. 

Daylight time delayed March 19 equinox in East  

And because the surplus added on to 365 is not exactly one quarter of a day (.2500), but a little bit less than one quarter (.2422), the occurrence of the equinox comes about 47 minutes earlier (on average) every four years: 

•  2000 March 20  2:35 a.m. EST 
•  2004 March 20  1:48 a.m. EST 
•  2008 March 20  1:48 a.m. EDT* 
•  2012 March 20  1:14 a.m. EDT 
•  2016 March 20 12:30 a.m. EDT 
•  2020 March 19 11:49 p.m. EDT 
•  2024 March 19 11:06 p.m. EDT 

The asterisk indicates that we were now observing Daylight Saving Time beginning in early March ... a practice that began in 2007.  In 2000, only those in the Pacific Time Zone observed the equinox on March 19.  In 2004, 2008 and 2012, those in the Pacific and Mountain Time Zones saw spring arrive on the 19th.  

In 2016, those in the Central Time Zone celebrated a March 19 springtime arrival. Were we still on the old system (when DST did not begin until early April), we would have been on standard time in 2016 and Easterners too would have observed the equinox on the 19th (at 11:30 p.m.) ... but daylight time pushed that off for another four years. Finally, in 2020, from coast-to-coast, spring arrived on March 19th, and so it will again in 2024.

And just as a point of record: In 1896, the vernal equinox arrived on March 19 at 9:29 p.m. EST. 

 Astronomical vs. meteorological spring 

Truth be told, there are really two springs: Astronomical spring and meteorological spring.

Astronomical spring is measured by the vernal equinox, but that’s only a marker in the big flow of time, set up by man; a sidereal milepost. Accurate as a ticking clock but only approximately timing the seasons. Meteorological spring supposedly has already started as of March 1, and runs through the end of May. In truth, however, meteorological spring ignores the clock and calendar, makes its own rules and creates a festival of song and blossom, all in its own time. 

The crocuses, early robins and other vernal phenomena pay no attention to the hairsplitting details marking the astronomical arrival of the vernal equinox. They all have their own way of knowing when spring truly begins. 

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.