Wednesday, March 18th 2020, 10:30 pm - The Leap Year and Daylight Saving Time teamed up to gift us an early spring
At precisely 11:49 p.m. EDT March 19 (3:49 UTC on March 20), the Sun will cross the celestial equator, from south to north, signifying the start of spring for the northern hemisphere!
Additionally, this is the earliest vernal equinox seen from North America in 124 years. The last time the equinox fell on March 19, was way back in 1896!
Why is this particular equinox so early?
The exact timing of the equinoxes and solstices changes from year to year, simply due to slight variations in Earth's orbit around the Sun. What's different this year, though, is a combination of two factors - the Leap Year and Daylight Saving Time.
When we added February 29 to the calendar this year, it pushed the calendar forward for everything after, and thus shifted the timing of every celestial event like this back a day, compared to last year. So, rather than the Equinox happening on March 20 again, it is on March 19.
Also, Daylight Saving Time factors into this because we're observing it a month earlier than we used to (before 2007). If it wasn't for DST beginning in early March this year, the equinox would take place on March 20, at 12:49 a.m. ET.
One remarkable aspect to all of this: if we hadn't added the leap day to this year's calendar, and we were still starting DST in April, Spring would have actually started on March 21, at 12:49 a.m.. That would have made this the latest start to spring since 1979!
WHAT IS GOING ON?
What does all that mean? Well, as our Earth travels in its orbit, the 23.5° tilt of the planet causes the angle of the Sun to change in our sky.
From late September to late March, the North Pole is angled away from the Sun, so that the Sun is positioned more directly over the southern hemisphere, and the Sun reaches its lowest point in the northern sky (and highest in the southern sky) on or around December 22.
From late March to late September, conversely, the South Pole is angled away from the Sun, so that the Sun is positioned more directly over the northern hemisphere, reaching its highest point in the northern sky (and lowest in the southern sky) on or around June 22.
At the two points in between these periods - specifically around March 20 and September 22 - it appears to us as though the Sun crosses the equator. In March, it crosses from south to north, and in September, it crosses from north to south.
The exact moment that the Sun appears to be over the equator, in either case, is known as an Equinox.
The view from geostationary orbit, care of EUMESAT's Meteosat-9 satellite, shows the solstices and equinoxes of 2013. Credit: NASA's Earth Observatory
Which hemisphere you're in at the time determines exactly which kind of equinox you're experiencing. In March, the northern hemisphere marks the vernal equinox, while the southern hemisphere marks the autumnal equinox. In September, it's the opposite.
As for why we have leap days and leap years, they are essential for stabilizing our calendar with respect to the seasons.
The Gregorian calendar used by much of the world tracks exactly 365 days every year, but a year on Earth is actually longer than that, by roughly 5 hours, 48 minutes and 45 seconds.
So, for the past 437 years, we have been following a specific pattern to offset that extra time.
Every four years, we add one day to the calendar - February 29 - thus, essentially pressing pause on the calendar to let everything catch up. There's a problem with that, however. If we just stopped with "once every four years", we'd be adding too much time. So, every three century years out of four, we skip adding the leap day. To put it in another way, if a potential leap year is equally divisible by 100, we skip it, except if that year is also equally divisible by 400. Thus, the year 2000 was a leap year, but we will skip 2100, 2200 and 2300, and have a leap year in 2400.
Daylight Saving Time, on the other hand, is not an essential practice. Ever year, those of us in the northern hemisphere shift the hours of the day forward by one hour in March, and move it back again in October or November. In the southern hemisphere, it is reversed. This puts us onto "Summer Time", which is supposedly to help us make more use of the daylight hours of each day.
There have been a lot of debate lately, about the merits of this system, with pushes to either discard it completely or move us all onto permanent Daylight Saving Time.
Teaser image: NOAA's GOES 13 geostationary weather satellite captured this equinox view of Earth's weather in 2013.
Sources: timeanddate.com | With files from The Weather Network