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SYFY WIRE Bad Astronomy

Today is the vernal equinox… the earliest one in 124 years!

By Phil Plait
The Earth on the vernal equinox of March 20, 2019. This image was taken by the DSCOVR satellite 1.6 million km from Earth in the direction of the Sun. The glow is sunlight reflecting off the Pacific Ocean, directly on the Equator.

Happy vernal equinox! Today is the day when the Sun rises due east and sets due west. For us in the northern hemisphere (about 90% of humans on Earth), that means spring is coming, with summer not far behind. Some people think of this as the first day of spring — which is fine, but I have other thoughts on that — and inversely it can be taken as the start of autumn in the southern hemisphere if you like. It's not when we have equal day and night, but it is the day when the length of the day is changing the fastest.

The reason for the season is the tilt of the Earth with respect to its orbit around the Sun. As our planet moves around the Sun once a year, the tilt changes where we see the Sun every day. The obvious one is that in the summer the Sun is higher in the sky, days are longer, and it gets hotter, with the opposite true in the winter.

It also means the places on the horizon where the Sun rises and sets change every day; the Sun rises and sets farthest south at the December solstice, and farthest north on the June solstice. If you got out at sunset every day from December to June you'll see the sun dip below the horizon to the south of due west, moving north a little every day until the June solstice, where it sets north of west. At some point it sets due west, and that's the vernal equinox.

There are other ways to look at it, too. For example, the vernal equinox isn't a day so much as a moment in time. It's defined astronomically as when the center of the Sun's disk is directly over Earth's equator. We use celestial coordinates on the sky which mirror those on Earth, as if we're embedded in a gigantic sphere: Project the Earth's north and south poles up to the sky and that's where you'll find the celestial poles, and the Earth's equator projected up is called the celestial equator. We use coordinates like longitude and latitude on the sky, except for historical reasons we call them Right Ascension (measured in hours instead of degrees, so there are 24 hours in a 360° circle) and declination. At the moment of the vernal equinox the Sun's celestial coordinates are 0 hours of RA and 0° declination. It's the basis of the entire sky coordinate system we use!

That moment occurs at 03:49 UTC tonight (late Thursday night/early Friday morning). For the United States, that makes March 19 the date of the vernal equinox across the country (Eastern time is UTC – 4 hours).

And that brings up an interesting point: This is the earliest vernal equinox since 1896, 124 years ago!

Why is that?

Leap years! But not because that 2020 is a leap year. Because 2000 was.

As you may recall from three weeks ago, 2020 is a leap year. There are roughly 365.25 days (the time it takes the Earth to spin once on its axis) in a year (the time it takes Earth to go around the Sun once, though that gets a bit complicated). Every year the calendar is a quarter day off, about six hours, and so every four years we throw an extra day into the calendar — February 29 — to account for it.

However, the year isn't exactly 365.25 days long, it’s more like 365.2422 days long. That difference adds up over time, so to correct for that we don't add in a leap day in years divisible by 100. Even that doesn't quite do the trick, though, so we also do add in a leap day every 400 years.

So 1700, 1800, and 1900 were not leap years, but 2000 was. So were 2004, 2008, 2012, 2016, and yes, the current year of 2020. And this is why the equinox is so early this year.

A diagram showing the Earth’s tilt over the course of the year. The axis stays pointed in the same direction in the sky, but dips toward the Sun in the summer and away in the winter. Credit: Colivine / wikipedia

The vernal equinox happens when the Earth is at the same point in its orbit every year — one way to think of it is that a line drawn from one side of the Earth on the equator through its center and out the other side points directly at the Sun. That's independent of our days and calendar.

So if you look at the times of the vernal equinox (I'll stick with UTC here for convenience) they are later every year by roughly six hours, that same quarter of a day left over from 365:

2017 March 20  10:28
2018 March 20  16:15
2019 March 20  21:58

But then every fourth year we add a leap day. This adds a day to the calendar, so the equinox (which happens at the same point in our orbit, remember) moves earlier by a day… but still by that additional six hours later:

2020 March 20 03:49

See? For the US east coast that's 19 March at 11:49 p.m., an earlier calendar day, but the time still jumped later by six hours. I know, it's a mess. And it gets worse.

2000 was a leap day. Up until then, the vernal equinox was on March 20 or 21 every year:

1992 March 20 09:47
1993 March 20 14:40
1994 March 20 20:27
1995 March 21 02:14
1996 March 20 08:03
1997 March 20 13:54
1998 March 20 19:54
1999 March 21 01:25

You can see again it's about six hours later every day, then in the leap year of 1996 jumps back 18 hours (24 back for leap day plus the usual six ahead).

But 2000 was a leap year, so the equinox that year was:

2000 March 20 07:35

That puts it very close to the March 19/20 line. The 18-hour pattern has continued since, too. Every non-leap year the moment of the vernal equinox moves later by six hours, and every leap year it moves earlier by 18.

That means that every leap year in the 21st century will be earlier than the previous one, so every leap year will be the earliest vernal equinox since 1896!

Isn't that weird? Keeping time is messed up.

The Earth on the vernal equinox of March 20, 2019. This image was taken by the DSCOVR satellite 1.6 million km from Earth in the direction of the Sun. The glow is sunlight reflecting off the Pacific Ocean, directly on the Equator.

That trend will end in 2100. Why? That's not a leap year. So we don't push the calendar another day, and the time of the equinox jumps ahead a day from 14:04 on March 19 2096 to 13:05 March 20 in 2100.

If I am thinking of this correctly, that vernal equinox time of 14:04 March 19 in 2096 will be the earliest leap day until the next 400-year-leap-year year of 2400. Then the pattern will repeat like it does for our own 21st century (I can't find an equinox time predictor for that far in the future to check, though).

Phew. I know, it's a little a lot. But one thing I absolutely love about astronomy and calendars and all that is the patterns, the cycles. They're so wonderful and beautiful, so predictable and understandable. It may not be easy to grok them, but we can.

It was easy back in the day to think of the Universe as some sort of mechanical clockwork, a giant machine where every part played a role like a cog in a gear system.

The funny thing is, that’s a pretty good analogy. Instead of teeth and axles we have gravity, and instead of gears we have moons and planets and stars and galaxies. And the housing is the Universe itself.

There's your thought for the day — this day of the vernal equinox.