New Moon, Solstice and for Africa and Asia an Annular Solar Eclipse

This months New Moon will occur at 16:42 on Sunday 21st June coinciding with the Winter Solstice which occurs at 7:43 am making it the shortest day and longest and darkest night of the year.

What is interesting is that at the time of the new Moon the Moon will pass perfectly between the Earth and the Sun, creating an annular solar eclipse for a myriad of observers across Africa and Asia. Indonesia, ONG and the top of Darwin and Cape York will see a partial eclipse. It’s a rare occurrence to have a solstice solar eclipse, but there’s a good reason for it this year. Here is why.

Where you can see the Annular eclipse on Sunday

Solstices occur twice a year, and the Moon cycles through its full set of phases every 29.53 days. In order to get a solar eclipse, the Moon needs to be in its “new” phase: the phase that it only achieves when the Moon passes between the Earth and the Sun.

Getting a new Moon to coincide with the solstice isn’t so rare: about 3.4% of all solstices will have a new Moon occurring within 12 hours of that moment of maximal tilt. That’s true for any specific phase you care to examine.

And given that we have two solstices a year, this means that 13.5% of all years will either have a new Moon or a full Moon on one of the solstices.

We came close to having new Moons on the solstice in 2014 and 2006, but each time we were offset by approximately 24 hours instead of 12 hours. You have to go all the way back to 2001 to find a new Moon that coincided with the solstice, which it did on June 21 of that year.

That solstice new Moon in 2001 also occurred with a total solar eclipse.

Photographed from many locations in Africa, the last solstice solar eclipse occurred on June 21, 2001, and was a total solar eclipse. There was a prior solstice solar eclipse in 1982, and there will be solstice solar eclipses again in 2039, and 2058.

So can you see the pattern?

The solar eclipse occurring on the June solstice of this year, 2020, occurs precisely 19 years after the last solar eclipse on a solstice: June 21, 2001.

Again there was a solar eclipse on June 21, 1982. There will be another solar eclipse on June 21, 2039. And jumping ahead by another 19 years, there’s still another solstice solar eclipse in our future on June 21, 2058.

In order to have a solar eclipse, you don’t just need a new Moon; you need a new Moon to occur precisely when the Moon crosses the Earth-Sun plane. The Earth traces an elliptical path around the Sun, and the Moon makes an elliptical orbit around the Earth, but those two ellipses are tilted with respect to one another. It isn’t much of a tilt ⁠— just 5.2° ⁠— but the Sun and the Moon are small enough as seen from Earth that most new Moons don’t result in any sort of eclipse at all.

With the Sun and Moon each making a circle about 0.5° in diameter on the sky, only about one in five or six new Moons results in either a partial, annular, or total eclipse of the Sun. In the 20th century, for example, there were a total of 1,237 new Moons and a total of 228 solar eclipses: about 18% of all new Moons.

When the Earth’s north pole is maximally tilted away from the Sun, it’s maximally tilted towards the full Moon, on the opposite side of the Earth, while when your hemisphere of the Earth is maximally tilted towards the Sun, it’s maximally tilted away from the full Moon. The Moon stabilizes our orbit but also slows the Earth’s rotation. The total inclination of the Moon’s orbit, of 5.1°, represents the maximum possible variance from a perfect Moon-Earth-Sun alignment.

So what should this mean for solar eclipses on the solstice? If we take into account that:

a new Moon occurs every 29.53 days,
there are two solstices each year,

3.4% of all solstices will experience a new Moon,

and that ~18% of all new Moons will result in a solar eclipse,

we can do the sums and determine that, on average, a solar eclipse should occur on a solstice just once every 82 years.

So back then to our apparent 19 year pattern. Why did we have one on the June solstice in 1982 and 2001; why are we having one in 2020; why will we have one in 2039 and again in 2058?

It’s simply that like everything involving the motion of bodies in our Solar System, events like these come in cycles.

This particular phenomenon of a 19 year eclipse cycle is known as a Metonic cycle, dating all the way back to Meton of Athens nearly 2500 years ago.

For every 19 years that pass, almost exactly 235 lunar months (full cycles, from new Moon to new Moon) pass as well, meaning that eclipses recur in a 19 year periodic cycle.

Well, maybe but there is a hitch!

For every 19 years that pass, we experience almost 235 lunar months: we fall short by just 72 minutes. So when we do get an eclipse on the solstice, we will get a number of them in a row with this 19 year period. But after enough of those “72 minutes” add up, we end up out of sync with the solstice once again.

We’re actually very lucky to be alive right now, when we’re experiencing a period where five solstice eclipses happen all in a row.

The June solstice of 1963 did not have one and neither will there be one on the June solstice in 2077. The eclipse cycles drift ever so slightly relative to our annual calendar over time, and this pulls events in and out of this 19 year pattern.

If you want to know when the last solstice solar eclipse was prior to 1982, you have to ego all the way back to the December 22, 1870 total solar eclipse.

And looking forward after the 2058 solstice solar eclipse, you need go all the way to the partial solar eclipse of December 22, 2242.

We’re actually very lucky to be alive right now, when we’re experiencing a period where five solstice eclipses happen all in a row. There wasn’t one, however, on the June solstice in 1963. Nor will there be one on the June solstice in 2077. The eclipse cycles drift ever so slightly relative to our annual calendar over time, and this pulls events in and out of this 19 year pattern.

If you want to know when the last solstice solar eclipse was prior to 1982, you have to extrapolate all the way back to the December 22, 1870 total solar eclipse. If you want to know when the next one will be after the 2058 solstice solar eclipse, you need to jump forward all the way to the partial solar eclipse of December 22, 2242. There will be no solar eclipses on any solstice for almost 200 years once 2058 passes us by.

There will be no solar eclipses on any solstice for almost 200 years once 2058 passes us by.

Donna the Astronomer

I am a keen astronomer lucky enough to live and work in Coonabarabran the Astronomy Capital of Australia! I am a ‘Drover’s Brat’ and discoverer of a couple of comets and asteroids. I operate Milroy Observatory and can show you how to best integrate dark sky experiences into your tourism, farm stay or AirBnB business.

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