Why is there an annular solar eclipse on the solstice?
There’s about to be an annular solar eclipse—one of the four types of solar eclipse—on the date of the northern hemisphere’s summer solstice.
That will happen at precisely 4:48 UTC on June 21, 2020, just a few hours after the instance of the solstice at 21:44 UTC on June 20, 2020.
Cosmically-speaking, it’s incredibly rare to have a ‘solstice ring of fire solar eclipse’, but for us 21st-century eclipse-chasers, it’s actually pretty common. It’s a story about cycles.
It does depend on what time-zone you’re in, but this coincidence of an annular solar eclipse on the same day as the June solstice will happen again this century. In fact, it happened in 1982 and in 2001, and it will happen again in 2039 and 2058. That’s five solstice eclipses in the space of 76 years—every 19 years—and then they’re gone until the year 2242.
Why do the two cyclical celestial events coincide? Is there any significance? Is there a pattern? The answer is both simple and complex.
“That an eclipse occurs on a selected day like solstice—or your birthday—is just coincidence,” says Robert Nufer, an amateur astronomer from Switzerland who has computed a chart showing how eclipses line-up with solstices and equinoxes.
Why, how and when do solar eclipses happen?
First, we need a short reminder about eclipses and eclipse seasons. Solar eclipses occur, of course, when the Sun, a New Moon, and Earth are lined-up. The reason this doesn’t happen every single New Moon is that the Moon’s orbit of Earth is tilted with respect to the plane the Earth orbits the Sun—the ecliptic. The Moon’s apparent path through our sky is therefore similar, but not the same as, the Sun’s apparent path through our sky. “If the Moon would orbit the Earth in exactly the same plane as the Earth orbits the Sun, solar eclipses would occur every lunar month—every 29.3 days, and two weeks later, a total lunar eclipse would occur,” says Nufer. That doesn’t happen.
The tilt of the Moon’s orbit is around 5º, but its path through our sky does intersect on two sides with the Sun’s path through our sky. They’re called nodes, and when the Moon is near those two nodes during New Moon or full Moon, solar and lunar eclipses, respectively, occur.
OK, so now we know why eclipses happen.
Given that there’s a New Moon every 29.5 days, there are two solstices each year and that about a fifth of New Moons cause some kind of solar eclipse, a ‘solstice eclipse’ ought to occur every 82 years.
The Metonic cycle
Not every 19 years? Cue the Metonic Cycle, a period of 19 years after which the phases of the Moon recur on the same day of the year. That’s 235 orbits of the Moon around Earth. “I have always been fascinated by the Metonic cycle,” says Nufer. “There was an eclipse on June 21 for the summer solstice in 2001 and of course this year in 2020, and the next eclipse on the same date will be in 2039 over the northernmost part of North America.”
The thing is, that 19-year gap is actually an hour and 12 minutes short, which over a few Metonic cycles means that eclipses and solstices eventually stop matching-up—and take many decades to come back into sync.