As the Earth moves around the Sun, it also spins on its axis. Twice a year, due to its 23.5-degree tilt, the Earth reaches points where it is maximally tilted either towards or away from the Sun—these moments are known as summer and winter solstices. Throughout this annual cycle, the Moon orbits the Earth, passing through the Earth-Sun plane and showcasing its phases, from new to full.

On June 20, 2020, the summer solstice will occur as the northern hemisphere is tilted most towards the Sun. Just nine hours later, the Moon will align perfectly between the Earth and the Sun, leading to an annular solar eclipse visible to many across Africa and Asia. Such a solstice solar eclipse is incredibly rare, and this year's occurrence has a scientific explanation behind it.

Solstices happen twice annually, while the Moon completes its cycle of phases roughly every 29.53 days. For a solar eclipse to happen, the Moon must be in its "new" phase, which occurs when it passes between the Earth and the Sun.

It's not uncommon for a new Moon to coincide with a solstice; approximately 3.4% of all solstices will see a new Moon within 12 hours of the peak tilt. This percentage holds true across different lunar phases.

3.4% of solstices will align with a first-quarter Moon.

3.4% will align with a full Moon.

3.4% will align with a last-quarter Moon.

With two solstices each year, this means that about 13.5% of all years will have either a new or full Moon on a solstice.

This frequency is significant! A new or full Moon occurs on a solstice roughly every 7 to 8 years. For instance, we just missed a full Moon on the December 2018 solstice, which occurred less than 24 hours after the peak tilt.

The last full Moon on a solstice happened in June 2016, while before that, it was December 2010.

In terms of new Moons, we nearly had them on the solstices in 2006 and 2014, but were off by about 24 hours each time. The last time a new Moon coincided with a solstice was on June 21, 2001.

Remarkably, that solstice new Moon in 2001 also aligned with a total solar eclipse.

This year's eclipse on the June solstice occurs exactly 19 years after the last one—June 21, 2001. You may wonder if there's a repeating pattern, prompting you to look at the years before and after in 19-year increments.

Indeed, there was another solar eclipse on June 21, 1982, and there will be another on June 21, 2039, followed by yet another in 2058.

However, this pattern may seem too good to be true. A solar eclipse occurring every 19 years is not what we should expect. Here's why.

To achieve a solar eclipse, a new Moon must happen precisely as the Moon crosses the Earth-Sun plane. Although the Earth follows an elliptical path around the Sun, the Moon's orbit around the Earth also follows an elliptical pattern, with a slight tilt of 5.2° between the two orbits. This tilt means that most new Moons do not lead to any eclipse.

Since the Sun and Moon each occupy about 0.5° in the sky, only about one in five or six new Moons results in a solar eclipse—partial, annular, or total. Throughout the 20th century, there were 1,237 new Moons and 228 solar eclipses, indicating that roughly 18% of all new Moons led to eclipses.

Considering these factors for solar eclipses at solstices:

• A new Moon occurs every 29.53 days.
• Two solstices occur yearly.
• 3.4% of solstices will witness a new Moon.
• Approximately 18% of new Moons will result in solar eclipses.

Calculating these figures, we find that a solar eclipse should happen on a solstice approximately once every 82 years.

So, why are we experiencing them every 19 years? Why did the June solstices of 1982, 2001, and 2020 have eclipses, with more to come in 2039 and 2058?

The answer lies in the cyclical nature of celestial events. This phenomenon of solar eclipses recurring every 19 years is known as the Metonic cycle, named after Meton of Athens from nearly 2500 years ago. In this cycle, approximately 235 lunar months pass every 19 years, which results in the periodic recurrence of eclipses.

However, there is a slight difference. Every 19 years accounts for almost 235 lunar months—falling short by just 72 minutes. Consequently, when a solar eclipse occurs on a solstice, several may happen consecutively every 19 years. Eventually, the accumulated "72 minutes" will misalign us with the solstice again.

Currently, we are fortunate to witness a series of five solstice eclipses consecutively. However, there was none in June 1963, nor will there be one in June 2077. The eclipse cycles gradually drift relative to our calendar, causing events to fall in and out of the 19-year pattern.

To find the last solstice solar eclipse before 1982, you would have to go back to December 22, 1870. The next one after the 2058 eclipse will occur on December 22, 2242. Following that, nearly 200 years will pass without a solar eclipse on any solstice.

Why do eclipses not merely shift gradually? Why do they fall out of sync with our yearly calendar?

This misalignment occurs because the nodes of the Moon's orbit—where it crosses the Earth-Sun plane—happen with a different frequency than the Moon's phases. The Moon traverses each node every 27.22 days, distinct from the 29.53 days of the lunar phase cycle.

However, when considering 19-year periods, both cycles nearly align perfectly. Nineteen calendar years equal approximately 235 lunar phases (with a 72-minute shortfall) and 255 node-crossing cycles known as draconic months. The node-crossing cycles, however, miss this periodicity by about 12 hours. After several eclipse cycles, they become misaligned enough to no longer coincide with the same time of year.

A solar eclipse that aligns with the solstice is a rare event, yet we are currently witnessing them relatively frequently. Over a long period, a solstice solar eclipse is expected to occur about once every 82 years—poor odds for any individual. Nevertheless, due to the clustering of these events every 19 years, we saw one in 1982, will see one in 2001, have one in 2020, and anticipate more in 2039 and 2058, all on the June solstice.

Following that, a long drought will ensue. For 184 years after the 2058 event, Earth will not experience another solstice solar eclipse. After a lengthy gap, a partial solar eclipse will return on December 22, 2242. Subsequent eclipses will occur every 19 years, followed by another drought until 2373, when a total solar eclipse will take place on the June solstice. Enjoy the series of June solstice solar eclipses while they last; after 2058, we will wait nearly two full centuries for their return.