A very famous quotes states that “Nature loves Change”. Much celestial change tends to come in cycles. One of the most interesting cycles is that of eclipse seasons.
An eclipse season is usually a 35-day period which encapsulates at least two (and possibly three) eclipses. Generally, there are two eclipse seasons in a year. They recur in cycles of 173.3 days.. In some years, it’s possible to have a third eclipse season hedging into the previous or following year; that’s how we get to seven eclipses in some years.
Year 2020 started in the midst of an eclipse season. There’d been a solar eclipse on December 26, 2019, and there was a lunar eclipse on January 10, 2020. We’ll also end 2020 with an eclipse season: a lunar eclipse on November 29-30 and a solar eclipse on December 14.
Another Eclipse season is in June-July 2020. The eclipse season of June and July 2020 will feature three eclipses: a lunar eclipse on June 5-6, a solar eclipse on June 21, and the third eclipse of that eclipse season, a lunar eclipse, on July 4-5. That eclipse season – June and July 2020 – will be the last eclipse season with three eclipses until the year 2029.
Eclipses are all about alignments
Consider that if the moon orbited Earth on the same plane that the Earth orbits the sun, then we’d have a solar eclipse at every new moon, and a lunar eclipse at every full moon.
But – in reality – the moon’s orbit is inclined by 5 degrees to the ecliptic (Earth’s orbital plane), so most of the time the new moon or full moon swings too far north, or south, of the ecliptic for an eclipse to take place. For instance, in the year 2020, we will have 12 new moons and 13 full moons, but only 2 solar eclipses and 4 lunar eclipses
Whenever the lunar nodes point directly at the sun, that momentous event marks the middle of the eclipse season. The alignment of the moon, sun and Earth is most exact when an eclipse happens at the middle of an eclipse season, and the least so when an eclipse occurs at the start, or the end, of an eclipse season. Any lunar eclipse happening early or late in the eclipse season presents a penumbral lunar eclipse, whereas any solar eclipse happening early or late in the eclipse season features a skimpy partial eclipse of the sun.
From the Earth’s perspective, an eclipse season is when the sun is close enough to a lunar node to allow an eclipse to take place. If the sun is close to a node at full moon, we see a lunar eclipse. If the sun is close to a node at new moon, we see a solar eclipse. A minimum of two eclipses (one solar and one lunar, in either order) happens in one eclipse season. A maximum of three eclipses is possible (either lunar/solar/lunar, or solar/lunar/solar).
If an eclipse happens at or near the mid-point of the eclipse season, as the upcoming solar eclipse on June 21st does, then we’ll have a central eclipse. If it’s a solar eclipse, the central eclipse presents either a total or annular eclipse of the sun; or if it’s a lunar eclipse, the central eclipse features a total eclipse of the moon. If the eclipse falls near the beginning or the end of the eclipse season, it’s either a penumbral eclipse of the moon or small partial eclipse of the sun.
Because the lunar eclipses occur so early and so late in the June/July 2020 eclipse season, the lunar eclipses on June 5, 2020, and July 5, 2020, will be extremely faint and hard-to-see penumbral lunar eclipses.