Enjoying EarthSky? Subscribe.

272,061 subscribers and counting ...

Middle of eclipse season on July 28

Above image via Ray Norris

Today – July 28, 2019 – the middle of the eclipse season comes at 23 hours Coordinated Universal Time (UTC). An eclipse season lasts somewhere around 34 to 38 days, and any full moon or new moon occurring within this period of time will undergo an eclipse.

Because the lunar month (the time period between successive new moons or successive full moons) is approximately 29.5 days long, it’s inevitable that at least one solar eclipse and one lunar eclipse should fall within the confines of any eclipse season. But in about 1 out of 7 eclipse seasons, the first eclipse of the eclipse season comes early enough to allow for a total of 3 eclipses in one lunar month.

The plane of the moon’s orbit is inclined at 5 degrees to the ecliptic (sun’s annual path in front of the constellations of the zodiac). The moon’s orbit intersects the ecliptic at two points called nodes (N1 and N2). It’s the middle of the eclipse season whenever this line of nodes points directly at the sun. In the above diagram, the line of nodes does NOT point at the sun .

That’s exactly what happens during the present eclipse season. The new moon presented a partial solar eclipse on July 13, 2018, followed by a total lunar eclipse on July 27, 2018, and then a second partial solar eclipse on August 11, 2018. That’s a total of 3 eclipses for this lunar month and eclipse season.

Worldwide map of the 2018 July 27 total lunar eclipse via EclipseWise. The full moon passed through the central part of the Earth’s shadow on July 27, 2018, because this full moon happened near the middle of the eclipse season.

Any eclipse near the midpoint of the eclipse season, whether it’s lunar or solar, presents a central eclipse. If it’s a lunar eclipse, it’s a total eclipse of the moon; and if it’s a solar eclipse, it’s either a total or annular eclipse of the sun.

Because the full moon on July 27, 2018, happened near the midpoint of the eclipse season, it staged a total lunar eclipse. Moreover, this full moon closely coincided with apogee – the moon’s farthest point from Earth in its orbit. All added up, that means a particularly long-lasting total lunar eclipse.

Century’s longest total lunar eclipse July 27

Any eclipse, whether it be solar or lunar, that happens near the beginning or the ending of an eclipse season presents a skimpy eclipse at best. If solar, it’ll be a partial eclipse of the sun; if lunar, it’ll be a partial or penumbral eclipse of the moon.

The next time that 3 eclipses will happen in one eclipse season will be 2020. The annular solar eclipse of June 21, 2020 – which occurs near the midpoint of this eclipse season – will be flanked by a penumbral lunar eclipse on June 5 and again on July 5, 2020. Penumbral lunar eclipses are so faint that people often don’t notice them even when they’re taking place.

The midpoint of the eclipse season (when the moon’s nodes align with the Earth and sun) recurs about every 173.3 days; and of the midpoint of one eclipse year (of 2 eclipse seasons) recurs every 346.6 days. But the moon returns to the same phase 6 times in about 177 days and 12 times in about 354 days. So the discordance between the eclipse season and the moon’s phases means a pair of eclipses (not 3 eclipses) during the eclipse seasons in 2019:

One eclipse season later:

January 6, 2019: Partial solar eclipse
January 21, 2019: Total lunar eclipse

Two eclipse seasons (one eclipse year) later:

July 2, 2019: Total solar eclipse
July 16, 2019: Partial lunar eclipse

However, 19 eclipse years (or 38 eclipse seasons) are nearly commensurate with 223 lunar months (223 successive returns to new moon or full moon). This eclipse cycle of 223 lunar months (6585.32 days) is known as the Saros cycle, and depending on the number of intervening leap years, represents a period of about 18 years 11.32 days (4 intervening leap years) or 18 years 10.32 days (5 intervening leap years). Therefore, some 18 years ago, in 2000, and 18 years from now, in 2036, a similar series of 3 eclipses happen in one eclipse season.

The year 2000:

July 1, 2000: Partial solar eclipse
July 16, 2000: Total lunar eclipse
July 31, 2000: Partial solar eclipse

The year 2018:

July 13, 2018; Partial solar eclipse
July 27, 2018: Total lunar eclipse
August 11, 2018: Partial solar eclipse

The year 2036:

July 23, 2036: Partial solar eclipse
August 7, 2036: Total lunar eclipse
August 21, 2036: Partial solar eclipse

The moon’s orbit intersects the ecliptic (Earth’s orbital plane) at two points called nodes. The middle of the eclipse season occurs at the instant that the moon’s line of nodes points directly at the sun, which happens on July 28, 2018. Because 19 eclipse years (38 eclipse seasons) are nearly commensurate to 223 lunar months (6585.32 days), the eclipses coming some 18 years later, in July and August 2036, will showcase 3 eclipses with similar geometries to those taking place in July and August 2018.

Bruce McClure