February Full Moon Enters Earthly Penumbra for 2017’s First Lunar Eclipse

Summary: The February full moon passes through the Earthly penumbra at 22:34 Coordinated Universal Time, Feb. 10, for 2017’s first lunar eclipse.

“Geometry of a Lunar Eclipse”: Sagredo, Public Domain, via Wikimedia Commons

The February full moon passes through the Earthly penumbra, beginning Friday, Feb. 10, at 22:34 Coordinated Universal Time (5:34 p.m. Eastern Standard Time), for 2017’s first lunar eclipse.

The February 2017 full moon’s passage through Earth’s faint, outer shadow, known as penumbra, occasions a penumbral lunar eclipse. The penumbra, which only blocks a portion of sunlight, subtly darkens the lunar surface.

A lunar eclipse only occurs during the moon’s full phase. The alignment of sun, Earth and moon, with Earth in the middle, that characterizes the full moon phase is responsible for the shadow cast upon the lunar surface by Earth’s partial or total blockage of sunlight.

The full phase does not trigger monthly lunar eclipses because of the tipped lunar orbit. The moon’s orbit is tipped about 5 degrees above or below the plane of Earth’s orbit around the sun. The orbital tip accounts for mostly misses of Earth’s shadows by the moon.

Also, for an eclipse to occur, the full moon needs to be near one of the two nodes, or points, where the lunar orbit intersects the plane of Earth's orbit around the sun. Two to four times each year, the full moon's nearness to a node catches the dark inner umbral shadow or subtle outer penumbral shadow cast by Earth during syzygy, an alignment of the three celestial bodies (sun, Earth, moon).

Penumbral lunar eclipses are rarely total. In a total penumbral lunar eclipse, the entire moon enters the penumbra without touching the umbral shadow. In 2017, the February full moon does not experience a total penumbral eclipse.

The 21st century, which spans Jan. 1, 2001, to Dec. 31, 2100, claims 86 penumbral eclipses. According to retired NASA astrophysicist Fred Espenak, known popularly as Mr. Eclipse, total penumbral eclipses, numbering just five, only account for 5.8 percent of the century’s penumbral eclipses. The first of the 21st century’s five total penumbral eclipses took place Tuesday, March 14, 2006. The remaining four occur in the last half of the century: Friday, Aug. 29, 2053; Friday, April 25, 2070; Saturday, Aug. 8, 2082; Tuesday, Sep. 29, 2099.

The penumbral eclipse magnitude expresses the fractional coverage of the lunar diameter by the Earthly penumbra. A penumbral magnitude of less than 1 indicates coverage of a small portion of the lunar diameter by the Earthly penumbra.

The penumbral magnitude for the February 2017 full moon during the penumbral lunar eclipse is 0.9884. The penumbral magnitude for the preceding penumbral lunar eclipse, which took place Friday, Sept. 16, 2016, was 0.9080.

Almost three years pass before the occurrence of the next penumbral lunar eclipse after February 2017’s event. On Friday, Jan. 10, 2020, the penumbral lunar eclipse has a penumbral magnitude of 0.8956.

Mr. Espenak’s identifies the total penumbral eclipse that occurs Friday, April 25, 2070, as the 21st century’s largest penumbral lunar eclipse. It has a penumbral magnitude of 1.0515.

The 21st century’s smallest penumbral lunar eclipse, which is not total, takes place Sunday, July 18, 2027. Its penumbral magnitude is 0.0014.

The February 2017 penumbral lunar eclipse happens four days after the month’s perigee, or closest center-to-center distance between Earth and moon. On Monday, Feb. 6, the waxing gibbous moon marked a perigee of 368,816 kilometers (229,171.638 miles).

Four eclipses, comprising two lunar and two solar, take place in 2017. The Feb. 10 penumbral lunar eclipse is succeeded a little over two weeks later, on Sunday, Feb. 26, by an annular solar eclipse.

In August, the year’s second set of lunar and solar eclipses happens. On Monday, Aug. 7, a partial lunar eclipse takes place. Two weeks later, a total solar eclipse, named the Great American Eclipse, takes place Monday, Aug. 21.

The takeaway for the February full moon as opener of 2017’s eclipse lineup, via its passage through the Earthly penumbra, is the penumbral lunar eclipse’s subtle visibility and its placeholder status as marking almost three years of no penumbral lunar eclipses until January 2020.

chart of the lunar path through Earth's shadow during the February 2017 penumbral lunar eclipse: Tom Ruen (SockPuppetForTomruen), Public Domain, via Wikimedia Commons