May 5th’s ‘Teaser’ Lunar Eclipse

A ‘barely there’ lunar eclipse will flirt with the shadow of the Earth Friday evening.

Out watching the Full Flower Moon Friday night? If you’re in the correct hemisphere, you might notice that the shade and hue of the Moon is a bit off its usual pearly white, especially along one edge.

It’s not your imagination: a subtle ‘penumbral lunar eclipse’ is indeed underway, as the Moon passes through the bright outer shadow edge of the Earth.

This is the first of two lunar eclipses (a penumbral and a slight partial) for 2023, and the end of the first eclipse season for the year. While April’s hybrid solar eclipse marking the start of the season wowed observers, a penumbral eclipse is a subtle event. Still, this eclipse has some unique aspects worth watching for.

The eclipse is visible on Friday evening May 5^th for observers in Africa, Europe, Asia and Australia; only North and South America will miss out. The eclipse technically runs from 15:14 Universal Time (UT) until 19:31 UT. At most, expect to see a slight ragged shading on the northeastern limb of the Moon around 17:24 UT.

A line across Europe and northwestern Africa will see the eclipse underway at moonrise in the evening. The Indian Ocean region will see the eclipse high near the zenith, and the Pacific will see it underway at moonset in the early morning hours.

Why Penumbrals Occur

Penumbrals occur when the Moon skirts through the outer bright shadow of the Earth, versus the dark inner umbral shadow where partials and total lunar eclipses occur. It helps to think of the celestial perspective to understand why this twilight show occurs. From the Earthward surface of the Moon, you would see a deep partial solar eclipse, with the Sun not quite obscured.

This eclipse just misses the umbra of the Earth. The Moon crosses its descending node along the ecliptic plane on May 4th at ~22:00 UT; if it had crossed near Full Moon, we’d have a total lunar eclipse, and if it had crossed much farther back, it would have missed the Earth’s shadow entirely.

How close to partial is Friday night’s eclipse? Well, the umbral magnitude (the fraction of the Moon’s central diameter versus the umbra) for this eclipse is -0.0457. The Moon juuust misses the dark inner umbral shadow of the Earth by over an arc minute. This is the closest umbral ‘miss’ since February 11^th, 2017, and narrowest until September 29^th, 2042. In fact, the 2042 eclipse is the closest for the 21^st century, with an umbral magnitude of only -0.0031. This week’s eclipse ranks as the 8^th closest ‘miss’ of the Earth’s umbra for the century.

Eclipses of like geometry (lunar and solar) are part of saros series, with each member occurring 18 years, 10-12 days, (depending on leap years) and 8 hours, or 223 lunar synodic cycles apart. Several saroses are active on any given year, and they’re generally over a millennium long.

Tales of the Saros

This week’s eclipse is also part of lunar saros series 141, which started on August 25^th, 1608 and runs out until October 11^th, 2888 AD. This is also the final penumbral-only eclipse for saros 141 before partial eclipses begin on May 16th 2041 with a shallow 6.5% partial lunar eclipse, the first in the series. Lunar saros 141 produces its very first total lunar eclipse on (mark your calendars) August 1st, 2067 AD.

Can you ‘observe’ a penumbral eclipse? In my experience, you might start to notice a slight tea-colored shading once the Moon is halfway immersed in the penumbra of the Earth. Here’s a fun project: if the Moon is high in the sky, take photos before, during and after the eclipse, using the same camera settings. You can indeed see the difference in the color and shade of the Moon:

Missing the eclipse? Fear not, as North America gets a ‘celestial consolation event’ on the morning of May 17^th. If skies are clear, watch as the slim waning crescent Moon occults (passes in front of) the planet Jupiter. The best seat in the house will be the southwestern United States at dawn.

…and there is indeed at least one live webcast of Friday’s penumbral lunar eclipse: astronomer Gianluca Masi and the Virtual Telescope Project will feature the eclipse live on May 5th starting at 18:45 UT.

Looking onward, the next eclipse is a fine annular ‘ring of fire’ eclipse across the western U.S. Mexico and Central America on October 14^th, 2023. This is followed by another lunar eclipse (a 12% partial) for Europe, Asia and Africa on October 28^th, 2023. The next total lunar eclipse occurs on March 14^th 2025, favoring the Americas.

To be sure, no one travels to the ends of the Earth for a penumbral lunar eclipse… but its always great to see the celestial dance of the Sun, Moon and Earth unfold.