The July 4th-5th Penumbral Lunar Eclipse

A lunar eclipse is expected on July 4th, 2020, though it won’t be a spectacular show as we’ve seen in the past with total lunar eclipses (“blood” moons).

Why? This lunar eclipse will occur before your eyes and will barely have any visible impact for most – a penumbral lunar eclipse.

Defined literally, an eclipse is: “An obscuring of the light from one celestial body (in this case, the moon) by the passage of another (the Earth) between it and the observer, or between it and its source of illumination (the sun),” according to the Oxford Dictionary.

Diagram: A Total Lunar Eclipse (the Umbra covering the moon) versus a Penumbral Lunar Eclipse (the Penumbra covering the moon) (timeanddate.com)
A Total Lunar Eclipse (the Umbra covering the moon) versus a Penumbral Lunar Eclipse (the Penumbra covering the moon) (timeanddate.com)

Quite often, with an impending eclipse of the moon looming, we are told that the moon will be eclipsed by the shadow of the Earth. But to be strictly precise, the Earth casts not one but two shadows out into space. The most noticeable is a dark, slender, tapering cone of darkness that extends out into space for some 857,000 miles (1.38 million kilometers), called the umbra. Surrounding the umbra is the penumbra, or partial shadow, also conical but much larger. 

In a total eclipse of the moon, the inner part of Earth’s shadow, called the umbra, falls on the moon’s face. At mid-eclipse, the entire moon is in shadow, which may appear blood red.

In a partial lunar eclipse, the umbra takes a bite out of only a fraction of the moon. The dark bite grows larger, and then recedes, never reaching the total phase.

In a penumbral lunar eclipse, only the more diffuse outer shadow of Earth – the penumbra – falls on the moon’s face. This third kind of lunar eclipse is much more subtle and much more difficult to observe than either a total or partial eclipse of the moon. There is never a dark bite taken out of the moon, as in a partial eclipse. The eclipse never progresses to reach the dramatic minutes of totality. At best, at mid-eclipse, very observant people will notice a dark shading on the moon’s face. Others will look and notice nothing at all.

Stages of the Eclipse

Astrophysicist Gianluca Masi of The Virtual Telescope Project in Italy captured these images of the penumbral lunar eclipse on Feb. 10-11, 2017. The July 2020 penumbral eclipse won't be this prominent, our columnist writes. (Image credit: Gianluca Masi/The Virtual Telescope Project)
Astrophysicist Gianluca Masi of The Virtual Telescope Project in Italy captured these images of the penumbral lunar eclipse on Feb. 10-11, 2017. The July 2020 penumbral eclipse won’t be this prominent, our columnist writes. (Image credit: Gianluca Masi/The Virtual Telescope Project)

Penumbral eclipse begins: 11:07 PM AST July 4th (0307 GMT)
Maximum eclipse: 12:30 AM AST July 5th (0430 GMT)
Penumbral eclipse ends: 1:52 AM AST July 5th (0553 GMT).

On Saturday night, the moon will skim partially through the southern part of the penumbra.  That means the upper part of the moon will be inside the penumbral shadow. However, the geometric magnitude of this eclipse is listed as 0.3546. That means that just over one-third of the moon’s diameter will be inside of the so-called “half-shadow” at the time of the maximum eclipse.

In general, most people don’t notice the penumbral shadow projected on the moon until at least 70% of its diameter is covered. Some people who have very acute vision and better-than-average perception might notice an ever-so-slight shading when only 50% of the moon is inside the penumbra.  

But in the case of Saturday night, the obscuration amounts to just a tad over 35%; not enough to make any kind of visual impact.

If you really want to see a penumbral eclipse that will make an impact (sort of), you will not have too long to wait. 

On Nov. 30, very early that Monday morning (the predawn hours) another penumbral lunar eclipse will take place. However, on this occasion the geometric magnitude will be 0.8285, or more than four-fifths of the moon’s diameter will be inside of the penumbra at maximum eclipse, likely resulting in a noticeable shading of the upper portion of the moon.

The Weather Forecast

On Saturday night, a high-pressure system remains across Trinidad and Tobago. Thus, mostly settled and clear skies are forecast, though mild to moderate Saharan Dust will remain present across the region, obscuring your view slightly.

The one possible hindrance – the difficult to forecast low-level cloud patches, but these partly cloudy periods and isolated showers will be brisk. Even if you aren’t able to see the penumbral eclipse, it’s still a great full moon to gaze upon!

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