Will You See The 4th Of July Penumbral Lunar Eclipse?

The penumbral shadow was very obvious over the top third of the moon before the start of the partial eclipse on Jan. 31, 2018. Bob King

Due to COVID-19 concerns many cities have postponed their 4th of July fireworks displays and celebrations. But as no human hand moves in the heavens, celestial events will go on according to the laws of physics. That includes a penumbra eclipse of the moon.

Total eclipses of the moon and sun are some of the most dramatic sights nature puts on our plate. Total and partial lunar eclipses occur only at full moon when the moon lines up on the opposite side of the sun from the Earth and enters the cone of shadow spilling out behind the planet. At least two and as many as five lunar eclipses can occur in a calendar year but never at every full moon. That’s because the orbit of the moon is tilted 5.1° with respect to Earth’s. During most months the full moon passes above or below Earth’s shadow and we see no eclipse. Only when the lineup is nearly precise does the moon enter Earth’s shadow.

If the sun were a point of light, Earth would only have a single, simple shadow, but because it’s an extended disk, parts of the sun are visible in the half-shaded shadow called the penumbra. This diagram shows you views looking back at Earth from within the penumbra (top) and umbra (bottom). Wikipedia

The Earth casts dual shadows: a dark, inner umbra where the ball of the planet completely blocks the glaring solar disk, and an outer penumbra where the globe only partially blocks the sun, creating a partial shadow. If you could stand in the umbra and look back toward the Earth the sun would be completely blocked from view. But if you stood in the penumbra and looked back, the globe would only partially block the sun — no matter how deeply you stood within the penumbra you’d have some amount of sun in your eyes — more near its edge and less the closer to the umbra.

During a total lunar eclipse the moon passes through the umbra; in a penumbral eclipse it only crosses the outer or penumbral shadow as it will on July 4. NASA

During a partial eclipse the moon glides through the penumbra and partway into the umbra and then departs. In a total eclipse it passes through the penumbra and then fully into the umbra. The moon’s red color during totality is caused by reddened sunlight refracted (bent) around the circumference of the Earth by the atmosphere that spills into the umbra. But during a penumbral eclipse the moon completely misses the umbra (because of its tilted orbit, remember?) and only makes it as far as the penumbra.

Anyone will notice the dark bite the umbral shadow takes from the moon, but the penumbral shadow, filtered by partial sunlight, can be subtle. During a deep penumbral eclipse, when the moon comes close but misses the umbra, the shadow is obvious as a dusky, gray shading covering about half the moon. But if the moon only dips partway into the outer shadow — as it will the night of July 4 — it can be tricky to see.

This diagram shows the moon crossing through the Earth’s penumbra on July 4-5, 2020. Look for a faint, gray shading on the top third of the moon around 11:30 p.m. CDT for the best view.

The eclipse begins at 10:07 CDT (3:07 UT), reaches maximum at 11:30 p.m. and ends at 12:52 a.m. (July 5). At most only 36 percent — about a third — of the moon will dip into the penumbra. Casual viewers won’t notice any change, but I hope you’ll take a look. The best time will be around 11:30 p.m. CDT (12:30 a.m. EDT; 10:30 p.m. MDT and 9:30 p.m. CDT) at maximum penumbral eclipse. Will you see a modest darkening over the upper third of the moon? Yes or no, I’d love to hear about your observation.

The moon before and during penumbral eclipse. Fred Espenak

As with all lunar eclipses, this one will be visible across half the planet but best viewed from the Americas, Western Africa and Western Europe. Click here for a complete coverage map. If you don’t have success with the penumbra this time, you’ll have another go at it on Nov. 29-30 during the year’s 4th and final penumbral eclipse. On that date 74 percent of the moon will wallow in the outer shadow, a sight that should be obvious to the well-heeled skywatcher.

A bright trio of solar system gems lines up on the 4th of July. Stellarium

Whether or not you see the shadow you’ll enjoy seeing July’s Full Buck Moon (or Thunder Moon if you prefer) in the company of two bright planets, Jupiter and Saturn. Clear skies on the 4th!

3 Responses

  1. Edward M Boll

    I hope to know the outcome of Comet Neowise as it exits Perihelion in about 6 more hours. Hopefully the magnitude 2 from 1 is not a certainty. We hope if it fades over the next few weeks as it edges on toward Earth, that that fade is reasonably slow.

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