Thoughts on a partial lunar eclipse

On the night of July 16/17, 2019, I watched — for the first time in decades — a partial lunar eclipse through an 8-inch telescope. Usually, I have monitored the visual phenomenon with either my naked eyes or binoculars. But the change cast some new light onto my impressions of Earth’s shadow.

Penumbral differences

I was particularly surprised by how difficult it was to perceive Earth’s penumbra (its lighter outer shadow) during the early stages of this 65-percent partial eclipse. The brilliance of the Full Moon is formidable through an 8-inch telescope without a filter. And although I knew some penumbral phases of partial eclipses go unseen, the shadow’s smoky yellow presence made itself obvious only about 25 minutes prior to the appearance of Earth’s umbra (its darker inner shadow), or about 70 percent into the penumbral phase.

While this is within the norm, I found the situation during the shadow’s egress different. Namely, I found it much easier to follow the penumbra’s departure almost to the Moon’s edge. My guess is that it is easier to follow something dark becoming lighter (because your eye and brain can gradually accustom to the brightening shadow) than something overpoweringly bright becoming partially dimmed.

However, I also noticed something different in my observing behavior: During penumbral ingress, I focused my attention only on the section of the Moon that was to receive the shadow — just as one would while awaiting the first nibble out of a solar eclipse. During egress, however, my eye did not stop comparing the shadowed region with the opposing bright lunar limb — so I was continually evaluating the shifting relative brightness of the two regions. Image 2 shows how the penumbral shading was apparent in images through my 3-inch refractor 50 percent into the penumbral phase, and other images show it earlier still.

Umbral irregularities

For the first five minutes after Earth’s umbral shadow slipped onto the lunar disk, it appeared as a small semicircular dark path surrounded by a dark gray aureole. Five minutes later, the central dark patch and aureole reversed intensities, with the core appearing brighter than its surrounding ring. Another five minutes into the event, and the entire umbral shadow was a uniform dark gray surrounded by an equally uniform collar of lighter gray. The reverse-Life Saver effect was a mystery, though I believe it to be a contrast effect between the bright highlands and the dark maria as the shadow moved over the region.

What was most apparent as the eclipse deepened was that the leading edge of the umbra appeared bluer than the deeper reddish umbral section. This is typical of an eclipse where the Danjon value (a scale that evaluates the color of total lunar eclipses) equals 4, which occurs when Earth’s atmosphere is clear of contaminants (such as volcanic aerosols). The blue collar, as shown in Image 3, is caused by light passing through the clear upper stratosphere and penetrating the ozone layer, which absorbs red light, thus making that section of the shadow appear bluer. Close inspection also revealed the deepest penumbral shadow causing a yellow band to appear beyond the blue one.

There was also something else at play, which made it difficult to judge the overall intensity of the shadow’s color. In Image 3, note how the southern highlands (at the top) appear so much brighter than the remainder of the disk in shadow. This is not due to irregularities in Earth’s atmosphere, but rather due to albedo effect. Lunar highlands reflect twice as much light as the darker maria, which dominate the shadowed region.

This same phenomenon caused the first half of the umbral eclipse to appear darker than the second half. As Image 4 shows, the reason for the intensity difference is largely due to the greater amount of highland material appearing in the deeper (redder) part of the shadow during the second half of the eclipse. 

One other aspect of the eclipse surprised me. To my naked eyes and through binoculars, the edge of the Moon’s shadow appeared irregular (a contrast illusion created as the shadow covers the irregular boundaries of the maria), as you can see in Image 5 taken through a 3-inch telescope at prime focus. Through the 8-inch, however, the shadow’s edge remained smooth throughout the eclipse, as it should.

Send your thoughts on any lunar eclipse to sjomeara31@gmail.com.