The Earth’s Shadow… Twice

One of the more sublime sights seen from Mauna Kea is the shadow of the mountain rising through the mist and haze at sunset. One of the more sublime sights in the heavens is the Earth’s shadow crossing the face of the Moon, a total lunar eclipse. It is possible to combine these two phenomena if the timing is right, the Earth’s shadow seen twice.

Eclipse in the Mauna Shadow
The eclipsed moon rising in the shadow of Mauna Kea, February 20, 2008, photo by Alex Mukensnable, used with permission
The moment of totality in a lunar eclipse occurs when the full Moon is directly opposite the Sun in the sky. By simple geometry this same anti-solar point is where the tip of the mountain’s shadow will be projected for an observer standing near the summit of the same mountain. If the eclipse is in progress at sunset, and you are standing on the summit of a suitably prominent mountain, you will see the Earth’s shadow both in the sky and obscuring the Moon.

Back on Feb 20th, 2008 the timing was right. A friend of mine, Alex Mukensnable, noted the timing and set up to catch the eclipsed Moon rising in the shadow. The result was a great set of photos. The photo is nice enough as a still, be he did more than that, he shot the event as a timelapse and assembled a video of the rising Moon.

There are several possible variations depending on the timing with this sort of event… If the Moon was at the height of totality rising it would also be right at the tip of the mountain shadow as it rises. This is a relatively rare event as the timing requirements are tight. Unfortunately it would also be quite dim, darkened by the shadow, and not easily seen as it rose.

As lunar eclipses are long events, taking several hours to complete, the likelihood of the Moon being in at least the partial eclipse phase at sunset is fairly good. Still a rare event, but not extraordinarily so. Thus for a single site, the summit of Mauna Kea this even happens in both 2008 and 2015.

As the Earth’s shadow is about 2.6° across at lunar orbit, the closest a partially eclipsed Moon will be seen from the the very tip of the shadow is about 1.3°. This is a bit less than three lunar diameters. Of course these numbers will vary a few percent depending in the distance to the Moon which changes as it makes its elliptical orbit.

The Moon moves slowly across the sky from west to east, thus before totality the Moon would be above the tip of the mountain shadow. After totality, with the eclipse ending, the eclipsed Moon will be in the shadow. Placing the Moon in the shadow also makes it easier to see, the bright crescent a better contrast to the dark shadow. This is the case for the 2008 eclipse captured by Alex.

Of course you could reverse all this timing and watch the event at moonset and sunrise. If the eclipse was just starting at dawn it would again place the eclipse in the mountain shadow.

Another important point to remember is that the shape of the mountain’s shadow has little to do with the shape of the mountain. The shadow will always be a neatly conical form due to the effects of projection.

What brings this event back to the fore is that the timing will soon be correct to see this same event again. The total lunar eclipse of Sept 27th, 2015 will be a bit of a dud for Hawaii, most of the eclipse already over as the Sun sets and the Moon rises over the islands. However, this event will feature very similar timing to the 2008 eclipse. The Moon will still be in partial eclipse when it rises. As it rises a short time after full Moon it will again be deep in the shadow of Mauna Kea as it comes over the horizon.

You know where you will find me on the evening of the 27th. Now I just need some clear weather that day.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on the island of Hawaiʻi.

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