From the December 2018 issue

Will there be blood?

Totality means something totally different during a lunar eclipse.
By | Published: December 14, 2018 | Last updated on May 18, 2023
The most awesome experiences — in or out of astronomy — make silence impossible. Shouts and gasps accompany the precious minutes of totality during a solar eclipse, or a brilliantly animated aurora, or a sudden exploding meteor when it cleaves the night in a shower of sparks.

A lunar eclipse does no such thing to most people. For this reason — to avoid overselling the event — one might use an adjective like “worthy” or “favorable” rather than “mind-blowing.” Still, eclipses of the Moon are fascinating enough, and they shouldn’t be missed.

The total lunar eclipse on the night of January 20/21 is very favorable. The entire event is visible from virtually all of North, Central, and South America. In most places, the Full Moon is very high up, with the partial phases of the eclipse beginning just after 10:30 P.M. EST and around dinnertime on the West Coast.

As the Moon pushes farther into Earth’s shadow at 2,290 mph (3,685 km/h), many describe the Moon’s quickly changing appearance as “a series of phases,” but it doesn’t look at all like the lunar shapes seen each month. These phases are alien. They’re seen on no other occasion. The weirdest occur during the half-hour before totality — shadows that slash diagonally across the Moon’s face, rather than the curved crescents we expect. If you must choose one viewing window to watch during the eclipse, it should be then.

The Danjon scale rates the color of the Moon during a lunar eclipse, from nearly black to rust red to copper-orange.
Astronomy: Roen Kelly
When the eclipse is total an hour later, the shadow has changed from the inky black at the beginning of the event to a coppery red. It’s beautiful. Anyone on the Moon would see the black cameo of a “New Earth” surrounded by a brilliant crimson circle: all our planet’s sunrises and sunsets combined into a single red ring.

There’s a dramatic difference between totality during a solar eclipse and the same word when applied to the Moon. Solar totality is almost a frantically intense affair, typically lasting two to four minutes. Many phenomena suddenly and briefly appear. Should you gaze at the delicate, lacy magnetic lines of the corona? The hot pink “flames” of prominences? Look for Mercury and Venus in the sky? Use binoculars? Watch the reactions of the stunned people around you? Check out the distant horizons where the Sun is still weirdly shining? Take pictures using exposure bracketing? There’s not much time for too many things.

But lunar totality lasts an hour. And nothing is happening. OK, from rural sites, the sky is now dramatically starrier than when it was washed away by the Full Moon an hour earlier. But besides that, what useful observations can you make?

The traditional answer is to determine lunar totality’s color and darkness.

When you think about it, there aren’t many celestial objects that change color. It’s a shame, since we astronomers are chronically tint-deprived. In dim light, only our retinal rod cells operate, and they’re colorblind. It’s why I consider colorful targets a don’t-skip part of every public viewing session. Otherwise it’s a firmament of off-whites. 

Sirius’ gorgeous saturated prismatic colors do take one’s breath away. Or, move your eyes from Sirius’ blue to Betelgeuse’s red. Of course, if we used a paint store color swatch, we’d have to admit that those stars are actually diamond (blue-white) and pastel yellow-orange — pumpkin diluted with white. But now on January 20, the Moon will have real color, deep color.

Or maybe it won’t. In 1921, André-Louis Danjon invented a five-point scale (which runs from 0–4) for judging lunar totalities because he believed the color varies with different parts of the sunspot cycle.

It’s still unclear whether he was right. But earthly atmospheric pollutants, such as volcanic dust, definitely affect the Moon’s appearance. After Mount Pinatubo erupted in the Philippines on June 15, 1991, the total eclipse of the Moon 18 months later was almost black. Danjon would have rated it a 0. And our skygazing group did just that as we watched from a hilltop outside of Woodstock, New York.

Backyard astronomers used to frequently contribute such appraisals, writing, “I rate this eclipse a Danjon 3,” which meant it was brick red with a yellow edge to the shadow. In the 2010s, we had some eclipses that looked like a 4, meaning a bright copper red or orange center with a bluish rim.

Yes, bluish.

A Blue Moon? No joke. It’s definitely more likely this month than the “Blood Moon” term you’ll see in the mass media, since it’s between rare and impossible for Earth’s shadow to appear dark red like hemoglobin. But hey, maybe it’ll happen. And there you have it — the eclipse’s exact color and darkness will be a definite, surefire “unknown” that’s far from oversold, coming up on January 20.

You have your work cut out for you. Send us your report.