What Color Is the Moon? A Simple Science Project For Sunday Night’s Eclipse

There are many ways to enjoy tomorrow night’s total lunar eclipse. First and foremost is to sit back and take in the slow splendor of the Moon entering and exiting Earth’s colorful shadow. You can also make pictures, observe it in a telescope or participate in a fun science project by eyeballing the Moon’s brightness and color. French astronomer Andre Danjon came up with a five-point scale back in the 1920s to characterize the appearance of the Moon during totality. The Danjon Scale couldn’t be simpler with just five “L values” from 0 to 4:

L=0: Very dark eclipse. Moon almost invisible, especially at mid-totality.
L=1: Dark Eclipse, gray or brownish in coloration. Details distinguishable only with difficulty.
L=2: Deep red or rust-colored eclipse. Very dark central shadow, while outer edge of umbra is relatively bright.
L=3: Brick-red eclipse. Umbral shadow usually has a bright or yellow rim.
L=4: Very bright copper-red or orange eclipse. Umbral shadow has a bluish, very bright rim.

The Danjon Scale is used to estimate the color of the totally eclipsed moon. By making your own estimate, you can contribute to atmospheric and climate change science. Credit: Alexandre Amorim

The last few lunar eclipses have been bright with L values of 2 and 3. We won’t know how bright totality will be during the September 27-28 eclipse until we get there, but chances are it will be on the bright side. That’s where you come in. Brazilian amateur astronomers Alexandre Amorim and Helio Carvalho have worked together to create a downloadable Danjonmeter to make your own estimate. Just click the link with your cellphone or other device and it will instantly pop up on your screen.

On the night of the eclipse, hold the phone right up next to the moon during mid-eclipse and estimate its “L” value with your naked eye. Send that number and time of observation to Dr. Richard Keen at richard.keen@colorado.edu. For the sake of consistency with Danjon estimates made before mobile phones took over the planet, also compare the moon’s color with the written descriptions above before sending your final estimate.

Graph showing the change in heating of the ground in fractions of degrees (vertical axis) as affected by volcanic eruptions and greenhouse warming since 1979. The blue shows volcanic cooling, the red shows greenhouse warming. Notice the rising trend in warming after 1996. Credit: Dr. Richard Keen

Keen, an emeritus professor at the University of Colorado-Boulder Department of Atmospheric and Oceanic Sciences, has long studied how volcanic eruptions affect both the color of the eclipsed moon and the rate of global warming. Every eclipse presents another opportunity to gauge the current state of the atmosphere and in particular the dustiness of the stratosphere, the layer of air immediately above the ground-hugging troposphere. Much of the sunlight bent into Earth’s shadow cone (umbra) gets filtered through the stratosphere.

Volcanoes like Mt. Pinatubo, which erupted in June 1991 in the Philippines, inject tremendous quantities of ash and sulfur compounds high into the atmosphere, where they can temporarily block sunlight and cause a global drop in temperature. Credit: USGS

Volcanoes pump sulfur compounds and ash high into the atmosphere and sully the otherwise clean stratosphere with volcanic aerosols. These absorb both light and solar energy, a major reason why eclipses occurring after a major volcanic eruption can be exceptionally dark with L values of “0” and “1”.

The moon was so dark during the December 1982 eclipse that Dr. Keen required a 3-minute-long exposure at ISO 160 to capture it. Credit: Richard Keen

One of the darkest in recent times occurred on December 30, 1982 after the spectacular spring eruption of Mexico’s El Chichon that hurled some 7 to 10 million tons of ash into the atmosphere. Sulfurous soot circulated the globe for weeks, absorbing sunlight and warming the stratosphere by 7°F (4°C).

Lithograph from the German astronomy magazine Sirius compares the dark, featureless lunar disk during the 1884 eclipse a year after the eruption of Krakatoa (left) with a bright eclipse four years later, after the volcanic aerosols had settled out of the stratosphere (right).

Meanwhile, less sunlight reaching the Earth’s surface caused the northern hemisphere to cool by 0.4-0.6°C. The moon grew so ashen-black during totality that if you didn’t know where to look, you’d miss it.

Two photos of Earth’s limb or horizon from orbit at sunset before and after the Mt. Pinatubo eruption. The top view shows a relatively clear atmosphere, taken August 30,1984. The bottom photo was taken August 8, 1991, less than two months after the eruption. Two dark layers of aerosols between 12 and 15 miles high make distinct boundaries in the atmosphere. Credit: NASA

Keen’s research focuses on how the clean, relatively dust-free stratosphere of recent years may be related to a rise in the rate of global warming compared to volcano-induced declines prior to 1996. Your simple observation will provide one more data point toward a better understanding of atmospheric processes and how they relate to climate change.

This map shows the Moon during mid-eclipse at 9:48 p.m. CDT. Selected stars are labeled with their magnitudes. Examine the Moon backwards through binoculars and find a star it most closely matches to determine its magnitude. If for instance, the Moon looks about halfway in brightness between Hamal and Deneb, then it’s magnitude 1.6. Click to enlarge. Source: Stellarium

If you’d like to do a little more science during the eclipse, Keen suggests examining the moon’s color just after the beginning and before the end of totality to determine an ‘L’ value for the outer umbra.  You can also determine the moon’s overall brightness or magnitude at mid-eclipse by comparing it to stars of known magnitude. The best way to do that is to reduce the moon down to approximately star-size by looking at it through the wrong end of a pair of 7-10x binoculars and compare it to the unreduced naked eye stars. Use this link for details on how it’s done along with the map I’ve created that has key stars and their magnitudes.

The table below includes eclipse events for four different time zones with emphasis on mid-eclipse, the time to make your observation. Good luck on Sunday’s science project and thanks for your participation!

Eclipse Events Eastern Daylight Time (EDT) Central Daylight Time (CDT) Mountain Daylight Time (MDT) Pacific Daylight Time (PDT)
Penumbra first visible 8:45 p.m. 7:45 p.m. 6:45 p.m. 5:45 p.m.
Partial eclipse begins 9:07 p.m. 8:07 p.m. 7:07 p.m. 6:07 p.m.
Total eclipse begins 10:11 p.m. 9:11 p.m. 8:11 p.m. 7:11 p.m.
Mid-eclipse 10:48 p.m. 9:48 p.m. 8:48 p.m. 7:48 p.m.
Total eclipse ends 11:23 p.m. 10:23 p.m. 9:23 p.m. 8:23 p.m.
Partial eclipse ends 12:27 a.m. 11:27 p.m. 10:27 p.m. 9:27 p.m.
Penumbra last visible 12:45 a.m. 11:45 p.m. 10:45 p.m. 9:45 p.m.
Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. I also write a daily astronomy blog called Astro Bob. My new book, "Wonders of the Night Sky You Must See Before You Die", a bucket list of essential sky sights, will publish in April. It's currently available for pre-order at Amazon and BN.

Recent Posts

After Swirling Around a Black Hole, Matter Just Falls Straight In

The physics surrounding black holes is just plain weird. A gravitational well so strong that…

23 hours ago

The Habitable Worlds Observatory Could See Lunar and Solar ‘Exo-Eclipses’

A future space observatory could use exo-eclipses to tease out exomoon populations.

1 day ago

New Shepard’s 25th Launch Carries Six to the Edge of Space and Back

Sending tourists to space is still relatively novel in the grand scheme of humanity's journey…

2 days ago

That Recent Solar Storm Was Detected Almost Three Kilometers Under the Ocean

On May 10th, 2024, people across North America were treated to a rare celestial event:…

2 days ago

More Evidence for the Gravitational Wave Background of the Universe

The gravitational wave background was first detected in 2016. It was announced following the release…

4 days ago

When Uranus and Neptune Migrated, Three Icy Objects Were Crashing Into Them Every Hour!

The giant outer planets haven’t always been in their current position. Uranus and Neptune for…

4 days ago