In the shadow of Earth

We are no strangers to the night. Precisely half of our existence takes place in the shadow side of our planet. But as familiar with night as we are, one intriguing facet of it is rarely observable: The shadow we call night is not confined to Earth.

Sun and Earth conspire to create the shadow. Every night every person on the planet is rotated through the base of a narrow cone of darkness cast a million miles into space. Most nights the cone is invisible to us. It disappears into the darkness as a flashlight beam into the day sky. No stars or planets betray its presence. Only one large object in space passes through our shadow: the Moon. And when it does, its transformation is breathtaking.

First, the full Moon’s eastern edge takes on a dusky hue as it approaches Earth’s outer shadow (penumbra). As it passes into the main shadow (umbra), the sharp outline of that edge begins to dissolve into the cosmic void. Then we see something familiar and yet not: a shroud of darkness is slowly pulled over the Moon’s face. This process normally takes four weeks, as the Moon orbits Earth and goes through its phases: full, gibbous, half, crescent, new and back again. Now, during the eclipse, the process takes only six hours.

A total lunar eclipse is striking in other ways: First, the border (terminator) of a normal crescent Moon’s night region has a familiar curvature, dictated by the curvature of the Moon itself. But during a lunar eclipse, the terminator’s curvature is much shallower than usual. It is the shadow of a larger sphere – Earth – projected onto the face of the Moon.

Also unforgettable is the rotundity of the Moon during a total eclipse. A normal full Moon is so bright that it appears two-dimensional, a wafer of searing white pasted onto the sky. In the softer light of the eclipse we see the Moon as it truly is: a massive sphere.

Chart from NASA’s Scientific Visualization Studio at https://science.nasa.gov.

And as the eclipse approaches totality, the Moon not only dims but changes color. Sunlight diffused through Earth’s atmosphere bathes the Moon in an eerie reddish glow. Were you standing on the Moon’s surface looking back on Earth, you’d see a black disc surrounded by a ring of numinous red – the combined sunrises and sunsets of our world.

For most of human history, lunar eclipses have inspired not only awe, but fear. In 1503, the Arawak tribe of Jamaica stopped supplying food to a shipwrecked crew of Europeans. The Arawak had grown tired of the trinkets they were getting in exchange for the fruit of their labor, and the crew was forced to live off the land.

The ship’s captain consulted his navigational tables and found that on Feb. 29, 1504, a total eclipse of the Moon would occur. On the eve of the eclipse, he warned the Arawak that God was offended by their behavior, and as punishment would remove the moon from the sky. As Earth’s shadow began eating away the face of the Moon, the Arawak panicked and relented. Miraculously, the Europeans’ god restored the Moon to the sky, and the captain and his crew escaped starvation long enough to be rescued and returned to Europe. The captain’s name was Christopher Columbus.

Pray for clear skies the night of Thursday, March 13, when the Western Hemisphere of planet Earth will be treated to the spectacle of a total lunar eclipse, the last before March 3 of next year. On the west coast, Luna will begin passing into Earth’s penumbra by 8:57 p.m. and into totality by 11:26; by 11:58 totality reaches maximum. But night owls can watch the entire pageant unwind, high overhead, till 3 a.m.

We’ll also be treated to bonus viewing, against the dark background of an eclipsed sky, of Mars in Gemini, Jupiter nestled against the Pleiades, and a concentration of the brightest stars at any time of year: among them, Sirius in Canis Major, Capella in Auriga, Betelgeuse and Rigel in Orion. And hanging from the hunter’s belt 1,350 light years distant but visible to the naked eye: the majestic Orion Nebula.

Lunatics: be there.