The Fascinating Science Behind Lunar Eclipses

Issue 7, Volume 112

By Arin Faruque 

Cover Image

If you found yourself staying up past midnight on November 19, you may have noticed a rusty reddish moon gracing the night sky. This resulted from the Beaver Moon lunar eclipse, the longest partial eclipse since 1440 that was visible in numerous regions worldwide.

Lunar eclipses occur when the earth blocks some of the sun’s light that would otherwise hit the moon, resulting in a partial eclipse, or all of the light, resulting in a total eclipse. Though lunar eclipses only happen when there is a full moon, they do not happen every full moon. Rather, most years are marked by two lunar eclipses. This infrequency results from the inclination of the moon’s orbit, 5.1 degrees with respect to the ecliptic, the plane of the earth’s orbit around the sun, causing the full moon to generally be above or below the ecliptic. However, there are two points, called nodes, where the moon’s orbit crosses the ecliptic. If the full moon takes place when the moon is at one of the two nodes, the planes are aligned, and the result is a lunar eclipse.

During a lunar eclipse, two shadows are cast: the penumbra and umbra. The former is the small, darker center of the shadow while the latter is the larger and lighter outer region of the shadow. Variations in umbra and penumbra intensity results in three main types of lunar eclipses. The first is the penumbral eclipse, where the moon passes through the earth’s penumbral shadow. This eclipse is the most subtle of the three as it results in a slight dimming of the moon’s surface, which may not be visible to the eye even on the clearest of nights. More visible are partial eclipses, where the earth, moon, and sun are not completely aligned so only a part of the moon passes into the umbra. Here, the earth’s shadow only appears dark on the side of the moon facing Earth, giving it the appearance of a “bite” being taken out of the moon. Also known as “blood moons,” total lunar eclipses are the most visible because of their prominent reddish-orange hue. This occurs when the earth, sun, and moon are perfectly or very closely aligned, leaving the entire moon within the umbra.

The appearance of the moon during a total lunar eclipse eclipse can be better understood in the context of our atmosphere. Though the moon is fully in the Earth’s shadow, some light is still able to pass through to the moon’s surface. This light, however, is filtered by the atmosphere by Rayleigh scattering, a phenomenon that scatters light according to its wavelength. Colors with shorter wavelengths, such as blue and violet, are removed before they hit the surface of the moon. Meanwhile, colors with longer wavelengths, primarily red and orange, are able to pass through the atmosphere to hit the surface of the moon, resulting in the red-orange appearance that characterizes total lunar eclipses.

While technically a partial lunar eclipse, our most recent eclipse is better classified as an almost-total eclipse. It was the longest lunar eclipse in nearly 600 years due to the moon’s orbital speed relative to its distance from the Earth. At the time of the eclipse, the moon was near the point of its orbit that is farthest from the earth—this large distance resulted in the moon traveling more slowly. Furthermore, the eclipse was closer to a total eclipse than a partial eclipse, which means the moon spent more time in umbra, lengthening the eclipse time. Its closeness to a total lunar eclipse also explains its reddish color, which was most prominent during the eclipse’s peak at around 4 a.m., when over 97 percent of the moon was in the earth’s umbra. This allowed viewers from various countries across Asia, Europe, and especially North America to witness our moon change in a matter of hours.

While this past eclipse is the last of 2021, more exciting things are in store with two upcoming total lunar eclipses next year—in mid-May and early November. For many, these are powerful times used for reflection or to get in touch with the spiritual aspects of our solar system. But for others, they may just be the time to admire the moon’s beauty. Regardless, lunar eclipses remain something we can all look forward to every year.