The colloquial term “Blood Moon” refers to the striking appearance of the Moon during a total lunar eclipse. This astronomical event occurs when the Sun, Earth, and Moon align perfectly, with the Earth positioned directly between them. While the Earth casts its shadow across the lunar surface, the Moon does not disappear completely but instead takes on a deep, reddish-orange glow. This dramatic coloration gives the phenomenon its non-scientific nickname.
The Science Behind the Red Hue
The reddish color results from Earth’s atmosphere acting as a lens and a filter. During totality, the only sunlight reaching the Moon passes through the ring of our atmosphere along the edges of the Earth. This process is governed by Rayleigh scattering, the principle that explains the blue color of the sky.
As sunlight travels through the atmosphere, shorter wavelengths (blue and violet) are scattered away by nitrogen and oxygen molecules. Longer wavelengths, specifically red and orange light, are less affected and pass through the atmosphere. This filtered, reddish light is then refracted, or bent, into the Earth’s umbral shadow, illuminating the eclipsed Moon.
The precise shade and brightness of the eclipsed Moon can vary significantly from one event to the next. French astronomer André-Louis Danjon created a five-point scale, known as the Danjon Scale, to classify this color variation. A Moon that appears dark red or brownish, ranked L=1, indicates a heavily dust-laden or cloudy atmosphere, which absorbs more light. Conversely, a bright, coppery-red Moon, ranked L=3, suggests a clearer atmosphere that allows more red light to pass through and refract.
The presence of significant atmospheric aerosols, such as ash from a major volcanic eruption, can dramatically darken the Moon’s appearance. These fine particles absorb and scatter even the red light more effectively, potentially leading to an extremely dark eclipse, sometimes causing the Moon to appear nearly invisible. Therefore, the coloration is a direct indicator of the global atmospheric conditions prevailing during the eclipse.
Frequency and Visibility of Total Lunar Eclipses
The question of how many total lunar eclipses have occurred is complex because the event is defined by its visibility across the globe. Total lunar eclipses happen often, averaging about 1.5 per calendar year. The actual number can fluctuate, with some years experiencing none, and others having as many as three.
The perfect alignment of the Sun, Earth, and Moon is required for an eclipse. This alignment does not happen every month because the Moon’s orbit is tilted by about five degrees relative to Earth’s orbit around the Sun. Eclipses can only occur when the Full Moon is near one of the two orbital intersection points, known as the nodes.
The total number of eclipses across history is an impractical figure to count. Astronomers instead track the rate of occurrence, noting that the 21st century alone will feature 85 total lunar eclipses. A total lunar eclipse is only visible to an observer if the event occurs while the Moon is above the local horizon.
Unlike a solar eclipse, which is visible only across a narrow path, a lunar eclipse can be viewed by anyone on the night side of Earth when it occurs. This broad visibility means that while the total number of astronomical events is predictable, the count of those witnessed by any single region depends on geography and time. The cycle of total lunar eclipses operates on patterns like the Saros cycle, which repeats similar eclipses roughly every 18 years and 11 days.
Historical Tracking: The Phenomenon of Tetrads
While a precise historical count is elusive, a specific pattern of total lunar eclipses provides a countable marker associated with the “Blood Moon” phenomenon. This pattern is known as a tetrad, defined as a sequence of four consecutive total lunar eclipses. Each eclipse in a tetrad is separated by six lunar months, with no partial eclipses occurring.
The occurrence of tetrads is not uniform throughout history; they appear in cycles that vary over long periods. For example, there was a significant 300-year interval from 1582 to 1908 during which no tetrads occurred at all. In contrast, the current era is rich with these events, with eight tetrads scheduled to occur during the 21st century alone.
A recent and well-observed tetrad took place between 2014 and 2015. This specific grouping provided a concentrated period of “Blood Moon” visibility for a large portion of the globe. Tracking these tetrads offers the closest historical count to what the public often seeks regarding this specific phenomenon. The eclipses occurred on the following dates:
- April 15, 2014
- October 8, 2014
- April 4, 2015
- September 28, 2015
The next complete tetrad is already predicted, beginning in 2032 and concluding in 2033. The cycles are tied to the slow, long-term changes in the Earth’s orbital eccentricity, meaning the frequency of tetrads shifts over millennia. These clusters of four total eclipses serve as distinct, identifiable events in the astronomical record.