A blood moon occurs during a total lunar eclipse when the Earth moves between the Sun and the Moon. This event takes place two to four times a year, according to NASA, and remains visible to nearly half the Earth. The Moon takes on a red or brown hue due to light refraction through Earth’s atmosphere.
How the Blood Moon Gets Its Colour
During a total lunar eclipse, the Moon passes into Earth’s shadow. While sunlight is blocked, some light refracts through Earth’s atmosphere, illuminating the Moon. Caleb Scharf, director of astrobiology at Columbia University, explains the process:
“The red component of sunlight passing through Earth’s atmosphere is preferentially filtered and diverted into the Earth’s shadow where it illuminates the eclipsed moon, making it appear red or ‘blood’ colour.”
Figure 1: A blood moon against the night sky in 2014 [NASA Ames Research Center/Brian Day]
NASA states that the exact shade depends on atmospheric conditions. Dust, pollution, and cloud cover can darken the Moon’s red glow. Volcanic eruptions often make the Moon appear darker during an eclipse due to increased particles in the atmosphere.
The Role of Earth’s Atmosphere
The Moon does not generate its own light. Its usual bright appearance comes from sunlight reflecting off its surface. During a lunar eclipse, Earth’s atmosphere filters light, scattering shorter blue wavelengths and allowing longer red wavelengths to reach the Moon.
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A person standing on the Moon during an eclipse would see a red ring around Earth. This ring represents all the simultaneous sunrises and sunsets happening across the planet at that moment.
Understanding the Different Types of Lunar Eclipses
Lunar eclipses occur when Earth blocks sunlight from reaching the Moon. However, not all eclipses turn the Moon red.
- Total Lunar Eclipse: The Moon moves completely into Earth’s shadow, creating a deep red hue.
- Partial Lunar Eclipse: Only part of the Moon enters Earth’s shadow, resulting in a darkened section.
- Penumbral Eclipse: The Moon passes through the lighter part of Earth’s shadow, causing a slight dimming.
Figure 2: Different types of Lunar Eclipse
Only experienced skywatchers can detect penumbral eclipses, as the change in brightness is minimal.
Lunar Eclipses and Their Frequency
NASA reports that two to four lunar eclipses occur annually. Each total lunar eclipse remains visible for several hours. Unlike solar eclipses, which require protective eyewear, lunar eclipses can be observed without any special equipment.
The Moon’s Gradual Drift from Earth
The Moon slowly moves away from Earth at approximately 1.6 inches (4 cm) per year. This shift will eventually change how lunar eclipses appear. Scientists estimate that in the distant future, Earth’s shadow may no longer fully cover the Moon.
Historical Interpretations of the Blood Moon
Ancient cultures often feared the blood moon, attributing it to omens or supernatural forces. In 1504, explorer Christopher Columbus used a lunar eclipse to manipulate the indigenous Arawak people in Jamaica.
According to an account by Columbus’ son Ferdinand, Columbus warned the Arawak that his Christian god would turn the Moon red as a sign of anger. When the eclipse occurred, the frightened people provided Columbus’ crew with food, hoping to appease the divine force.
Figure 3: Formation of Blood Moon Effect
The Popularity of the Term ‘Blood Moon’
The Old Farmer’s Almanac clarifies that ‘blood moon’ is not a scientific term but a widely used popular phrase. The term originates from the Moon’s reddish appearance during a total lunar eclipse.
Some also use ‘blood moon’ to describe a Moon appearing red due to environmental factors such as smoke or haze. During autumn, full moons can take on a red hue due to the changing colour of leaves and atmospheric conditions.
Viewing a Blood Moon
A total lunar eclipse remains one of the easiest celestial events to observe. It requires no telescopes or protective gear. AccuWeather meteorologist and astronomy blogger Dave Samuhel highlights its accessibility:
“It’s one of the best astronomical events to witness without any equipment, and we know exactly when it’s going to happen.”
Cloud-free skies improve visibility. Observers across the globe can witness the Moon’s transformation without additional tools.
Scientific and Cultural Significance
A blood moon carries no unique astronomical significance, but its striking appearance continues to captivate audiences. Scientists study lunar eclipses to understand Earth’s atmosphere, while historians examine how past societies interpreted these events.
Lunar eclipses provide insight into Earth’s position in space and offer a reminder of the planet’s changing relationship with the Moon.
