Why are “blood moons” like we had tonight red-orange in color?
Because – during a total lunar eclipse – the Earth is between the sun and the moon, and so it blocks most of the sunlight from hitting the moon.
What light can reach the moon is sunlight which squeezes or bends around the Earth’s atmosphere and travels on to the moon.
Which light is that?
It’s the same exact light which causes sunsets and sunrises on Earth … the long wavelengths bending around the Earth. So – literally – the blood moon is red-orange because it is illuminated by thousands of sunsets and sunrises on Earth.
The sun is not a point source of light [it’s a ball], so its light leaks around the edge of the Earth, and results in an unsharp shadow. In passing through the Earth’s atmosphere, the light turns red or orange, so that the light that actually reaches the moon is tinted by thousands of sunsets and sunrises all around the periphery of the Earth.
One result of these multiple sunrises and sunsets is that the moon during an eclipse is often tinted red, which is the origin of the idea of a lunar eclipse being a “Blood Moon.”
During an eclipse, the moon may turn a red or coppery color as it reflects sunrises and sunsets happening around the world. While the moon is in shadow, some light from the sun shines through Earth’s atmosphere. Red light (unlike other colors that are blocked and scattered) is better able to penetrate the atmosphere, which creates this “bloody” effect.
And the Christian Science Monitors points out:
At the height of the eclipse — during what’s called “totality” — the moon won’t disappear. Rather, sunlight will bend through Earth’s atmosphere, with only the red light making it to the moon and reflecting back to us. (The shorter-wavelength blue light gets absorbed by Earth’s atmosphere, which is why on any given evening, a sunset can be red.)
In effect, all the world’s sunrises and sunsets are casting their glow on the moon.
“Picture it from the point of view of an astronaut standing on the moon,” said Alan MacRobert of Sky & Telescope magazine. “They would see the dark Earth in the sky thinly ringed with brilliant orange from the sun hidden behind it. The ring is bright enough to illuminate the lunar landscape an eerie red.”
So rather than being a portent of doom – as some people used to believe – a blood moon is really a projection of the beauty of Earth’s sunsets and sunrises onto the temporarily dark screen of the eclipsed moon.