Recently (April 15th), we had the first of four total lunar eclipses that are going to occur in the next several months. And the media kept calling it a “blood moon”, apparently from the red color of the moon when the eclipse was total. I’m not sure when this started to be done. When I was young and we watched lunar eclipses, the Moon turning red was how we could tell the eclipse was total. It was just part of the deal.
The moon has always turned red when there is a total eclipse. The reason why is because of the Earth’s atmosphere. The way a lunar eclipse happens is that the Sun, Earth and Moon line up, with the Earth in between so that its shadow lands on the Moon. Since the Moon goes around the Earth every month, we have the potential for a lunar eclipse each time. But because the Moon’s orbit around the Earth varies up and down some, the Moon doesn’t go into the shadow every time. So we only get the eclipse now and then.
And the red color is also understandable. It happens for the same reason that sunrises and sunsets give us those amazing colors. sunlight as it passes through the Earth's atmosphere gets scattered. During the day this scattering is what makes the sky blue. And in the evening, or early morning, the sunlight is coming through more of the atmosphere so gets scattered more and we get red skies.
Because of this scattering of the Sun’s light, the Earth’s shadow, which the Moon enters, isn’t completely black. Think about your own shadow. You block the light coming directly from the Sun, but your shadow isn’t completely black. There is still light being reflected from other objects being reflected into the shadow. The light in the Earth’s shadow gets there by refraction, not reflection,but you get the idea.
Now, since the composition of the Earth’s atmosphere changes, the amount of red on the Moon can change, but it has always turned red. But the recent trend of calling a lunar eclipse a “blood moon” seems to be more marketing than science.
Of course calling it that makes it sound ominous and some folks look to make it a sign of something. And maybe it is. It’s a sign of how good our understanding is of how the orbits of the Earth and Moon work. We can predict eclipses years in advance. So that people trying to say that a “blood moon” is a sign of something supernatural don’t even pay attention to the fact we know exactly how they happen.
But we can still look up at these “blood moons” with just as much awe as cave dwelling humans might have tens of thousands of years ago. But we don’t have to feel any fear about it. So we can stagger out of our houses at midnight (or whatever time the eclipse occurs) and look up and see that once again things have lined up just right, just like we knew they would.