What is a Blood Moon?

0:46 Welcome to Science is Not Boring by Codopoly.com. I am Mira. And I am Finn. A big hello to Luciana and Cecilia in Vicenza, Italy. Luciana actually asked what the blood moon is, so this episode is for you both.

1:03 And a big hello to Henson and Haven in San Diego. Plus, a huge hello to four-year-old Arthur, who loves the show and asked all about magnets. Arthur, magnets are awesome! We will definitely have to do an episode on those.

1:19 If you haven't already, tap follow wherever you're listening so you never miss an episode. So, what are we exploring today, Mira? Today, we are answering Luciana's amazing question. We are travelling 238,855 miles out into space to look at a glowing, deep red phenomenon, the blood moon.

1:45 Wait, blood? Is this going to be gross? You know I don't like blood.

1:51 Don't worry, Finn. There is no actual blood up there. But the moon does turn a deep dark red colour, like a dusty brick or a glowing ember in a campfire.

2:03 Okay, phew. But wait, how does a giant grey rock just change colour?

2:09 It is all about shadows, light, and geometry, and it gets intense. During a blood moon, the moon's temperature drops from a boiling 260 degrees Fahrenheit all the way down to negative 280 degrees in just a couple of hours.

2:27 That is a 540 degree drop. The rocks must literally crack and pop from the freezing cold.

2:35 They actually do. It is one of the most dramatic visual events in our solar system, and it happens right above our heads. Let's blast off. To understand why the moon turns red, we first have to understand what a lunar eclipse is. A lunar eclipse? Is that when the moon disappears? Sort of. It happens when the sun, the earth, and the moon line up perfectly in space.

3:01 The earth gets exactly in the middle, between the sun and the moon.

3:05 So the earth completely blocks the sun's light from hitting the moon?

3:09 Exactly. The sun is massive, about 93 million miles away from us, and when its light hits the earth, it casts a giant shadow into space behind us. How big is the shadow? Earth is pretty big. It is enormous! The darkest inner part of Earth's shadow is called the Umbra. That Umbra shadow stretches out into space for about 870,000 miles.

3:37 Whoa! 870,000 miles? But you just said the Moon is only 238,855 miles away.

3:48 Right! So the Earth's shadow reaches almost four times further than the Moon. When the Moon, travelling at 2,288 miles per hour, rolls right into that giant shadow, we get a total lunar eclipse.

4:04 Okay, but wait. If the Earth is completely blocking the sunlight, shouldn't the Moon just go pitch black? Why does it turn red?

4:13 That is the big mystery. If the Earth had no atmosphere, if we had no air, no clouds, and no sky, the moon really would go pitch black. It would vanish completely in the night sky.

4:26 But we do have an atmosphere. So the air does something weird to the light.

4:31 You got it. Our atmosphere wraps around the earth like a glowing blanket of gas. And that blanket acts like a giant, curved magnifying glass.

4:41 A magnifying glass? So it actually bends the sunlight?

4:46 Yes! Sunlight looks bright white to us, but it actually contains all the colours of the rainbow combined: red, orange, yellow, green, blue, indigo, and violet.

4:59 Oh, like when you shine a light through a glass prism and you get a rainbow on the wall.

5:04 Exactly! Back in the year 1871, an English scientist named John William Strutt, who was also known as Lord Rayleigh, figured out how our atmosphere plays with these different colours. What did Lord Rayleigh discover? He found that particles of gas in our atmosphere scatter light with short wavelengths, like blue and violet.

5:27 They bounce all over the place like tiny pinballs.

5:30 Wait, is that why the sky is blue during the day?

5:35 Yes, that is literally called Rayleigh scattering. But colours with longer wavelengths, like red and orange, do not scatter as easily. They plough straight through the thickest part of the Earth's atmosphere. So the red light just crashes right through the air and keeps going. Exactly. The Earth's atmosphere filters out the blue light and refracts, or bends, the red light, right into the dark shadow and straight onto the moon's dusty surface.

6:04 That is wild! It is like the earth is shining a giant red spotlight onto the moon. It gets even cooler.

6:11 Think about where that red light is actually coming from. It is passing through the very edges of the Earth, the exact places where the sun is just rising or just setting. Wait, you mean? Yes! When you look up at a blood moon, you are actually seeing the glowing red light from every single sunrise and every single sunset happening all around planet Earth, projected onto the moon at the exact same time.

6:40 No way! Millions of sunrises and sunsets all happening at once. That is the most beautiful thing I have ever heard.

6:48 Now, I want you to use your imagination, Finn. Picture yourself in a spacesuit, standing on the moon during the total lunar eclipse. Maybe you're standing right in the sea of tranquility.

7:00 Okay, I am there. There is no wind, just silent grey dust everywhere. I look up at the Earth. What do I see?

7:09 You see a massive black disk in the sky. Earth looks almost four times larger than the moon does to us. The planet is completely dark, but it is surrounded by a blazing, glowing ring of red fire.

7:25 A ring of fire! Because that is all the sunrises and sunsets shining right through the edges of the atmosphere.

7:31 Exactly! But the blood moon does not always look the same. Sometimes it is bright copper orange, and sometimes it is so incredibly dark you can barely see it. Why does it change colour? Does our atmosphere change? It sure does. In 1921, a French astronomer named Andre Louis Danjeon created something called the Danjon Scale to measure this.

7:57 It goes from L0 to L4. What is an L0 eclipse? L0 is a very dark eclipse, almost black or deep grey. This happens when our atmosphere is completely filled with dust, clouds or smoke. Where does enough dust come from to cover the whole Earth? Usually volcanoes. In 1991, a massive volcano called Mount Pinatubo erupted in the Philippines.

8:27 It shot 10 billion tonnes of ash and 20 million tonnes of sulfur dioxide gas high into the sky.

8:34 Ew, 10 billion tons of ash! That sounds like a gigantic choking mess.

8:41 It was. All that ash wrapped entirely around the Earth and physically blocked the red light. The next lunar eclipse after that eruption was so unbelievably dark, people thought the moon had disappeared completely.

8:56 So the colour of the moon actually tells us what is happening on Earth. That is genius.

9:01 It really does! And scientists have been using the lunar eclipse to learn secrets about the universe for thousands of years.

9:09 Thousands of years? Like before they even had telescopes?

9:14 Yes. Way back around the year 350 BC, which is over 2,300 years ago. An ancient Greek philosopher named Aristotle watched a lunar eclipse very closely. What was he looking for just staring at a shadow? He noticed that the edge of the Earth's shadow moving across the moon was perfectly curved. From that one simple observation, he proved that the Earth was a round sphere, not flat.

9:42 He figured out the entire shape of the Earth just by looking at a shadow on the moon. That guy was brilliant.

9:48 He sure was. Later, an astronomer named Aristarchus even used geometry to calculate the huge distance between the Earth and the Moon. But you might be wondering, if the Moon takes about a month to orbit the Earth, why don't we get a blood moon every single month?

10:06 Yeah. If the Earth is always in the middle, shouldn't there be an eclipse every 30 days like clockwork? That is a great question.

10:14 It is because the Moon's orbit around Earth isn't perfectly flat. It is tilted by about five degrees. Five degrees? That doesn't sound like very much at all. It is enough. Because of that tilt, most months the moon passes just a little bit above or just a little bit below the Earth's huge shadow. They only line up perfectly about twice a year.

10:37 That makes them super rare and special. But I am still stuck on how unbelievably cold it gets.

10:44 Oh, it is extreme. Because there is no air on the moon to trap any heat, the moment the Earth's shadow hits it, the temperature plunges. NASA actually has to power down some of their lunar robots during eclipses so their batteries don't freeze solid and break.

11:00 Hahaha. Note to self always pack a heavy winter coat if I ever go to the moon.

11:05 Alright, Finn, it is time for the quiz. Are you ready? Okay, I am ready. Lay it on me. First question. What is the name of the effect that scatters blue light in our atmosphere? Next one. What is the name of the scale that measures how dark or bright a lunar eclipse is? Question three. Who was the ancient Greek philosopher who used the eclipse to prove the Earth was round?

11:43 Here's number four. How many degrees is the moon's orbit tilted? Last question. What shot 20 million tonnes of sulfur dioxide gas into the sky in 1991 and made the moon look dark grey? Great job! Let's see how you did. The first answer is Rayleigh scattering. For question two, it is the Danjon scale.

12:17 The answer to number three is Aristotle. For question four, the orbit is tilted by five degrees. And the final answer is the Mount Pinatubo volcano.

12:31 I am never going to look at the moon the same way again. Knowing it's lit up by millions of sunrises all at once is just awesome.

12:41 It really is. Science is full of beautiful surprises like that. And listeners, if you loved learning about the Danjeon scale and Rayleigh scattering as much as we did, please leave us a five-star review.

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