Discover the podcast behind this transcription in Science or Fiction. © Futura
Remember one number already: 384,400. This is the number of kilometers that separate us from the moon. Did you see ? In 1969, Neil Armstrong’s team traveled so far to set foot on the lunar surface. To learn all about this event, I refer you to our episode of Science or Fiction where we deciphered the true from false about the rumor that man never went to the moon.
A History of Reflective Panels
However, five reflective plates were installed on the lunar soil during the Apollo missions. why will you tell me Well, they were used to measure the Earth-Moon distance with a laser system. These reflective panels are made up of a network of small glass blocks designed to focus and reflect the light in the direction it came from. Brilliant! Well yes, especially since you got it roughly speaking, all you have to do is send a laser beam from Earth to one of its panels, measure the time it takes for the round trip and the distance that separates us The star is calculated quickly. We derive the Earth-Moon distance thanks to the little mathematical formula D = (c*T)/2, where D is the distance, c is the speed of light, and T is time. So the distance is equal to the speed of light multiplied by the time it takes for the laser to go back and forth, all divided by two. Yes, because since it’s a round trip, we don’t want to measure the route twice. So remember that thanks to these reflective panels if you have some equipment at home you can take this measurement altogether from your backyard!
Why does the moon flee from us?
So, knowing this, you can imagine that scientists were able to analyze the distance separating us from our satellite and easily find the YES! The moon is gradually moving away from us. First, remember that the distance between it and us is not constant, for the simple and good reason that the moon’s orbit is not a perfect circle. As a result, its closest past us is about 362,600 kilometers away, while its furthest is about 405,400 kilometers away. Yes, that’s still almost 43,000 kilometers difference, that’s not nothing! And thanks to the laser system and the little calculation I talked about earlier, the scientists found that the moon is actually moving away at an average of 3.8 centimeters per year. It’s crazy, isn’t it?
In question, a major factor: the tide! Yes, you may have known that the moon has an influence on the tides, but obviously the latter also have an influence on the moon. Hold on, it’s a bit complicated. I had to bang my head on several articles on the subject for a while, which really weren’t clear, but luckily Laurent Sacco, our national astrojournalist at Futura, was able to clarify everything in a few sentences. As you know, the moon’s gravitational pull pulls the oceans together, causing something called tides. So far, so good. Yes, except that if you live near the sea you may have already discovered that the moon is not necessarily over the shore at high tide. Uh yes! This is because the earth continues to revolve around itself, much faster than the moon revolves around our planet. So our satellite attracts the oceans, but at the same time they move faster than it. Due to the gravity of the earth, the sea floor and the distribution of the continents, the water masses on the earth’s surface remain relatively in place and continue their course in front of our satellite. All the better, because if the moon were strong enough to crowd out the oceans, it would be difficult to build our cities in the dry. Fine, but this contrast between the terrestrial gravitational pull and the lunar gravitational pull on the oceans is not without consequences. As the moon continues to pull the oceans towards it, it creates friction on the earth’s surface, which, to hold on, causes our planet’s rotation speed to decrease. You heard me right! To put it more schematically, by pulling on the oceans, the moon slows the earth down, isn’t that crazy?
But it doesn’t stop there. Because imagine if we didn’t just snap our fingers and slow the movement of the earth, hoping nothing would happen. This phenomenon boils down to reducing the so-called angular momentum. However, physics teaches us that the sum of the angular momentum of the Earth’s rotation and the Moon’s rotation around the Earth is necessarily constant. Result ? As the Earth’s rotation rate decreases, the Moon’s rate of rotation around our planet is bound to increase. And that’s exactly what’s happening: the moon starts spinning faster around our blue planet, resulting in a larger orbit. In short, the moon is actually moving away from earth at about 3.8 centimeters per year! Okay, but having seen all that, does the moon’s distance really affect us earthlings?
What is at stake for us?
Of course, with all these forces at play, we could tell ourselves that if anything succeeds in driving the moon off, we won’t last long! But don’t worry, it’s been like this for millions of years, so if something had to happen, it would have happened by now! But it’s true: If we look closely at what that means, we can already see that our days are getting longer by about 1.8 milliseconds every century. Well that’s ok, it’s not much, basically it means that in 3.3 million years a day will be 24 hours and a minute. Not enough to add the spleen to the court broth. On the other hand, it also means that the moon and earth used to be closer together, so the earth rotated a little faster and the days were a little shorter! Well, that still has to be taken with a pinch of salt because it’s not easy to estimate the Earth-Moon distance one or two billion years ago, especially since that distance wasn’t always made so constant. Geoscientist Stephen Meyers calculated by analyzing rocks about 1.5 billion years old that the moon was therefore closer at that time, about 340,000 kilometers from Earth, and that the days lasted 18.68 hours! However, it should be noted that these results need to be compared to other analyzes performed by other means in order to be validated. But at least it gives you a little idea!
Another consequence of the moon’s distance: total solar eclipses could occur very rarely or not at all. In the past, the moon, which was closer to us, appeared larger than the sun in the sky and therefore had to hide it completely. Today, being a little distant and the same size as the Sun, we are entitled to eclipses where the perfect alignment of the two stars allows us to see the magnificent solar corona. Nature is well done, isn’t it? Yes, but not for long, because after a while the moon will be so far away that we won’t be able to see it from Earth covering the sun. This is already the case with the so-called annular eclipses that occur when the moon is furthest from our planet! The sun then forms a ring of light around our satellite. It gives you an idea of what to expect, but hey, as long as it’s just that and not a huge catastrophe decimating all life on earth, it’s fine!