ClassX Video Lessons Youtube Channel MinuteEarth Our Definition For “Moon” Is Broken (Collab. w/ MinutePhysics)
Hi there! This is Kate from MinuteEarth. A long time ago, people thought everything in the sky was moving around the Earth. But actually, the Moon is the only thing orbiting Earth, while Earth itself orbits the Sun. Other planets orbit the Sun too, and they have their own moons. So, we decided to call all these natural satellites “moons.” When we say “moon” today, we mean a natural satellite that orbits a planet, which orbits a star.
Here’s the tricky part: space is full of rocks of all shapes and sizes, and not all of them fit our idea of a moon. We could just call everything a moon, but we think “moon” should be a special term. Let’s explore why.
With better technology, we’ve found smaller and smaller moons. Saturn, for example, has over 60 moons, some of which are tiny rocks just a kilometer wide, hiding in its rings. If we don’t set a size limit, should we call every tiny rock and dust particle in Saturn’s rings a moon? What about every speck of dust orbiting Earth?
On the other hand, some moons are too big to be called moons. Take Pluto and its companion Charon. They’re so close in size that they orbit each other. Technically, when two objects orbit, they both circle around a common center of mass, called the barycenter. If one object is much bigger, like Earth, the barycenter is inside it, making it feel right to say the smaller one orbits the larger one. But when they’re nearly the same size, like the binary asteroid 90 Antiope, the barycenter is between them, and it doesn’t make sense to call one a moon.
So, where do we draw the line on what we call a moon? Maybe size isn’t the best measure. Perhaps it’s about how round the object is, if we can see it from its planet, or if it has a regular orbit. Or maybe there’s no single definition that captures all the ways objects can orbit each other and what we think of as a moon.
Unlike moons, some space objects, like stars that have died, have clear definitions. If you’re curious about black holes, white dwarfs, and neutron stars, check out MinutePhysics! Let’s leave the moon debate behind and dive into the fascinating world of space!
Use clay or playdough to create models of different moons in our solar system. Pay attention to their size and shape. Compare your models to understand why some objects are considered moons and others are not. Discuss with your classmates how size and shape might influence the definition of a moon.
Divide into groups and hold a debate on whether small rocks and dust particles should be classified as moons. Use evidence from the article to support your arguments. This will help you understand the complexities of defining what a moon is.
Research a specific moon in our solar system and create a presentation about it. Include details about its size, orbit, and any unique features. Present your findings to the class and discuss how this moon fits into the current definition of a moon.
Use an online simulation tool to explore how moons orbit planets and how planets orbit stars. Experiment with changing the size and mass of the moons and planets to see how it affects their orbits. This will give you a hands-on understanding of the concepts discussed in the article.
Draw a comic strip that illustrates the journey of a moon in our solar system. Include interactions with its planet and other moons. Use humor and creativity to show the challenges of defining what a moon is. Share your comic with the class to spark discussion.
Sure! Here’s a sanitized version of the transcript:
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Hi, this is Kate from MinuteEarth. Until relatively recently, humans believed that everything we could see in the sky was orbiting around the Earth. Of course, the Moon was the only object actually orbiting the Earth, while the Earth itself was orbiting a star. However, there were other objects orbiting that star that also had things orbiting them, so we decided to call all of those secondary satellites “moons” as well. Nowadays, when people say “moon,” they essentially mean a natural satellite of a satellite of a star, which seems to capture our collective understanding of what distinguishes a moon.
However, there’s a challenge: space rocks come in all sorts of shapes and sizes and behave in many ways, many of which don’t fit into our traditional idea of a moon. At this point, we could give up and say, “Okay, all these things are moons,” but in this video, we’re going to defend the idea that “moon” is a special title and we shouldn’t use it too broadly.
To start with, through improvements in technology, we’ve ended up calling smaller and smaller things moons. We’ve already identified more than 60 increasingly small moons orbiting Saturn alone, some of which are essentially kilometer-wide rocks hiding in its rings. It’s inevitable that we’ll keep identifying ever smaller individual chunks orbiting planets. So if we don’t set some lower limit on moon sizes, are we willing to consider each of the billions of tiny rocks and dust particles in Saturn’s rings – and every last speck of dust orbiting the Earth – as moons?
At the other extreme, some “moons” are too large to be considered moons. Pluto and its companion Charon are close enough in size that it’s fairer to say they both orbit each other. In fact, technically, any time anything orbits another thing, they’re both orbiting their common center of mass, called the barycenter. The location of that point depends on the relative masses of the two objects. If one is significantly larger than the other, like the Earth, the barycenter will be close to or even inside it, making it feel right to say the smaller one is orbiting the larger one. But when they’re almost the same size, like with the binary asteroid 90 Antiope, the barycenter will be almost halfway between, and it doesn’t make sense to call one or the other (or both) moons.
So where in the continuum of size ratios should we stop calling something a moon? Maybe the ratio of sizes doesn’t accurately capture your understanding of what a moon is – perhaps absolute size would be better, or the roundness of the candidate moon, or whether it can be seen from the surface of its planet, or whether it has a regular elliptical orbit. Or maybe there’s simply no single definition for “moon” that captures both the complexity of the different ways objects can orbit each other and our intuitive sense of what makes a moon.
Unlike moons, there are many other objects in space, like the different things a star can become when it dies, that are actually delineated quite naturally. You should come over to MinutePhysics to find out more about them! Enough of this discussion about whether a moon around a moon is still a moon – come over to MinutePhysics, where we’re going to talk about black holes, white dwarfs, and neutron stars!
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This version maintains the original content while ensuring clarity and appropriateness.
Moon – The natural satellite that orbits a planet, particularly Earth’s moon. – The moon reflects sunlight, which is why we can see it shining brightly in the night sky.
Orbit – The curved path of a celestial object or spacecraft around a star, planet, or moon. – Earth’s orbit around the Sun takes approximately 365 days to complete, which defines our year.
Planet – A celestial body that orbits a star, is spherical in shape, and has cleared its orbit of other debris. – Jupiter is the largest planet in our solar system and is known for its Great Red Spot.
Satellite – An object that orbits a planet or star, which can be natural, like a moon, or man-made. – The International Space Station is a man-made satellite that orbits Earth and serves as a space laboratory.
Space – The vast, seemingly infinite expanse that exists beyond Earth’s atmosphere, where stars, planets, and other celestial bodies are found. – Astronauts travel to space to conduct experiments and explore the universe beyond our planet.
Rocks – Solid mineral material that can be found on planets, moons, and asteroids, often studied to learn about the history of these celestial bodies. – Scientists study moon rocks brought back by astronauts to learn more about the moon’s composition and history.
Size – The physical dimensions or magnitude of an object, often used to compare celestial bodies like planets and stars. – The size of the sun is much larger than any planet in our solar system, making it the central star around which they orbit.
Barycenter – The center of mass where two or more celestial bodies, such as a planet and its moon, balance each other in their orbits. – The barycenter of the Earth-Moon system is located inside the Earth, but not at its center.
Stars – Massive, luminous celestial bodies made of gas that emit light and heat from nuclear reactions in their cores. – The Sun is the closest star to Earth and provides the energy necessary for life on our planet.
Dust – Tiny particles of matter found in space, often forming clouds that can lead to the creation of stars and planets. – The dust in space can form beautiful nebulae, which are regions where new stars are born.
