The Artemis ll mission will end tomorrow, when the astronauts splash down near
San Diego. Below, the NASA site on their journey:
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Here, a never before seen by humans view, and a great picture. I had it up as my computer
background, but this morning I changed back to a solid pale color. Just too powerful. One astronaut
said it: 'We are now a two-planet species!'
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Chatting with Copilot:
ROTATION
Why do planets rotate on themselves?
Planets rotate on themselves because they inherit spin from the swirling cloud of gas and dust that formed the solar system, and nothing in space really stops that spin. The core idea is the conservation of angular momentum, a physical law that keeps rotation going unless something actively slows it.
🌌 Why planets start spinning in the first place
When the solar system was forming, everything began inside a giant, slightly rotating cloud of gas and dust called the solar nebula. Even a tiny initial swirl matters:
As the cloud collapsed under gravity, it shrank.
When something shrinks while rotating, it spins faster — the same way a figure skater spins faster when pulling in their arms.
This process amplified the nebula’s rotation dramatically.
The collapsing cloud flattened into a rotating disk, and planets formed inside this disk.
The forming planets inherited that rotation.
🔁 Why planets keep spinning
Once a planet is spinning, it tends to stay spinning because:
Space is almost a perfect vacuum.
There’s no air resistance or friction to slow it down.
So the rotation continues for billions of years.
Only small effects — like tidal forces from moons or the Sun — gradually slow a planet’s rotation over immense timescales. For example, Earth’s day is getting a tiny bit longer every century.
🌍 Why some planets spin strangely
Not all planets rotate the same way:
Venus spins backward (retrograde).
Uranus spins on its side.
These oddities are likely due to massive collisions early in the solar system’s history that altered their original spin.
🧭 The essence
Planets rotate because they were born from a rotating system, and physics keeps that rotation going.
Angular momentum is the key: once you have it, you keep it unless something takes it away.
If you want, I can also explain this using visuals, equations, or a step‑by‑step physical model — whichever helps you build the intuition you’re aiming for.
So how did the Moon end up in tidal lock?
The Moon became tidally locked to Earth because gravity stole its rotation energy over billions of years until the Moon’s spin slowed enough that one rotation took exactly the same time as one orbit. After that point, the same side always faced Earth — and it became locked.
Here’s the full, intuitive picture.
🌕 1. The Moon did not start tidally locked
When the Moon first formed (likely from the giant impact that created the Earth–Moon system), it was:
much closer to Earth
spinning much faster
still molten or partially molten
So originally, the Moon did rotate freely.
🌊 2. Earth’s gravity raised “tidal bulges” on the Moon
Just like the Moon raises tides on Earth, Earth raised huge tidal bulges on the young Moon.
But because the Moon was rotating, those bulges were not perfectly aligned with Earth — they were dragged slightly ahead by the Moon’s spin.
This misalignment is the key.
🔧 3. Gravity pulled on those misaligned bulges
Earth’s gravity tried to pull the bulges back into alignment.
That gravitational tug acted like a brake on the Moon’s rotation.
The bulges tried to “snap back” toward Earth
That created torque
Torque slowed the Moon’s spin
This is the same mechanism that slows Earth’s rotation today (our days get ~1.7 milliseconds longer per century).
🕰️ 4. Over millions of years, the Moon’s rotation slowed
As the Moon’s spin slowed, the tidal bulges became less misaligned.
Eventually the rotation period matched the orbital period:
𝑇 rotation = 𝑇 orbit
At that moment, the bulges lined up perfectly with Earth.
🔒 5. Once aligned, the system “locked”
When the bulges aligned, the torque dropped to zero.
No more braking
No more change in rotation
The Moon became tidally locked
This is a stable configuration — if the Moon tried to rotate faster or slower, gravity would pull it back.
🌌 6. Why this happens so often
Tidal locking is extremely common in the universe because:
Gravity always tries to minimize energy
Locking is the lowest‑energy configuration
Over long enough timescales, almost all close moons become locked
In our solar system, almost every major moon is tidally locked to its planet.
🧠 The essence
The Moon is tidally locked because:
Earth’s gravity raised tidal bulges on the Moon → those bulges created torque → torque slowed the Moon’s rotation → rotation matched the orbit → locking became permanent.
If you want, I can also show:
a step‑by‑step physical model
a diagram of the tidal bulges
the equations for tidal torque
a simulation using Python
Which direction do you want to explore next?
GRAVITY
From a chemical point of view, what is gravitational attraction?
How is the mass of an object determined?
How are general relativity and quantum mechanics incompatible?
If the universe is expanding, how come I weigh the same on my scale every morning?
How does dark energy drive expansion?
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Read the Wikipedia page on Gravity in Farsi:
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