# Tag Info

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### The moon has just the right speed not to crash on the Earth or escape into space. What are the odds?

There isn't a "Goldilocks speed" for orbit. If you put two objects in space, and give them a velocity relative to each other, then provided that velocity is less than the escape velocity (...
• 104k
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### Is it possible to achieve a stable "selenostationary" orbit around the Moon?

First off, such an orbit wouldn't be a geostationary orbit since geo- refers to the Earth. A more appropriate name would be lunarstationary or selenostationary. I'm not sure if there is an officially ...
• 14.6k
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### Does the Milky Way orbit around anything?

The object which has less mass and gravitational pull orbits around the nearest object with more mass and gravitational pull. Actually, both the heavier and the lighter object orbit around their ...
• 17.8k

### Is it possible to achieve a stable "selenostationary" orbit around the Moon?

As the answer by zephyr describes very well, there are very few stable orbits around the moon, and none of them are stationary. But the moon is tidally locked to Earth. That means that all of the ...
• 766
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### What is the **actual** average distance of the Moon from Earth?

Mean distance averaged over time for any Keplerian orbit is $a(1+\frac{1}{2}e^2)$, where $a$ is the semi-major axis and $e$ is the eccentricity. Using your NASA fact sheet, I get about 384,979 km for ...
• 15.6k
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### Is Earth's Surface "In orbit"?

1. Is material on Earth's surface not in free fall around Earth's center? No. Material on the Earth's surface -- or inside it -- is not in orbit, and so is not in free fall. You can temporarily put ...
• 15.7k
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### What makes protoplanetary disks start rotating? (Initial energy needed to rotate)

Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the ...
• 4,420
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### If the Moon were impacted by a suitably sized meteor, how long would it take to impact the Earth?

As several people have said, this is incredibly unlikely. Part of the reason why is that the "circling the drain" effect you describe doesn't really happen for solid objects much less dense than black ...
• 10.1k
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### Why do the planets orbit in the same direction?

The same reason (almost) all of them rotate in the same direction: because of the conservation of angular momentum. Before a star and its planets exist, there’s just a cloud of disorganized gas and ...
• 7,870
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### Does the Earth constantly lose mass?

You are wrong that "to keep any object in circular motion requires energy" In a circular orbit, the force of gravity is always perpendicular to the motion of the moon, so no work is done by ...
• 104k

### Simple experimental evidence that Earth revolves around Sun

The answer is ironic: Without good instruments, there is no evidence. The people who thought that the Sun went around the Earth were perfectly correct as far as the actual evidence went until the ...
• 7,450
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### Why do satellites arcs end abruptly when observed from Earth

Because satellites are only visible when they are in sunlight, they are not visible when they go into the Earth's shadow. The app most likely predicts where this occurs and ends the arc. In other ...
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### Can a planet with no atmosphere be orbited at extremely low altitudes?

Yes, but. Firstly the 400km orbits of the ISS are already extremely low, in comparison to the 6400km radius of the Earth. See https://what-if.xkcd.com/58/ for pictures of orbits. So if you rephrase ...
• 104k

### What if Earth and Moon revolved around each other like Pluto and Charon?

They do, but due to the ratio of masses being vastly different, they seem like they would not to do so as moon seems to rotate just around (the centre of) Earth. The ratio of Earth and Moon's masses ...
• 556
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### Was the Geocentric Model correct at all?

Ptolemy's epicyclic, geocentric model, in use until the Renaissance, was very accurate in terms of predicting the positions of planets and the times of eclipses. What it couldn't account for were ...
• 132k
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### Is it possible to have a stable 3 body system that orbits in a perfect circle?

Yes and No. — It depends on what you mean by "stable". To be precise, "stable" means immune to small perturbations. "Equilibrium" can be either "stable" or &...
• 104k

### How did Kepler "guess" his third law from data?

Kepler's account of how the third law came to be is as follows (Caspar p.286; emphasis mine): On the 8th of March of this year 1618, if exact information about the time is desired, it appeared in ...
• 7,784
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### Why are there so many binary systems?

Collapsing gas clouds fragment into multiple cores because the Jeans mass, that determines the minimum mass that becomes gravitationally unstable to collapse, becomes smaller if the cloud is able to ...
• 132k
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### If the Sun disappeared, could some planets form a new orbital system?

The issue here is whether pairs of planets can become gravitationally bound to each other. In the two-body problem the trajectories or orbits are ellipses (bound orbits), parabolas and hyperbolas (...
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### Is it at all possible for the sun to revolve around as many barycenters as we have planets in our solar system?

The short answer is no; there is only one barycenter. Yes, you can count the Sun/Jupiter barycenter or the Sun/Saturn barycenter, or whichever barycenter you want, but the net effect of all Solar ...
• 6,071
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### Can a photon orbit around a galaxy?

No. The milky way has a lot of mass — about 1.5 trillion times the mass of the sun. For light to orbit it would have to follow a null geodesic, ie the photon sphere at 1.5 times the Schwarzschild ...
• 104k

### How do planets retain momentum?

Such forces do exist. They are utterly negligible in most circumstances. If you imagine a planet "running into" matter whilst travelling in its orbit. Let's say that on average that matter is at rest ...
• 132k
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### When did people first measure that the Earth was closest to the Sun during January?

Hipparchus, not Kepler Kepler got the conic sections right, and Newton gave us the mechanics. But the question is about when people knew that the Earth was closer to Sol in one part of the year than ...
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### The newly discovered comet NEOWISE has been given the name C/2020 F3. What is the meaning of the F3 and the C in this name?

C means "comet": it has a coma which means that volatiles are being released due to solar heating. Other possible letters are "A" for asteroid, "P" for (short) periodic ...
• 104k
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### Why does the Earth have a z-component at the start of the J2000 epoch?

You made the same fundamental mistake that Anton Gromov made in his question on the sister Space Exploration StackExchange network site: You used the solar system barycenter rather than the Sun as the ...
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### If suddenly "knocked" or perturbed from its orbit, would gravity eventually return the Earth to its original orbit?

There's a few parts to this question so there's more than one answer. Earth gets knocked a little bit out of its orbit all the time by gravitational influence of other planets in our solar system. ...
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### Does a parabolic trajectory really exist in nature?

No, but nor do elliptical or hyperbolic trajectories. These are features of a model we have for gravity, not part of nature. The modelling process involves constructing a mathematical framework that ...
• 104k
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### How did Kepler "guess" his third law from data?

Kepler's third law is trivial (in my opinion) compared to his first law. I am quite impressed that he was able to deduce that the orbits were ellipses. To get that, he had to go back and forth ...
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