66 votes

Where do we have it from that the Moon is migrating away from Earth?

There are, I think, at least four parts to this argument: the first being the theoretical argument that ties it all together and the remainder being observational evidence for the Moon's orbit ...
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  • 14.7k
52 votes
Accepted

Why won't the Sun set for days at N66.2 which is below the arctic circle?

From the PyEphem Quick Reference Guide: Rising and setting are sensitive to atmospheric refraction at the horizon, and therefore to the observer’s temp and pressure; set the pressure to zero to turn ...
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  • 2,980
34 votes
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How can Io be tidally heated while it is in tidal lock?

How can Io be tidally heated while it is in tidal lock? It is tidally locked in a mean motion sense of "tidally locked". That Io is in an eccentric orbit rather than a circular orbit means ...
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  • 27.1k
26 votes

Is Venus in some way tidally locked to... Earth?

@BMF's comment links to (Gold & Soter 1969) Icarus 11, (3), November 1969, pp 356-366 Atmospheric tides and the resonant rotation of Venus. Since it is paywalled I'll add a short summary: From ...
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  • 31.5k
22 votes

Why won't the Sun set for days at N66.2 which is below the arctic circle?

Correct me if I am wrong, but if we count sunsets by the center of the Sun apparently crossing the horizon then the Sun is supposed to set every day at latitudes under the arctic circle. That is not ...
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  • 27.1k
14 votes

Is there any way for a planet orbiting a red dwarf in the habitable zone to not be tidally locked?

Leconte et al. (2015) suggested that the presence of an atmosphere could prevent or at least slow tidal locking. The star should exert two separate torques: one on the atmosphere and one on the solid ...
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  • 33.7k
14 votes

Is Pluto a "proto-planet"?

Pluto will never be a planet. There are a number of technical papers that give more precise meaning to the concept of "clearing the neighborhood". It's not just now, it's can the object in question ...
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  • 27.1k
14 votes

What pulls the Sun above the galactic plane and pulls it below the galactic plane?

Your contention is like saying that finding a pendulum on the upswing must mean there is a mysterious force pulling it in that direction - untrue, the only force acting is a restoring force back ...
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  • 114k
13 votes

Is Venus in some way tidally locked to... Earth?

This is a fun little problem that's remarkably close and the math is pretty easy when you use the right periods. Venus' synodic period, relative to Earth, is 583.92 days on average. He uses 584, but ...
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  • 22.9k
13 votes

How can Io be tidally heated while it is in tidal lock?

That sentence on wikipedia continues "from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean moons". There's an orbital resonance (with the other ...
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  • 478
12 votes
Accepted

Coordinates in the Night Sky

The altazimuth system is directly linked to your position on the Earth. It is the “left-right/up-down” coordinates system; the azimuth being the “left-right” (with 0° for North, 90° East, etc.), and ...
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12 votes
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What's the meaning of virial in Astronomy, and in particular the expression "a virialized cluster of galaxies"?

A slightly modified version of the virial theorem that you cite states that for a system of N bodyes (galaxies in a cluster) autogravotating $${1 \over 2} \ddot I = 2K + V$$ Where $K$ is the total ...
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  • 2,676
9 votes

Why won't the Sun set for days at N66.2 which is below the arctic circle?

Wikipedia's article on the Arctic Circle provides the explanation. Firstly, it says: because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the ...
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9 votes

What's the meaning of virial in Astronomy, and in particular the expression "a virialized cluster of galaxies"?

According to the Wikipedia article on the Virial Theorem: The word virial for the right-hand side of the equation derives from vis, the Latin word for "force" or "energy", and was ...
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  • 1,997
9 votes
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How to calculate the altitude of a star given the hour angle, declination, and latitude?

You can use this fundamental formula in spherical astronomy[1] $$\sin a=\sin \phi \sin \delta + \cos \phi \cos \delta \cos H$$ where $a$ is the wanted altitude, $\phi$ is your latitude, $\delta$ is ...
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  • 2,660
8 votes

Where do we have it from that the Moon is migrating away from Earth?

The orbit of an astronomical body around another astronomical body is an ellipse, with the primary in one of the two focal points of the ellipse. Thus the orbiting body gets closer to the primary ...
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8 votes
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Spotting the booster rocket of the Chinese space station?

The name of the module is Tianhe (天和), and the name of the rocket is Long March 5B, but it got a special designation of 2021-035B. It is not really official, but I think it is good enough to get some ...
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  • 2,660
8 votes
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Degrees of freedom in restricted circular coplanar three body problem

In the most general case, there are three (spatial) degrees of freedom for each body, for a total of 9 degrees of freedom. The circular restricted three-body problem forces the two larger masses to be ...
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  • 14.7k
7 votes
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What causes objects to become tidally locked?

Tidal locking occurs because the planet deforms the satellite into an oval, with long axis pointing towards the planet. If the satellite is rotating the long axis will move away from being pointing ...
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  • 87.6k
7 votes

The deduction of the Hill Sphere formula

Hill sphere is named after John William Hill (1812–1879) and its simple logic follows from the presence of three bodies (let's assume Sun is the largest mass with Earth as the secondary mass and a ...
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  • 633
7 votes
Accepted

Is there any way for a planet orbiting a red dwarf in the habitable zone to not be tidally locked?

Yes: It has a companion planet or an excessively large moon, with the two bodies orbiting their common center of mass (much like the Earth and the Moon). They could be tidally-locked to each other, ...
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  • 102
7 votes
Accepted

Is Pluto a "proto-planet"?

The question says a few interesting things: The system orbit isn't on the ecliptic The system hasn't cleared its neighbourhood These are not going to change in the next few million years - or ever. ...
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  • 2,377
7 votes
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Has any moon achieved "retrograde equatorial orbit"?

This is directly related to another question: Why are asteroids with zero orbital inclination rare? If captured, irregular moons are randomly oriented in space then there is very little chance of them ...
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  • 114k
7 votes

Why are solar systems stable and not chaotic?

The solar system is chaotic, but it is also stable! The fixed and linkages between the bars of a double pendulum allow for very rapid energy transfer between the arms. This makes the chaotic motion ...
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  • 87.6k
7 votes

What pulls the Sun above the galactic plane and pulls it below the galactic plane?

Nobody says that nothing exists "above" or "below" the galactic plane. The stars are thickest at the galactic plane and get scattered thinner and thinner with increasing distance ...
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7 votes

Spotting the booster rocket of the Chinese space station?

I found an openly available CZ-5B Rocket Body (ID 48275) reentry prediction: Most intersting I found the following graph which shows how slowly error bars of the predicted reentry time decrease over ...
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  • 5,284
6 votes

Is Venus in some way tidally locked to... Earth?

According to Wikipedia, the orbital relationship between Venus and the Earth is coincidental and not because they are locked in a true orbital resonance, but it may be due to a true resonance in the ...
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  • 8,862
5 votes

Is there any way for a planet orbiting a red dwarf in the habitable zone to not be tidally locked?

The more likely case is actually a spin-orbit resonance that is not 1:1 but a half odd multiple, like the 3:2 case of our own Mercury. Having eccentricity in the orbit encourages this situation. I’...
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