# Tag Info

## Hot answers tagged celestial-mechanics

66

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 increasing in size. 1. The underlying theoretical argument. This, of course, is the idea that tidal braking causes the Earth to slow down in its rotation and the Moon ...

52

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 off refraction. It seems likely that, if you're using the default settings, the result returned is including atmospheric refraction, giving the results you ...

26

@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 the abstract: The observed spin-orbit resonance of Venus, whereby the same side of Venus faces the Earth at each inferior conjunction, cannot be explained ...

25

In the protostar stage of the Sun, it was surrounded by a (spinning) gas cloud. This cloud behaved like a fluid (well, a gas is a fluid), so it flattened out into an accretion disk due to conservation of angular momentum. The planets eventually formed from the dust/gas in the disk from compression of the dust in the disk. This process won't end up moving the ...

22

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 how PyEphem defines sunrise and sunset. It defines sunrise as the time the top of the Sun would nominally first appear above an unobscured horizon (no mountains)...

14

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 body of the planet: $$T_a=-\frac{3}{2}K_ab_a(2\omega-2n),\quad T_g=-\frac{3}{2}K_gb_g(2\omega-2n)$$ where K_a\equiv\frac{3M_*R_p^3}{5\bar{\rho}a^3},\quad K_g\...

14

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 clear the neighborhood of its path while the Sun is still a star. In the case of Pluto, Ceres, Eris, and a host of other not-quite-planet objects, that will not ...

13

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 lets strive for accuracy. Venus' solar day is 116.75 days, so 5 solar days is 583.75 days - Venus does 5 rotations in nearly the same amount of time that ...

12

The motion of the Sun in the direction perpendicular to the Galactic plane is perfectly well understood. The mass of the stellar component, which dominates over dark matter at the solar galactocentric radius, is strongly concentrated towards the plane - that's why it is called the Galactic plane. Very roughly, you can characterise the density as $\rho_0 \exp(... 12 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 the altitude is the “up-down” (with 0° being the horizon, 90° being overhead, also called zenith). Of course, a star’s (or planet’s, comet’s, galaxy’s, etc.) ... 9 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 northern summer solstice up to about 50 minutes (′) (90 km (56 mi)) south of the Arctic Circle. As the Arctic Circle is currently at roughly 66°34′N, this means a ... 8 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 until it reaches its closest point, and then gets farther away from the primary until it reaches its farthest point, and then gets closer again. When an ... 8 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 data. I found this website for current position (live on globe). Your userpage says you live in Europe, so the rocket will fly over Greece at 00:30 CEST (22:30 ... 8 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 in perfectly circular orbits defined by their masses and the chosen orbital radii (with the third body having negligible mass and thus no influence on their ... 7 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 towards the planet, and the gravity of the planet will tend to pull it back, slowing the rotation until one face is permanently facing the planet. Tidal locking ... 7 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 satellite of negligible mass orbiting the Earth as the third mass), where the radius of the Hill sphere will be the largest radius at which a satellite could orbit ... 7 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, but they cannot be tidally-locked to their star. 7 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. Orcus is an interesting counter-example. It is in a similar orbit to Pluto - similar aphelion, perihelion and eccentricity, similar orbital period (to within ... 7 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 having either inclination angles near zero or near$180^\circ\$. This is because, if they are uniformly distributed in space, the fraction of orbits within a ...

7

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 develop rapidly. The interactions between planets are gravitational and much much weaker, moreover, the planets are heavier and it takes a lot more energy to ...

7

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 from the galactic plane. It is often said that the disc of the galaxy is only 1,000 light years thick, but that is a round figure. The galactic halo is a ...

7

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 time (as obtained from The Aerospace Corporation Tweet): Last but not least, I also learned from a Business Insider story called "The US is tracking an ...

6

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 past, or the system may be evolving toward a resonance in the future. A number of near-integer-ratio relationships between the orbital frequencies of the ...

5

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’ve been meaning to write this up on the Worldbuilding.SE but I have not re-found enough references. But see this video.

5

There may be some three-body periodic solutions that orbit around a common point, but in general they get a little crazy-looking and may not always contain an immediately obvious center of mass from casual observation. But of course it will always exist. If you watch closely, you'll see that whenever one body reaches the intersection point, all three are on ...

5

The sun still rises in the East and sets in the West. So you quickly identify the cardinal directions. By observing the point around which the stars rotate each night you can find the altitude of the pole, which gives you your latitude. You can't find your absolute longitude, but with good time keeping you can find your longitude relative to your starting ...

5

Your understanding of the illumination is correct. k is the ratio of the illuminated length BC to the diameter AC. New Moon has an illumination of 0, Full Moon an illumination of 1, and the quarter phase an illumination of 0.5. The position angle is measured counterclockwise from north (celestial north, along a line of right ascension) to the bright limb C. ...

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