# Tag Info

## Hot answers tagged rotation

53

It's a matter of how "day" is defined. Wikipedia's article on Jupiter cites this IAU/IAG paper for the length of a Jupiter day. In it, footnote (e) of table I has the following: The equations for W for Jupiter, Saturn, Uranus and Neptune refer to the rotation of their magnetic fields (System III) The radio emissions of the gas giants have well-defined ...

52

In a similar way, we could ask... No beams can be exactly 1 meter long. No beams can be exactly straight. The material making up a beam cannot be truly isotropic. So why should we bother calculating the stress in a 1 meter straight beam having isotropic material? Because knowing how to perform this calculation is a building block for doing more ...

51

Yes, the shape of the constellation does and will change over time. All the stars have their own peculiar velocities and have some random motion which over time will ruin all the constellations. However, Even though the stars are moving at rapid speeds, to us, in our sky, due to their enormous distance from us, they appear to move extremely slowly and the ...

47

All models are approximations, we judge a model on how useful it is. Understanding the collapse of a non-rotating star to a black hole gives insight into the nature of gravitational collapse. Much of the physics of collapse does not depend on spin. The formation of an event horizon, for example. Models can be refined, and in this case, considering ...

32

First, we need to decide which definition of "day" to employ. There are several types of days: Apparent solar day: the time between two successive culminations of the Sun (apparent Noon) from an fixed Earth-based observer; Mean solar day: a more uniform, averaged solar day without seasonal variations; Stellar/Sidereal day: the time needed for the Earth to ...

30

For objects on the scale of a planet, the state of matter doesn't really matter much. A colliding planet or planetesimal would not "just pass through" The amount and density of the gas would prevent that. Some easy experiments: Put your hand outside a car as you are driving along, you'll feel that "gas" (ie air) has real substance and ...

29

There is very likely to be a random scatter. Unlike planets orbiting the Sun in the Solar System, most of the stars in the Galaxy did not form at the same time as the Galaxy itself. There is therefore no strong reason to suspect that the angular momentum vectors would be aligned for similar reasons. On the other hand, the Galactic gravitational potential ...

28

What you're asking, basically, is whether there are any proofs for the heliocentric model of the Solar System. A literal naked-eye observation of the Earth revolving around the Sun would be rather difficult, since human beings have never gone to another planet yet, and have only been to the Moon briefly, decades ago. Here are several proofs; some of them ...

23

"It's believed that the Earth was rotating about once every 5 hours before the theorized collision with a Mars sized coorbiting object referred to as Theia." Almost. Theia did not have to be co-orbiting, just an intersecting orbit. We have no idea what the Earth's spin was before the collision, but it is theorized that the Earth rotation had a 5 hour ...

22

This is a bit more complicated than it seems. First off, the definition of a day that matters to us earthlings is the average amount of time from one solar noon to the next (or alternatively, the time it takes for the Sun to appear above the same meridian from day to day); it is called a solar day. The sidereal day, which is the time it takes for some given ...

22

There is no rotation speed which can achieve that - globally. For a local analysis see below. The global equilibrium surface temperature in the absent of any atmospheric greenhouse effect and with an albedo of 0 is around 450K, thus 180°C: Distance from the Sun: $d_{merc} = 0.38$ AU Solar constant at earth radius: $S_{earth} = 1360$ W/m² Thus solar constant ...

21

I don't think you'll find a single agreed shape for a rotating neutron star, not least because we don't have an agreed single model for the equation of state of the material in a neutron star (which is more complex than the name suggests). I found one openly available paper (I'm sure there are more) which will give you a rough flavor for the complexity of ...

20

Leap years exist for two reasons: There are not an integer number of days in a year. People perceive a need to keep the seasons where they are on the calendar. Given the above, there is no way to avoid leap years, or something similar. Defining the calendar year as being a fixed number of days (e.g., 365 days) would result in the seasons shifting by one ...

20

Another consideration is that the physics that describe a rotating black hole was much harder to develop. The maths describing the Schwarzschild (uncharged, non-spinning) black hole was developed in 1916. This was expanded to charged, non-spinning black holes in 1918 (The Reissner–Nordström metric) It wasn't until 1963 that the Kerr metric for uncharged ...

20

Here's an animation I found that gives you an idea of the movements and timescales involved: It depicts the estimated movement of the Orion Constellation from 3 million years in the past to 3 million years in the future.

17

There are two different things going on, and they aren't too related (from what I can see). For the hot air balloon situation, you would think that if you hovered above where you are for 6 hours, the Earth could spin under you, and you would land back in a completely different place. Unfortunately, because the hot air balloon was on the Earth to begin with, ...

16

Even though I'm a professional astronomer (but not an expert on this field), I don't have a good answer. AFAIK, this question is still open. Actually, the fact that most planets in the Solar system rotate in the same sense as they orbit the Sun is an important constraint on possible planet formation scenarios (of which still several are presently discussed ...

16

You are correct, and in fact the clocks are offset. But most people never notice this because of the way days are defined. If you measure the "day" length by the stars (instead of the sun) you will see it's not 24 hours at all. It's around 23 hours 56 minutes. (Check this by going out at night, picking a bright star, and noting the exact time it passes due ...

16

It's hard to feel the Earth's rotation personally, but only because we are not very sensitive to very gradual changes and very weak "forces". However, there are everyday objects that can show us the rotation of the Earth. The balloon may have been a bad choice because even a slight wind over time will dominate the problem. (also see] A pendulum One object ...

15

Although its too late to reply to this interesting question but trying to add few more points. Yes the sun rotates. Now the question arises as to how we can check that? We can observe this by observing sunspots. All sunspots move across the face of the Sun. This motion is part of the general rotation of the Sun on its axis. Observations also indicate that ...

15

The Yarkovsky effect is the thrust on a small object in space that has been heated by sunlight, created from radiant energy. The YORP effect is when the thrust produces rotation, likely due to uneven heating or differences in surface projections (IE: A mountain acts as a better solar collector than a flatter portion of a spherical object on the opposite side)...

15

As with any claim of possibility, it really comes down to whether we are able to measure it or not. Since we're not talking about quantum mechanics, this is not too difficult to speculate. All celestial bodies I can think of rotate. The sun, the planets, the moon, the galaxies, clusters of galaxies, the supermassive black hole at the center if the Milky Way,...

14

You are right that the tilt of the asteroids are distributed in very random way, and that the rotation of the Solar nebula is a minor contributor to that tilt, and only skews it a little. However, you are not right that randomness simply adds up. The randomness does in fact cancel out more and more when you combine a large amount of asteroids, until the ...

14

Would this even be theoretically possible? No. The law of conservation of angular momentum prevents this for a planet with reasonable inner structure in a gravitational field like that around the sun. The angular momentum of the planet points in a certain direction. Changing this direction requires a torque perpendicular to the rotation axis. If a torque ...

14

The answer to this is certainly tidal forces, but that doesn't explain the exact mechanism for how tidal forces result in tidal locking, i.e., an orbiting body showing the same face to the central body as it orbits due to the rotation rate and revolution rate being equal. I'll describe this mechanism using the Earth-Moon system so I can be specific, but it ...

14

Can it be confirmed that it is exactly 360.0 degrees using official data? TL;DR: No, it can not. Instead it can be confirmed to be 361.0 degrees. To my knowledge: The Earth's rotation period is very close to 23 hours, 56 minutes, 4.1 seconds or 86164.1 sec. That's called a sidereal day A day is defined as 24 hours, or 86400.0 sec. So in one day it turns ...

14

Synchronous rotation is when the orbit of a body has the same period as its spin. If the inclination and obliquity are the same, then the same face of the body will always point towards the barycenter. Here is an animation of Pluto and Charon Credit: Stephanie Hoover Wikimedi commons. Tidal forces impart torque to orbiting body rotations. An orbiting body ...

13

Very cool question. I want to get into a little bit of detail here because otherwise there would be a one-paragraph answer, and I don't think that would cut it. So here goes. The planets in the solar systems have orbits with pretty low eccentricities (see this for more eccentricity values). At the upper end is Mercury, with an eccentricity of 0.2056. At the ...

13

You're correct in assuming the net angular momentum of the system in question here will remain constant. The Moon's orbit around Earth is responsible for the slowing of Earth's rotation. This effect is extremely small. The decrease in Earth's angular momentum is transferred to the moon, which resultantly sees it's orbit accelerate. This acceleration also ...

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