55
votes
Accepted
Why does the Sun have different day lengths, but not the gas giants?
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 ...
- 3,042
52
votes
If all stars rotate, why was there a theory developed that requires non-rotating stars?
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 ...
- 785
51
votes
Accepted
How are constellations intact if the stars are rotating around galactic nuclei?
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. ...
- 1,085
47
votes
If all stars rotate, why was there a theory developed that requires non-rotating stars?
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 ...
- 104k
38
votes
Accepted
If planets are ellipsoids why don't we have 3 diameters?
It is possible for a rotating body in hydrostatic equilibrium to be a triaxial ellipsoid. This solution was found by Jacobi in the mid 1800s, thus it's known as the Jacobi ellipsoid. An example in the ...
- 11.7k
32
votes
Accepted
Official degrees of earth’s rotation per day
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 ...
- 986
32
votes
Do planets lose energy while rotating?
The energy a rotating planet has is derived from the rotation the initial interstellar cloud of gas and dust had prior to it flattening into a protostellar disk.
The rotation of a planet is bound by ...
- 2,051
30
votes
Accepted
How is it possible for a collision to be responsible for Uranus's axial tilt?
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 ...
- 104k
29
votes
Accepted
Do the axes of rotation of most stars in the Milky Way align reasonably closely with the axis of galactic rotation?
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 ...
- 132k
23
votes
Accepted
How did Mars come to have a 24 hour 39 minute day?
"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-...
- 1,070
23
votes
Accepted
Does the Sun shift periodically between prograde and retrograde rotation about its axis?
That does not happen. That cannot happen on grounds of conservation of angular momentum.
You might confuse this with a text talking about the Sun's magnetic field which undergoes a constant change and ...
- 14.3k
22
votes
Accepted
Shape of neutron stars
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 ...
- 5,878
22
votes
Official degrees of earth’s rotation per day
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 ...
- 1,361
22
votes
Accepted
What rotational speed would Mercury need to have to achieve a temperature comfortable for humans
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 ...
- 14.3k
22
votes
Accepted
Rotation direction of Pulsars
Does the Earth rotate clockwise or anticlockwise?
If you are floating above the north pole of Earth you would see the Earth rotating anticlockwise(*) if you were floating above the south pole, you ...
- 104k
20
votes
Accepted
How can we avoid needing a leap year/second?
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 ...
- 32.1k
20
votes
Accepted
If all stars rotate, why was there a theory developed that requires non-rotating stars?
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 ...
- 4,037
20
votes
How are constellations intact if the stars are rotating around galactic nuclei?
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 ...
- 4,037
17
votes
Why don’t we feel the Earth rotating beneath us?
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 ...
- 903
17
votes
Why don’t we feel the Earth rotating beneath us?
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 ...
- 31.1k
16
votes
Accepted
Why do our clocks not go out of sync with the day over a year?
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 ...
- 2,397
16
votes
Accepted
What is the difference between the Yarkovsky effect and YORP effect?
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 ...
- 2,563
16
votes
How scientists find the direction of rotation of planets?
I'll focus first on the question of the title: "how do we find / measure rotation?"
The easiest method is the same as everyone else: look and see. Take images some time apart and you will ...
- 14.3k
15
votes
Are there non-rotating objects in the universe?
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 ...
- 4,309
14
votes
Why do planets tend to rotate in the same direction although they have formed from tumbling asteroids?
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, ...
14
votes
Accepted
Are there any known planets whose axis is oriented such that one pole always faces its star?
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....
- 694
14
votes
Why do certain moons have their rotational period equal to their orbital period?
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 ...
- 14.6k
14
votes
Official degrees of earth’s rotation per day
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 ...
- 31.1k
14
votes
Accepted
Is there any difference between tidal locking and synchronous rotation?
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 ...
- 15.6k
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