56 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 ...
Mark's user avatar
  • 3,103
53 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 ...
James's user avatar
  • 795
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. ...
Aryan Bansal's user avatar
  • 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 ...
James K's user avatar
  • 116k
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 ...
PM 2Ring's user avatar
  • 13.1k
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 ...
FSimardGIS's user avatar
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 ...
Fred's user avatar
  • 2,169
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 ...
James K's user avatar
  • 116k
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 ...
ProfRob's user avatar
  • 146k
23 votes
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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-...
Eubie Drew's user avatar
  • 1,070
23 votes
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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 ...
planetmaker's user avatar
  • 17.7k
22 votes
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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 ...
StephenG - Help Ukraine's user avatar
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 ...
BillDOe's user avatar
  • 1,371
22 votes
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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 ...
planetmaker's user avatar
  • 17.7k
22 votes
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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 ...
James K's user avatar
  • 116k
21 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 ...
Ingolifs's user avatar
  • 4,137
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 ...
Ingolifs's user avatar
  • 4,137
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 ...
User24373's user avatar
  • 913
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 ...
uhoh's user avatar
  • 31.4k
16 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, ...
SE - stop firing the good guys's user avatar
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 ...
Andy's user avatar
  • 2,467
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 ...
Rob's user avatar
  • 2,616
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 ...
planetmaker's user avatar
  • 17.7k
15 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 ...
zephyr's user avatar
  • 15k
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 ...
Daddy Kropotkin's user avatar
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....
aventurin's user avatar
  • 734
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 ...
uhoh's user avatar
  • 31.4k
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 ...
Connor Garcia's user avatar
  • 16.2k
13 votes
Accepted

Decreasing rate of Earth's rotation: where does the power go?

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 ...
Brumder's user avatar
  • 246

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