Skip to main content
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
  • 3,423
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
  • 1,095
48 votes
Accepted

How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

This is not so hard to do. All you really need is a bit of care, a few sticks and a convenient place to observe the sunrise or sunset. So note that there is a cycle of the year and you want to know ...
James K's user avatar
  • 125k
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
  • 125k
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
  • 15k
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
  • 1,006
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
  • 125k
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
  • 156k
25 votes

How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

It's easier than you think to notice the fraction... simply because the rounding error compounds over the years! After a good amount of years, you notice that solstices are drifting from their ...
Offirmo's user avatar
  • 351
24 votes

Is it coincidence that the earth's rotation and revolution are in the same direction?

Not a coincidence, most other planets rotate the same way. It is a consequence of how planets were formed from a disc of gas and dust that was orbiting around the sun as it formed. The sun formed ...
James K's user avatar
  • 125k
24 votes
Accepted

Earth is Tilted, but in reference to What Object / relative to what?

The orbit of the Earth around the Sun defines an orbital plane. The equator of the Earth is (currently) tilted at an angle of 23.4 degrees to this orbital plane. Or to put it another way, the spin-...
ProfRob's user avatar
  • 156k
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 ...
planetmaker's user avatar
  • 19.9k
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 ...
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
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 ...
planetmaker's user avatar
  • 19.9k
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 ...
James K's user avatar
  • 125k
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
  • 4,165
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
  • 4,165
18 votes

How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

The ancient Egyptians appeared to know that the year was 365.25 days. This was established by observing on what day the heliacal rising of Sirius occurred (i.e. on what day that it first becomes ...
ProfRob's user avatar
  • 156k
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
  • 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 ...
uhoh's user avatar
  • 30.7k
17 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
  • 19.9k
17 votes
Accepted

Is it coincidence that the earth's rotation and revolution are in the same direction?

I think it is coincidence, apart from that there may be anthropic arguments for the spin axis inclination (the difference between the spin and orbital axes) to be small but non-zero. i.e. A small, but ...
ProfRob's user avatar
  • 156k
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
  • 2,477
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
  • 2,676
16 votes
Accepted

Are galaxies growing, shrinking or stable?

Right now big galaxies like the Milky Way are growing by absorbing smaller galaxies - this is happening right now to the Sagittarius Dwarf and in the past to several other galaxies. Plus, there has ...
Anders Sandberg's user avatar
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

Only top scored, non community-wiki answers of a minimum length are eligible