Equinox

[Knighty2112]

OK, let's give it one final go then I'm done too;

The 25,920 precessional cycle of the earths axis does not affect the yearly cycle of spring, summer, autumn, winter on earth, and the seasons do not come about because of it. All through a precessional cycle of 25,920 the tilt of the earth is always at 23.5 degrees, However in relation to the sun if we could see a cycle of pressesion we would see the earths axis spin around in a circle like a gyroscope. During all this time during a full pressesional cycle the tilt of the earths axis remains at 23.5 degrees in relation to earths orbit around the sun, even though it spins or wobbles around in its pressesional cycle, this wobble in relation to the earths orbit around the sun is always 23.5 degrees. So this is why it is is fixed, as the tilt is always 23.5 degrees regardless of where in the 25,920 year pressesional cycle we are.

Spring and Autumn occur yearly at the two equinoctal points in the earths orbit when the earths Northern and Southern Hemisphere lie square on to the sun, so both hemispheres receive the same amount of daylight, and the day and night are both 12 hours long. The tilt of the earths orbit however is still at 23.5 degrees, but this tilt lies parallel to the sun at these two equinoctal points. These two equinoctal points lie 180 degrees apart in the earths orbit around the sun.

At one point in its orbit around the sun, the Northern hemisphere will be tilted towards the sun by 23.5 degrees as the northern pole tilts towards the sun. This gives us summer in the Northern hemisphere, and winter in the Southern Hemisphere. In the northern hemisphere days are longer, and nights are shorter.

180 degrees later in the earths orbit, the Southern Hemisphere will be tilted more towards the sun by 23.5 degrees as the Southern pole titles towards the sun. This gives us summer in the Southern Hemisphere, and winter in the northern hemisphere. In the northern hemisphere days are shorter, and nights are longer.

At no point in any of the above does the earths tilt in relation to its orbit around the sun change from been at 23.5 degrees during a yearly orbit, or during any of the 25,920 of a pressesional cycle as it orbits the sun.

Unless of course we are struck by a large asteroid to enable this!  

[harrym]

3 hours ago, Knighty2112 said:

At no point in any of the above does the earths tilt in relation to its orbit around the sun change from been at 23.5 degrees during a yearly orbit, or during any of the 25,920 of a pressesional cycle as it orbits the sun.

Actually it changes by a few degrees in a 41000 year cycle. This is caused by changes in the inclination of the Earth's orbit, rather than anything to do with the axis itself.

[Mariusz]

I checked again informations about Uranus and there is four seasons not two (its been wrongly stated in this discussion)  during a Uranus year(84 earths years). I have to apologise you and correct myself and say that Uranus has not fixed axis. Otherwise only one Hemisphere of Uranus was in light and the other in darkness all year (84 earths year).

https://en.m.wikipedia.org/wiki/Uranus

[Moonshane]

I didn't say Uranus has two seasons, I said that a planet which has a polar axis pointing directly at the sun has two seasons. Uranus has a tilt of 97.7 degrees relative to orbit (according to your wiki link) so will as previously stated have four seasons and is effectively the same as Earth if you ignore the angle of tilt. The seasons of spring and autumn on Uranus will be very subtle I suspect compared with Earth.

[Stu]

I suspect it might be a good idea to agree on the Earth's situation before trying to work in Uranus!

Has the OP actually clarified what he is trying to find out yet? I'm finding this all extremely confused and lacking in a desired outcome, but perhaps that's just me!

[Moonshane]

True Stu but they are the same ?

There does seem to be a lack of willingness to accept logic on the part of the op 

[Knighty2112]

Knocked up the diagram below to show the earth's orbital plane looking down from above on the northern side with positions 1, 2, 3 & 4 of the points in earths orbit. The two smaller diagrams above and below this show the earth's position viewed from the side for positions 1, 2, 3 and 4 in earths orbit.

Positions 1 & 3 show the spring (vernal) equinox position & the autumnal equinox position respectively.

Positions 2 & 3 show th northern hemisphere positions for summer & also winter respectively.

At position 2 the northern hemisphere can be clearly seen to be tilted towards the sun, whilst the Southern one points away from th sum.

At position 4 the northern hemisphere tilts away from the sun, whilst the Southern Hemisphere tilts towards it.

Positions 1 & 3 clearly show that the tilt of the earth lies parallel with the sun, so both day and night have equal lengths.

Positions 2 & 4 show both the northern hemisphere and the southern hemisphere pointing towards the sun respectively.

As can be clearly seen the 23.5 degree tilt of the earth with regards to earth's orbital plane always remains at 23.5 degrees reagardles of the position in the earths orbit.

HTH.

[Mariusz]

How to describe axis state and difrences between normal situation when we have four seasons and hipotetic situation when we have two seasons ( for example winter in the northern hemisphere and summer in the southern) during all year?

Tilt of the axis in regards to orbital plane doesn't change in both situation during a year.

[Moonshane]

The only way you can have permanent seasons in two hemispheres is when you have a solar system situation like our moon. This is called synchronous rotation and if you replace the Earth with the sun and the moon with Earth you have fixed seasons on one hemisphere although it's eastern and western not northern and southern in this case. I don't think it's possible with northern and southern hemisphere unless the planet is wobbling and spinning wildly over the orbit.

[Moonshane]

If you believe it or possible despite all the evidence in this thread to the contrary I'll be happy to read your sources to educate myself. 

[Stu]

2 minutes ago, Moonshane said:

I don't think it's possible with northern and southern hemisphere unless the planet is wobbling and spinning wildly over the orbit.

Was trying to think how this could be possible and it looks pretty unfeasible doesn't it? I think the earth's rotation axis would have to be always pointing at the sun as it orbited, not going to happen for a number of reasons.

[Moonshane]

The problem with that is that the poles are by definition the axes of rotation and in an orbit the axis of rotation cannot always point at the orbital axis unless it changes direction.

[Knighty2112]

3 hours ago, Mariusz said:

How to describe axis state and difrences between normal situation when we have four seasons and hipotetic situation when we have two seasons ( for example winter in the northern hemisphere and summer in the southern) during all year?

Tilt of the axis in regards to orbital plane doesn't change in both situation during a year.

Just to check still Mariusz; have you now got an idea now of how the equinox's work, and how the seasons come about from the 23.5 degree 'fixed' orbit of the earth in relation to its orbital as it orbits around the sun?

[Mariusz]

Yes, but I am still not sure what's going on exactly with the axis, I am only sure that 23.5 degree tilt doesn't change. If you answer for my quation above, will be easier to understand.

[Stu]

1 hour ago, Moonshane said:

The problem with that is that the poles are by definition the axes of rotation and in an orbit the axis of rotation cannot always point at the orbital axis unless it changes direction.

You said that far better than I could ever have done!! 

[Knighty2112]

18 minutes ago, Mariusz said:

Yes, but I am still not sure what's going on exactly with the axis, I am only sure that 23.5 degree tilt doesn't change. If you answer for my quation above, will be easier to understand.

I cannot see what you are failing to see here as the axis of rotation of the earth and it position in relation to the sun has been plotted in diagrams and graphs here, and discussed fully in this thread, so I am at a loss to see what you fail to grasp here, sorry! 

[Mariusz]

 

2 hours ago, Stu said:

I think the earth's rotation axis would have to be always pointing at the sun as it orbited, not going to happen for a number of reasons

In this case earth' s rotation axis (northern end)have to be always pointing opposite direction than the Sun, but You got the idea. so

What are the reasons it is not happening? 

[Moonshane]

We are a long way from "what's an equinox" but the reason it is not happening is that objects in space travel in a straight line unless affect by the mass of another object . Earth is effectively captured by the sun but is still attempting to travel in a straight line. If you actually look at the content of the sources and clips above, you'll understand that the orbital characteristics are dependent on a number of factors such as distance, difference in masses of the objects etc. Eventually, the orbit of the Earth, like the moon, will change as the solar system eventually breaks down.

http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/zoom-travel.html 

[Moonshane]

Or more basically, because Earth, due to its mass relative to the sun, distance from the sun and speed, does not (currently) have a synchronous rotation. If you actually read my reply above, it's not very likely that a planet could ever have a northern/southern hemisphere fixed season situation but would be more likely to have a eastern/western hemisphere fixed season situation but only when there is synchronous rotation.

[harrym]

1 hour ago, Mariusz said:

 

In this case earth' s rotation axis (northern end)have to be always pointing opposite direction than the Sun, but You got the idea. so

What are the reasons it is not happening? 

In order to change the direction of the Earth's rotational axis, something would have to apply a torque to the Earth (or the Earth would have to change shape, but we'll ignore that). You can think of a torque as a "turning force". An example is how sitting on one end of a see-saw causes the see-saw to turn. This is because the force applied to the see-saw acts on a point that is not aligned with the centre of mass of the see-saw (which is just the middle). Sitting on the middle of a see-saw doesn't cause it to turn.

So how does this apply to the Earth? Well, the Earth is already turning, but a torque (caused by an off-centre force) would cause change in either the rate or axis of rotation, or both. However, almost all the force on the Earth is gravitational (mainly from the Sun, and to a lesser extent the Moon), and a uniform gravitational force always acts on the centre of mass of an object. So these forces don't produce any torque, and the direction of the Earth's axis doesn't change.

(In fact, the gravitational forces are slightly non-uniform, because the gravitational force due to the Moon/Sun is slightly stronger on the side of the Earth facing the Moon/Sun. This causes very small torques, due to the Moon and Sun's gravity acting on the Earth's tidal and equatorial bulges. The torque on the tidal bulge slows down the Earth's rotation very slightly, and the torque on the equatorial bulge causes precession.)

[Mariusz]

 

2 hours ago, Mariusz said:

5 hours ago, Stu said:

I think the earth's rotation axis would have to be always pointing at the sun as it orbited, not going to happen for a number of reasons

In this case earth' s rotation axis (northern end)have to be always pointing opposite direction than the Sun, but You got the idea. so

What are the reasons it is not happening? 

1 hour ago, harrym said:

In order to change the direction of the Earth's rotational axis, something would have to apply a torque to the Earth (or the Earth would have to change shape, but we'll ignore that). You can think of a torque as a "turning force". An example is how sitting on one end of a see-saw causes the see-saw to turn. This is because the force applied to the see-saw acts on a point that is not aligned with the centre of mass of the see-saw (which is just the middle). Sitting on the middle of a see-saw doesn't cause it to turn.

Thanks for all your answers.

Does it mean that in this case we can describe rotational axis as fixed, because it doesn't change direction?

[Moonshane]

More accurate would be fixed until it changes direction due to one or more of the factors described by Harry.

[Mariusz]

Video shows fixed axis and fixed? axis.

 

[Stu]

5 minutes ago, Mariusz said:

Video shows fixed axis and fixed? axis.

 

The second example shows a fixed axis as it always point in the same direction. The first example is a rotating axis, always pointing towards the sun, unlikely (impossible??) to happen I think.

[Moonshane]

The first shows the equivalent of a synchronous rotation as the same face shows to the sun all the time like our moon. The axis of rotation is fixed and the Earth in the first example rotates once per year however the axis on the globe is not the rotational axis in that case rather it is vertical. In that case a day is the same length as a year. 

In the second example he more or less reflects what the Earth does which is that it has a fixed rotational axis that always points in the same direction as it orbits the sun.