Equinox

[harrym]

For planets and major moons (basically anything that's round or close to round), rotational axes are ALWAYS fixed, except over very long periods when there may be precession. This is because, as I mentioned earlier, there is very little torque on such an object. (For smaller objects that are asymmetrical, such as Saturn's moon Hyperion, the rotation can be chaotic.) Even when the rotation is synchronous, as with the Moon and the Earth, the axis is still fixed. In the case of the Moon, this leads to "libration in latitude": over the course of a month, the Moon's north pole (for example) tilts towards and then away from the Earth. The Moon's axis is tilted only 6.7 degrees to its orbital plane, so this effect isn't very obvious.

[Mariusz]

If you extend axis from both side to the infinity, as I see, Axis in the first example draws shape like at the first picture, axis,in the other example, draws shape like at the second picture

 

 

[Paul M]

So Polaris whizzes round at superluminal speed in a huge distant orbit to keep itself in alignment with Earth's axis? 

Or maybe it stays put amongst the rest of the stars and Earth's axis just conveniently remains pointed towards it?

Only one of those state of affairs can be correct.

[Moonshane]

11 hours ago, Mariusz said:

If you extend axis from both side to the infinity, as I see, Axis in the first example draws shape like at the first picture, axis,in the other example, draws shape like at the second picture

 

 

If you extend the axes from both sides to infinity you would have two infinitely long parellel lines not three dimensional shapes, the first pair vertical and the second tilted at 23.5 degrees.

If you extend the whole axial orbits you would get tubes one vertical in the first video and one tilted at 23.5 degrees in the second video. This is because both videos SHOW FIXED ROTATIONAL AXES.

Your graphics suggest infinity is at the poles or centre of the Earth?

[Mariusz]

First video shows axis tilted 23.5 degrees, I don't know how you spot that it is vertical? These two videos show what's going on with axis, not how fast the Earth spins. The one reason you don't see the earth spinning is because I have't got three hands and the other reason is that this video represents what's going on during 525600 minutes when the Earth orbits the Sun in a few seconds, so really your eyes should not spot any spinning.

 

 

[Moonshane]

I have already explained this. It is because it has a synchronous rotation.

[Moonshane]

The be clear, the axis is the centre line of rotation of the planet and in the first part of the video this is vertical and not that of the "atlas globe' as it is not rotating on that 'axis'.

[Moonshane]

Watch this video 

Ignoring minor differences the first part of your video shows the orbit of the moon.

[Mariusz]

First part of the video doesn't show the orbit of the moon, and rotational axis is not vertical, it's tilted 23.5 degrees regards to orbital plane. 

Use a laser as pencil ended from both sides which represent axis. Holding it on the outstretched hand, tilt it from the vertical at an angle of 23.5 in such a way that the north end is pointing in the direction where you look at in the front of you. Southern end of laser points on the floor, the northern end on the ceiling. Make a move around yourself. The axis moving on the cone's side elevation will draw the shape as below.

 

 

 

[Moonshane]

I completely disagree and refer you to my previous answer.

[Paul M]

10 minutes ago, Mariusz said:

First part of the video doesn't show the orbit of the moon, and rotational axis is not vertical, it's tilted 23.5 degrees regards to orbital plane. 

Use a laser as pencil ended from both sides which represent axis. Holding it on the outstretched hand, tilt it from the vertical at an angle of 23.5 in such a way that the north end is pointing in the direction where you look at in the front of you. Southern end of laser points on the floor, the northern end on the ceiling. Make a move around yourself. The axis moving on the cone's side elevation will draw the shape as below.

I've no idea where we are up to.

So to find some common ground, do you agree that Earth's axis is always pointing towards Polaris, day and night, season after season?

Of course there is some parallax to consider but so far as the apparent position of Polaris is concerned it's effectively zero. If Polaris was only as far away as your lounge ceiling then the effect I think you are alluding to would indeed be a factor.

[Knighty2112]

 

First part of the video doesn't show the orbit of the moon, and rotational axis is not vertical, it's tilted 23.5 degrees regards to orbital plane. 

Use a laser as pencil ended from both sides which represent axis. Holding it on the outstretched hand, tilt it from the vertical at an angle of 23.5 in such a way that the north end is pointing in the direction where you look at in the front of you. Southern end of laser points on the floor, the northern end on the ceiling. Make a move around yourself. The axis moving on the cone's side elevation will draw the shape as below.

 

 

 

This again is the precessional cycle you are describing of the earth on its axis that takes over 25,920 years to complete one cycle, not the earths yearly cycle around the sun. In the earths yearly rotation around the sun the axis is fixed in the same position in all its 360 degree circuit around the sun. I repeat, it is fixed in the same position. The very slight movement that does occur is negligible on a yearly basis, and would take over 72 years to get even 1degree of movement in the full 360 degree 25,920 year pressesional cycle. 

[Stu]

I actually think the double conic shape is correct for the first example not the second, even though in practise this arrangement is impossible. The earth's axis is tilted on the globe at 23.5 degrees and it is rotating axis is orbits the sun so a line extended out from both sides will describe a double cone shape.

In the second example the axis remains at 23.5 degrees pointed in the same direction so would describe a thin tube pointed at 23.5 degrees.

I really do think this thread needs a re-statement of what the OP does or does not understand, and what they want to establish because so far  it is about as confused as anything I've seen on SGL!

[Mr Spock]

This topic defies logic. I can't even...

[Moonshane]

The two polar orbits would surely form a tube? I cannot see how two poles in orbit can describe anything but. You could only form a cone as far as I can see if considering the orbital relationship between the south pole and north pole for an axis tilted to the orbital plane and from the perspective of the sun but this is getting very abstract. Eg if you consider our south pole and take its orbit as fixed round the sun you could then map the north pole's orbit and as the Earth is tilted it has an orbit relatively offset from that of the south pole so you could then say this forms a cone but I doubt this is waht Marius is suggesting.

If anyone disagrees with me and can explain why I'd be genuinely grateful to understand if I am incorrect.

 

 

[harrym]

Trying to work out what's going on from the point of view of an observer on the surface of the Sun (as in the video) is confusing. When you try and analyse the motion of ANY object from the perspective of a rotating reference frame, things can get confusing (eg Coriolis effect).

I say we should forget about that video (I'm not sure what the first part of it is even trying to show) and think about what's actually going on.

The direction Earth's rotational axis is fixed, at least over normal timescales. By 'fixed' I mean fixed relative to the effectively fixed background stars.

If I spin around on the spot, the Earth's axis won't appear fixed to me. But it's still fixed. If I stand on the Sun (yeah I know that's unlikely) and always face the Earth, the Earth's axis won't appear fixed to me. But it's still fixed.

[Mariusz]

I made the diagrams to explain more details. First diagram represents first video. The axis is tilted 23.5. There is one(?) force affecting the Earth orbits the Sun. Axis remains fixed(?), all the way around we observe solstice.

The  second diagram represents second video. The axis is tilted 23.5. There is two(?) forces, one is affecting the Earth orbits the Sun, the other force is affecting the axis to be fixed(?), which allows us to have four seasons. In the second video, it can be seen, that an extra force has to be applied to keep changing the seasons.

[Paul M]

9 minutes ago, Mariusz said:

 

The  second diagram represents second video. The axis is tilted 23.5. There is two(?) forces, one is affecting the Earth orbits the Sun, the other force is affecting the axis to be fixed(?), which allows us to have four seasons. In the second video, it can be seen, that an extra force has to be applied to keep changing the seasons.

 

On the contrary, it would require a force to shift Earth axis in the way you are thinking in drawing no.2. The mass of the Earth is in a free-fall orbit around the Sun. There is no force nor any need for a force to keep Earth axis pointing almost directly at Polaris. It would require a huge force to act on the Earth to shift its axis annually as you suggest. 

Of course, as already discussed, there are a number of gravitational forces and interactions at work in the Earth/Moon/Sun system which introduce perturbations, processions and tidal friction etc. The effects of those are mostly quite small or happen over long timescales.

The simple fact remains that centuries of observation and the laws of physics support the views of every respondent to this thread other than yours and I'm still struggling to work out your position or proposition.

[Knighty2112]

57 minutes ago, Mariusz said:

I made the diagrams to explain more details. First diagram represents first video. The axis is tilted 23.5. There is one(?) force affecting the Earth orbits the Sun. Axis remains fixed(?), all the way around we observe solstice.

The  second diagram represents second video. The axis is tilted 23.5. There is two(?) forces, one is affecting the Earth orbits the Sun, the other force is affecting the axis to be fixed(?), which allows us to have four seasons. In the second video, it can be seen, that an extra force has to be applied to keep changing the seasons.

For diagram two I am not sure what this extra force you think is needed to create the seasons with? Is this force marked in green the pressessional rotation of the earth that you think is needed here? If that is the case then you are totally wrong. There is no extra force required to create the seasons, they come about purely from the positions of the earth in orbit around the sun yearly in its 23.5 degree tilt to earth's own orbital plane, which at the two solstice points cause unequal amounts of sunlight falling on both the northern and Southern hemispheres, causing summer/winter, and at the equinoctal points when sunlight is equal in both hemispheres causing spring/autumn. There is no other force required on earths orbit required to cause the seasons here, so I am not sure why you can't see this or where your confusion comes from in regards to this which is something that is not in dispute by anyone except you. 

[Mariusz]

Why the Earth does't behave like in the Video/Diagram 1?

 

 

[harrym]

On 27/08/2017 at 14:10, 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.

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.)

I've explained this already.

Angular momentum (https://en.wikipedia.org/wiki/Angular_momentum) is always conserved.

[Moonshane]

People seem to be getting as frustrated as me explaining and re-explaining something so simple to someone so apparently intelligent.

[Mariusz]

Exactly it is -Angular Momentum in Classical Mechanics- the same source you referred to, and it applies to Video/Diagram 1

 

 

 

[Moonshane]

I think you are confusing orbit and rotation.

[Moonshane]

The Earth is traveling through space in a straight line but the mass of the sun bends that straight line into an orbit but to all intents and purposes with respect to many things including angular momentum it is still traveling in a straight line. If you walked in a straight line with a rotating gyroscope would the axis change?