Equinox

[Mariusz]

35 minutes ago, harrym said:

The clearest way to see that the camera is rotating is to observe that all the background stars appear to be rotating around the Earth.

I know that camera is rotating. My question was about something else.

[Mariusz]

13 minutes ago, Moonshane said:

Ok I get that now. If you watch the 'equator ' this points to the centre of the sun only once each side of the orbit around the sun. These are the equinoxes so in effect you have answered your own question. You seem to know so much about the subject I am surprised you didn't understand it in the first place?

Thanks, I did some research and next quations appeared. Gif model doesn't show equinoxes, because there is allways summer in the Southern Hemisphere and winter in the Northern Hemisphere .

[Moonshane]

Sorry yes I clarified my answer with further expansion.

[Mariusz]

Ok, I'd like to leave equinox for now and focus  more what's going on with axis during a year and come back to equinoxes at the end.  

[Moonshane]

See the illustration above. There's no change to the axis in real terms over the year. Put a skewer through an orange, hold it at an angle so the pole approximates that of Earth and the walk around a fixed object. There's no change in the axis as you walk round but the poles are either closer or further away depending which side you are on hence seasons. The closer to the equator you are the less variation  in seasons but there is still subtle variation.

[Mariusz]

Thanks,  I found the same explanation in many sources, but I am really confused, because all of them in the same time says that the Earth and it's axis behave like giroscope (principles).  In the hipotetic situation-with gif model, which was discribe above, I clearly sea axis and where they are pointing, there is a changes in the axis. (?). Summer on the Southern Hemisphere and winter in the northern all year.

 

[Stu]

I think you are over complicating this.The gyroscope example is just plain wrong as far as I can see. The earth's axis remains fixed (ignoring long term precession), it does not wobble like the gyroscope. Shane has explained it well in a number of different ways but you don't seem to be listening to the key point. The Earth's axis does not wobble over the course of a year. Simples.

[Moonshane]

"The hypothetico-deductive model or methodis a proposed description of scientific method. According to it, scientific inquiryproceeds by formulating a hypothesis in a form that could conceivably be falsified by a test on observable data. A test that could and does run contrary to predictions of the hypothesis is taken as a falsification of the hypothesis. A test that could but does not run contrary to the hypothesis corroborates the theory. It is then proposed to compare the explanatory value of competing hypotheses by testing how stringently they are corroborated by their predictions."

 

Using your stated method, the Earth does not have permanent summer in one hemisphere and permanent winter in another hemisphere nor does the axis wobble wildly over a year so the gyroscope hypothesis is false.

 

[Mariusz]

1 hour ago, Stu said:

 

I think you are over complicating this.The gyroscope example is just plain wrong as far as I can see. The earth's axis remains fixed (ignoring long term precession), it does not wobble like the gyroscope

 

Gyroscope model is right, because it describes exactly what's going on in this hipotetic situation and everything match. From perspective of the sun axis here remains fixed, like your elbow and wrist should stay fixed to make this move with your glob around the Sun.

[Moonshane]

We are going to have to agree to disagree about the relevance of the gyroscope model relative to the Earth. It is only correct if you are on the sun. Equinoxes don't happen on the sun they happen on Earth.

[Knighty2112]

The earth does spin around on its axis with the northern and southern projections of the poles changing position in space, but this takes 25,000 years to complete one full rotation. All this rotation does is shift the dates when the equinoxes, and also the summer and winter solstices occur. But the tilt of the earth is always at 23.5 degrees during all this slow precessional drifting of the apparent position of the poles, just as in a yearly orbit the earth is tilted always at 23.5 degrees, so (regardless of the longer shifting of the equinoxes over 25,000 years) in every year there will always be a point twice in the earths yearly orbit where the sun is level with the equator at mid day, so when standing at the equator on these dates the sun will appear directly overhead, so the length of the day and the night are equal. At two other points in earths orbit the earths equator will be shifted  23.5 degrees northwards or southwards at a certain date at mid day, so we get the summer or winter solstices of the respective hemisphere that you live in. At no point in the earths orbit does the inclination of the orbit change from 23.5 degrees. However, with regard to the position of the sun at mid day the sun will appear twice in its orbit at 90 degrees above the horizon at the equator, and once each year at a point at 23.5 degrees more at the equator at mid day at the suns most northerly point (the northern summer solstice), and also 23.5 degree less at the equator at mid day at the suns most southernly point (the northern winter solstice). 

Hard to explain, yes. Much easier to visualise rather than explain, so the diagram below might make the above description much easier (hopefully!)  

[Moonshane]

2 minutes ago, Knighty2112 said:

Much easier to visualise

Hence my addition of an illustration at the start of the thread. ?

[Stu]

12 minutes ago, Mariusz said:

Gyroscope model is right, because it describes exactly what's going on in this hipotetic situation and everything match. From perspective of the sun axis here remains fixed, like your elbow and wrist should stay fixed to make this move with your glob around the Sun.

I'm with Shane here, we will have to agree to disagree.

[Moonshane]

Anyway I am off to look at the sun ?

[Stu]

3 minutes ago, Moonshane said:

Anyway I am off to look at the sun ?

Lucky you! I'm stuck at work!

[Knighty2112]

6 minutes ago, Moonshane said:

Hence my addition of an illustration at the start of the thread. ?

Yep, saw that Shane. Wasn't trying to take away anything from your post, so hopefully 2 diagrams are better than one! Hehe!  

[Mariusz]

5 minutes ago, Stu said:

Hard to explain, yes. Much easier to visualise rather than explain, so the diagram below might make the above description much easier (hopefully!)  

It works well for some one who is outside of this diagram, but when you place yourself where the sun is, you will see axis in difrent positions during a year. 

 

[Stu]

33 minutes ago, Mariusz said:

It works well for some one who is outside of this diagram, but when you place yourself where the sun is, you will see axis in difrent positions during a year. 

 

I get that, but viewing it like that is clearly not helping you to understand it so I don't see how it is relevant?

[Moonshane]

You seem to change your argument to suit when someone explains what is going on. What are you trying to understand currently?

[Moonshane]

47 minutes ago, Knighty2112 said:

Yep, saw that Shane. Wasn't trying to take away anything from your post, so hopefully 2 diagrams are better than one! Hehe!  

Of course I realise that buddy ?

[Mariusz]

47 minutes ago, Moonshane said:

 

You seem to change your argument to suit when someone explains what is going on. What are you trying to understand currently?

 

I am not changing my argument to suit anything. I am describing what I see. Below there are real pictures of Nasa which makes me confused even more.

 

 

[Knighty2112]

OK, try to think of the equinox points when viewed from another view point; when either at one of the two equinox points in the earths orbit, even though the orbit of the earth is tilted still at 23.5 degrees, as it is now lines up parallel with the sun it appears square on to the earth (in relation to the poles), so the sun will appear to an observer X at noon at the equator as been straight above the observer (90 degrees overhead, so no shadows will be cast of the observer), so the lengths of both day  and night wil be equal for both northern and Southern Hemispheres  (see image below below);

 

If we now got to the point in the orbit when a solstice occurs, viewed from the same angle as above above, the tilt of the earth (poles) is not square on with the sun at the equator at mid day, so the observer X sees the sun as higher up at mid day at this point, a shadow will be cast (image showing the norther hemisphere solstice), so at this point in its orbit the sun appears in the sky for a shorter period as from the point for an equinox, so the days are longer at this point in the earths orbit for the majority of the  northern hemisphere, but shorter for most of the Southern hemisphere;

 

HTH!

[Moonshane]

22 minutes ago, Mariusz said:

I am not changing my argument to suit anything. I am describing what I see. Below there are real pictures of Nasa which make me confused even more.

 

 

Again you have referenced something looking from 1m  miles away rather than Earth. This has no relevance to equinoxes. What is it you are trying to understand?

[Mariusz]

Yes, but it has relevance what I see is going on with axis.

[Moonshane]

From what perspective? I.e. from what point in space? It matters. Things look different when looking from one point or another. Just look at the recent eclipse. It looked different from various points on Earth and even happened at different times at different points along the path of totality.