Equinox

[Moonshane]

Look at this site. https://www.nesdis.noaa.gov/content/dscovr-deep-space-climate-observatory The satellite you reference sits in a fixed position (effectively) 1 million miles away and films the sunny side of Earth. They must rotate the images to show Earth 'upright' for aesthetic reasons. If you look at the short eclipse run on the above page it shows the start

and the end of the eclipse

from the satellite's perspective. The angle of inclination appears to be what you'd expect i.e. 23.5 degrees.

 

 

[Paul M]

The only way to visualise the mechanics of Earth's axis is from a fixed point that is not connected to either the Sun or Earth. If your position changes during observations it is easy to misinterpret relative motions of the two bodies.

So scrap the wobbling gyroscope GIF, it isn't even a good representation of precession and precession is an whole other subject beyond the concept of the equinoxes and takes place over thousands of years not annually.

For the purpose of the subject in hand, Earth's axis is fixed in space and currently, in the northward direction, points almost exactly (and very conveniently) at Polaris. As Earth circles the Sun its axis remains pointed at Polaris. Neither pole "wobbles" about relative to any reference point.

Because of the axial tilt, at some point in the year, the north pole is tilted towards the Sun. That is the June solstice and as a result the northern hemisphere experiences summer. The Arctic regions experience perpetual daylight and the Antarctic experiences perpetual night. 

6 months later the Earth is on the opposite side of the Sun, as seen from our arbitrary fixed point in space. The south pole is now tilting towards the Sun and the seasons are reverses. That is the December Solstice.

Exactly half way between the solstices we have the equinoxes. These are where Earth's axial tilt is neither towards or away from the Sun. The polar regions both experience 12 hours day and 12 hours night (nominally). They are equal. The Sun appears overhead at the equator at local noon on the day of the Equinox. On the June solstice the Sun appears overhead at local noon on the tropic of Cancer. On the December equinox the Sun appears overhead at local noon on the tropic of Capricorn. This geometry defines the position of the tropics.

Earth's axial tilt is thereabouts 23.5 deg from perpendicular to its orbital plane so it comes as no surprise that the latitude of the tropic of Cancer is 23.5 deg north and the tropic of Capricorn 23.5 deg south. The Arctic and Antarctic circles are, unsurprisingly, 66.5 deg north and south!

Nothing wobbles (on human timescales), it's like a well oiled and rigid machine. The seasons shift smoothly and predictably as Earth experiences solstice after equinox, after solstice, after equinox on its journey round the Sun

Listen closely and you can hear the gears purring away.

[Moonshane]

If you look, it's the same on the NASA video.

[Mariusz]

2 hours ago, Moonshane said:

The angle of inclination appears to be what you'd expect i.e. 23.5 degrees.

I know, it's corect, tilt of the axis respect to orbital plane doesn't change.

2 hours ago, Paul M said:

The only way to visualise the mechanics of Earth's axis is from a fixed point that is not connected to either the Sun or Earth. If your position changes during observations it is easy to misinterpret relative motions of the two bodies.

Sun is a fixed point, if you' d like to say that it doesn't matter because camera is rotating, place 365 cameras around the sun, each of them separetly takes one pictures every day, as soon as the Earth apears in the frame. Everything stay motionless. If you get these pictures and look at them you will see the same changes with axis like pictures from NASA shows.

 

2 hours ago, Paul M said:

So scrap the wobbling gyroscope GIF, it isn't even a good representation of precession and precession is an whole other subject beyond the concept of the equinoxes and takes place over thousands of years not annually.

From fixed point (Sun) this Gyroscope model doesn't wobble, axis doesn't change position, becaouse it represents situation, when there is summer in the Southern Hemisphere and winter in the northern all the time during all year. It shows the true, I can't scrap it.

[Moonshane]

 

12 minutes ago, Mariusz said:

2 hours ago, Paul M said:

The only way to visualise the mechanics of Earth's axis is from a fixed point that is not connected to either the Sun or Earth. If your position changes during observations it is easy to misinterpret relative motions of the two bodies.

Sun is a fixed point, if you' d like to say that it doesn't matter because camera is rotating, place 365 cameras around the sun, each of them separetly takes one pictures every day, as soon as the Earth apears in the frame. Everything stay motionless. If you get these pictures and look at them you will see the same changes with axis like pictures from NASA shows.

By definition the sun is not a fixed point unconnected to the sun or the Earth. In your example above, by taking photos at a different point around the sun every day you are changing position and therefore take a different perspective every day. From here read my previous response about perspective. I still cannot understand what you wish to learn. The NASA camera does not orbit the Earth as such, it sits in a more or less fixed position between the sun and the Earth (taking photos of the constant sunny side) every two hours and guess what, the axis of tilt does not change.

[Moonshane]

39 minutes ago, Mariusz said:

3 hours ago, Paul M said:

So scrap the wobbling gyroscope GIF, it isn't even a good representation of precession and precession is an whole other subject beyond the concept of the equinoxes and takes place over thousands of years not annually.

From fixed point (Sun) this Gyroscope model doesn't wobble, axis doesn't change position, becaouse it represents situation, when there is summer in the Southern Hemisphere and winter in the northern all the time during all year. It shows the true, I can't scrap it.

You are misunderstanding what the gyroscope shows. It is a representation only of the orbit of the Earth around the sun and not the Earth's globe from the perspective of the sun. The 'equator' is not the equator of the globe. The fixed 'south pole' merely reflects the orbit of the south pole around the sun and the variable north pole reflects the track of the orbit of the north pole from the sun's perspective and relative to the orbit of the south polar orbit. It is not very helpful. Mapping the two orbits I have created a rough illustration below (exaggerated for effect) which explains why the gyroscope example moves like it does. The large blue circle is the sun. The small blue circle shows the Earth with assumed tilt. The blue orbit is the south polar orbit and the red one is the north polar orbit which varies in relation to the south polar orbit from the sun's perspective. The key thing to understand is how much perspective affects what you see.

I hope this makes this point clearer.

 

Book1.pdf

[Moonshane]

was https://en.wikipedia.org/wiki/Precession your source for the gyroscope gif? if it was then it is being used to show precession in an exaggerated manner. This is completely different to orbits and equinoxes and happens over very long periods. My comments about assumed you were looking at a site which relates to the topic we were discussing as you referred to seasons and hemispheres. Perhaps you could confirm the source if it's not as above? It might help us stop guessing.

[Stu]

1 hour ago, Moonshane said:

was https://en.wikipedia.org/wiki/Precession your source for the gyroscope gif? if it was then it is being used to show precession in an exaggerated manner. This is completely different to orbits and equinoxes and happens over very long periods. My comments about assumed you were looking at a site which relates to the topic we were discussing as you referred to seasons and hemispheres. Perhaps you could confirm the source if it's not as above? It might help us stop guessing.

It's clear that this is the source of the gyroscope gif Shane, and as you say, relates to something quite different and not relevant to this conversation. I am as confused as you as to what the OP is trying to find out!

[Moonshane]

I should point out that my post above with the pdf was the only way I could get the gyro to make sense in the context of orbits, seasons and equinoxes hence my comments. This is further evidence of perspective creating a different impression!

[Mariusz]

How are you going to get difrent seasons during a year with a fixed axis? With a fixed axis you only get   Summer in the one Hemisphere and winter in the other or atumn in the one and spring in the other during all year. Simple phisical laws applies  here, and this situation exactly represents Gyroscope. If you know any better model in motion showing that what we are talking about, I' d like to see  it.

[Moonshane]

PLEASE forget the gyroscope. It is COMPLETELY irrelevant to the seasons and to orbits. The reason we have seasons with a fixed axis is because the Earth's fixed axis is tilted relative to the sun which is in a (relative to the Earth) fixed position.

 

[Moonshane]

 

[Moonshane]

Think of it this way. If the Earth's axis was not tilted but pointed straight up, then we would all have 'no seasons' or if you like a permanent summer as each day, the Earth would rotate and receive the same amount of sunlight through the whole year. So vertical axis relative to the orbital plane = no seasons for all. Turn the Earth's axis through 90 degrees so the axis points directly at the sun. Now you still have permanent seasons but one pole will never get any light (the one facing away) and one will be permanent 'summer' as it always faces the sun. Any degree of tilt other than these two scenarios will create seasons. It really is as simple as that.

If you can find any evidence that disputes this I'd be equally interested to hear about it.

[Mariusz]

11 minutes ago, Moonshane said:

Any degree of tilt other than these two scenarios will create seasons

Two seasons with fixed axis and four seasons with not fixed axis during one year. The rest is correct.

[harrym]

12 minutes ago, Moonshane said:

Think of it this way. If the Earth's axis was not tilted but pointed straight up, then we would all have 'no seasons' or if you like a permanent summer as each day, the Earth would rotate and receive the same amount of sunlight through the whole year. So vertical axis relative to the orbital plane = no seasons for all. Turn the Earth's axis through 90 degrees so the axis points directly at the sun. Now you still have permanent seasons but one pole will never get any light (the one facing away) and one will be permanent 'summer' as it always faces the sun. Any degree of tilt other than these two scenarios will create seasons. It really is as simple as that.

If you can find any evidence that disputes this I'd be equally interested to hear about it.

No, if the axis is tilted 90° there are still seasons - everywhere has half a year of continuous daytime and half a year of continuous nighttime, so there's summer and winter but not really any spring or autumn. Uranus is pretty much like this.

[Moonshane]

5 minutes ago, Mariusz said:

Two seasons with fixed axis and four seasons with not fixed axis during one year. The rest is correct.

I give in. Are you still seriously suggesting the Earth has four seasons so does not have a fixed axis? Did you even watch the videos?

 

4 minutes ago, harrym said:

No, if the axis is tilted 90° there are still seasons - everywhere has half a year of continuous daytime and half a year of continuous nighttime, so there's summer and winter but not really any spring or autumn. Uranus is pretty much like this.

That's actually what I said Harry. If the axis is vertical (i.e. aligned with the sun's axis) there will be constant summer. If rotated (I should have said from that point) 90 degrees the axis is then pointing at the sun and as you  (and I) describe there are just two seasons, permanent summer and winter.

[harrym]

1 minute ago, Moonshane said:

That's actually what I said Harry. If the axis is vertical (i.e. aligned with the sun's axis) there will be constant summer. If rotated (I should have said from that point) 90 degrees the axis is then pointing at the sun and as you  (and I) describe there are just two seasons, permanent summer and winter.

I understood your post as suggesting that if the axis is tilted by 90° then one pole is permanently pointed at the sun. Sorry if I misunderstood.

[Moonshane]

The first example was with the axis vertical rather than tilted and the second example was horizontal (so rotated through 90 degrees from vertical).

[harrym]

Yes I understood that, but I thought your post suggested that when the axis is horizontal one pole is permanently pointed at the sun, which isn't the case (it still has half a year of sunlight and half a year of darkness). Once again sorry if I'm misunderstanding you.

[Moonshane]

I don't normally accept defeat but am unsubscribing from this thread as I believe that I have done all I can to explain this. Hopefully further reading by the OP or simply actually reading the content of this thread will eventually lead to enlightenment.

[Stu]

I think you have been extremely patient Shane!

[Mariusz]

8 hours ago, harrym said:

if the axis is tilted 90° there are still seasons - everywhere has half a year of continuous daytime and half a year of continuous nighttime, so there's summer and winter but not really any spring or autumn. Uranus is pretty much like this.

Uranus is like that because it has fixed axis. 

[Stu]

1 minute ago, Mariusz said:

Uranus is like that because it has fixed axis. 

Earth has a fixed axis too, unless you are considering long term precession.

[Moonshane]

Sorry I cannot resist. Please can you explain the difference between the fixed axis of Uranus and the not fixed (sic) axis of Earth? 

I dont mean that it contains the word 'not'. I would like you to explain your logic in your own words so we can understand where your confusion lies.

 

Quote this

 

[Moonshane]

8 hours ago, harrym said:

Yes I understood that, but I thought your post suggested that when the axis is horizontal one pole is permanently pointed at the sun, which isn't the case (it still has half a year of sunlight and half a year of darkness). Once again sorry if I'm misunderstanding you.

Sorry Harry but yes you are right I was getting carried away with myself and suspect my brain caved in.