Equinox

[harrym]

4 hours ago, Mariusz said:

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

In Video/Diagram 1, the Earth's angular momentum is changing, so angular momentum is NOT conserved unless it is transferred somewhere else. But there's no way for that to happen, as that would require large torques on the Earth that don't exist.

Do you agree with everything in my post about torque? If not, what do you disagree with?

[Mariusz]

Ok I get that, the fact that the obcject is spinning making rotational axis fixed for observer standing outside the orbit. Only when the Earth was not spinning the diagram 1 is true.

[harrym]

In diagram 1 the Earth is still spinning, but it rotates at exactly the same rate that it orbits the Sun, so it always shows the same face (well, almost - there will still be a bit of libration) to the Sun. This situation is similar to the Moon's orbit around the Earth. The axes that you drew in diagram 1 are therefore incorrect as the Earth isn't rotating about them.

[Mariusz]

Let's leave the axis for now and back to the Equinox. 

Looking at the picture below I don't understand how observers all over the world see the Sun rising in the same point where exactly east is, but when it comes to the high noon, they see it in difrent places on the sky.

[Moonshane]

An equinox is a single moment in time in two different places on the equator over the course of a year. The answer to your question though is because the Earth is tilted to the orbital plane and rotating on that axis. We seem to be back to square one. I suggest with the knowledge you have gained, or perhaps not gained, you re-read the thread as all the answers are there.

[Mariusz]

I reapit the quastion, there is no answer in the thread for it.  

Looking at the picture I can't see why observers (wherever they are in the glob) can see the Sun rising exactly where the cardinal east point is, but if so, they should see the Sun in the high noon in the zenith as well( vertically above a head) wherever they are in the glob. How to explain it?? 

 

[Paul M]

29 minutes ago, Mariusz said:

I reapit the quastion, there is no answer in the thread for it.  

Looking at the picture I can't see why observers (wherever they are in the glob) can see the Sun rising exactly where the cardinal east point is, but if so, they should see the Sun in the high noon in the zenith as well( vertically above a head) wherever they are in the glob. How to explain it?? 

 

 

I'll admit that now you have submitted a question I can get a handle on the answer is actually quite complex and not easily understood in terms of momentary observations from any given location.

On the day of the Equinox the Sun will pass directly overhead for an observer on the Equator. But for them (depending on whether spring or autumnal equinox) the Sun will rise either north or south of due east and set (opposingly) south or north of due west. Can't align my brain cells to work out which way is which. 

My ability to run 3D models in my head after a 12 hour shift at work is diminishing!

Very interesting question and requires some time playing with Stellarium!

 

[Mariusz]

26 minutes ago, Paul M said:

On the day of the Equinox the Sun will pass directly overhead for an observer on the Equator. But for them (depending on whether spring or autumnal equinox) the Sun will rise either north or south of due east and set (opposingly) south or north of due west.

Yes, these changes are not visible, so we can say that Sun will pass over equator from east point, vertical high noon and set on the West Point. But I can't see how it works at the picture, rise for observer at the equator seems to be shifted 23.5 degree to the south, set the same, only high noon seems to be in the right place.

[harrym]

This is quite hard for me to explain in words rather than physically. I can tell what's going on but I can see how it might be counterintuitive.

Is that your globe, or if not, do you have a globe? If so, I suggest you cut out a small piece of card (or paper if you don't have any, but card would work better as it's stiffer), and draw a cross on it, marked with the four compass directions. Stick this to the globe on the equator with north pointing towards the North Pole. You will find that the Sun rises in the east, passes directly overhead and sets in the west. Then stick the card to the globe somewhere other than the equator. You will find that the Sun still rises in the east and sets in the west, but does not pass overhead.

2 hours ago, Paul M said:

On the day of the Equinox the Sun will pass directly overhead for an observer on the Equator. But for them (depending on whether spring or autumnal equinox) the Sun will rise either north or south of due east and set (opposingly) south or north of due west. Can't align my brain cells to work out which way is which.

This may be true due to the motion of the Earth around the Sun during that particular day. But the Sun will rise very close to due east and set very close to due west everywhere on Earth.

[Paul M]

47 minutes ago, harrym said:

 

This may be true due to the motion of the Earth around the Sun during that particular day. But the Sun will rise very close to due east and set very close to due west everywhere on Earth.

I based my thoughts on the idea that at sunrise and sunset on the day of the equinox, an observer on the Equator is  some degrees above or below the ecliptic plane and due to the local geometry of the horizon relative to the ecliptic plane the and Sun would appear offset from E & W as seen by the observer... I thinked.

Doubting myself, I just set my location as 0 long, 0 lat in Stellarium and looked at sunset and sunrise on the dates of the Equinoxes and proved myself to be talking almost complete, well no, complete nonsense!

 

[Knighty2112]

3 hours ago, Mariusz said:

Yes, these changes are not visible, so we can say that Sun will pass over equator from east point, vertical high noon and set on the West Point. But I can't see how it works at the picture, rise for observer at the equator seems to be shifted 23.5 degree to the south, set the same, only high noon seems to be in the right place.

Best I can describe what's happing at the equinox is with the drawing below, which shows as looking through the earth (if that were ever possible right!) towards the sun, showing the earth's orbital plane, the 23.5 degree tilt of the earth which at this point in the earths orbit lies parallel to the sun. The sun is seen directly in the centre of the earth at this position if we could see through the earth towards the sun. Observers are placed at position X (at the equator), and Y further north in the northern hemisphere.

At sunrise, observer X at position X1 sees the sun rise due east as the earth rotates on its axis. At position X2 on the earths globe the sun will be directly overhead, and at sunset at position X3 they will see the sun set due west. The sun takes the same amount of time to climb from position X1 to X2 (6 hrs), and then from X2 to X3 (6hrs), so the day is 12 hrs long, and hence the night is also 12hrs long.

Observer Y will see the sun also rise  due east at Y1 position, but at midday at position Y2 they will not see the sun directly overhead but lower down in the sky. They will then also see the sun set at Y3 position. 

For all observers on earth at the equinoxes the sun will take the same time to rise and set, so both day and night will be equal at all points.

After either of the equinoxes, the tilt of the earth does not lie parallel with the sun anymore, so either the northern hemisphere or the Southern Hemisphere starts to receive more or less sunlight as they tilt closer, or further away from the sun, and the sun at midday rises further north or south because of this, so daytime and nighttime are no longer equal anymore.

And that is my final word on the topic in this thread!  

[Moonshane]

Bear in mind that an equinox is a specific moment that does not necessarily happen at 'noon' / midday  in the traditional sense I.e. 12pm 

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

[Mariusz]

Knightly2112 your model shows the same only from the other perspective. As you see direction for the Sun shows straight line running on the orbital plane from the centre of the Earth to the centre of the Sun. Parallels point direction east( including equator and tropics). The only reason I can accept that observer at the equator see the Sun straight in the cardinal point east, ( as observers at every parallels) is ,that according to the scale, Sun will be 1,5 mile from the picture/diagram, so for all observers  in the world Sun will rise in the same point ( this principle is not just for equinox, only degrees will vary during a year). But according to the distance between the Earth and the Sun and that observers at every parallel see the Sun rising in the same point as observer in the equator, all observers doesn't matter if they are at the tropic or 55 N or whatever, they should see Sun overhead in the noon, as observer see at the equator.

 

[Knighty2112]

27 minutes ago, Mariusz said:

Knightly2112 your model shows the same only from the other perspective. As you see direction for the Sun shows straight line running on the orbital plane from the centre of the Earth to the centre of the Sun. Parallels point direction east( including equator and tropics). The only reason I can accept that observer at the equator see the Sun straight in the cardinal point east, ( as observers at every parallels) is ,that according to the scale, Sun will be 1,5 mile from the picture/diagram, so for all observers  in the world Sun will rise in the same point ( this principle is not just for equinox, only degrees will vary during a year). But according to the distance between the Earth and the Sun and that observers at every parallel see the Sun rising in the same point as observer in the equator, all observers doesn't matter if they are at the tropic or 55 N or whatever, they should see Sun overhead as observer see at the equator.

 

No. At midday when the sun is at its highest the sun will appear offset from the observers local zenith point because of the earths curvature. With your statement the sun would also appear directly overhead at the  north pole too, which is patently absurd and impossible for the sun to appear like this for all observers on the earth. Eratosthenes worked out the principle to work out the approximate circumference of the earth way back in Ancient Greece. The diagram below shows that an observer at Syene sees the sun at midday directly overhead, but due to the circumference of the earths curvature, the suns rays which hit the earth parallel at all points on the earth, an observer at Alexandria at the same time due north of Syene sees the sun offset from zenith at Alexandria by 7 degrees. The further north and south you are from the equator at these points make the sun appear shifted from the local observers zenith point by the degrees north or south they are from the equator. For an observer near the North Pole the sun appears at mid day at nearly 90 degrees on the horizon. 

 

[Mariusz]

According to what you said and evidence you provided, observers should see the  Sun rising in the difrent points depend where they are on the glob.

[Knighty2112]

32 minutes ago, Mariusz said:

According to what you said and evidence you provided, observers should see the  Sun rising in the difrent points depend where they are on the glob.

No. Refer back to my previous diagram posted yesterday. At sunrise observers X & Y at position X1 & X2 see the sun rise due east. For an observer at the north pole too sun would still rise in the east. At midday the observer at position X2 sees the sun directly overhead. The observer at Y2 sees the sun offeset from their local zenith point by however many degrees further north they are from the equator. An observer at the North Pole would see the sun at midday at the horizon due south from then.

The below quick diagram shows that at sunrise the sun appears to rise due east for all observers, as the suns rays lie parallel here also. At any points on the terminator shadow line on the earth the sun appears to rise in the east at sunrise. See above diagram in last post above as to why the sun appears at a different point when at its highest point in the sky for an observer.

 

[Moonshane]

http://curious.astro.cornell.edu/about-us/161-our-solar-system/the-earth/day-night-cycle/184-how-does-the-location-of-sunrise-and-sunset-change-throughout-the-year-advanced

http://curious.astro.cornell.edu/about-us/52-our-solar-system/the-sun/observing-the-sun/186-why-does-the-location-of-sunrise-change-intermediate

 

[Mariusz]

Where dissapeared the orbital plane at your picture, showing exactly direction to the Sun?

[Moonshane]

I understand what Marius is saying about the latitude affecting the sun's angular position in the sky and why this should also be the case (due to parallel sunrays) for the longitudinal aspect (I e. From above the north pole) but this is not the case because of the rotation of Earth on that axis. The terminator position is what represents sunrise and this is perpendicular to the sun's rays at equinox and therefore eyeryone sees the sun rise in the east and set in the West.

[Knighty2112]

42 minutes ago, Mariusz said:

Where dissapeared the orbital plane at your picture, showing exactly direction to the Sun?

Carry on the centre line of the suns raysto go through this the eastern point on the earth, centre of the earth, and to the West Point of the earth horizontally. That is still the earths orbital plane. 

[Mariusz]

For me now your picture looks like the equator's plane is tilted 0 degree regards to orbital plane and the rotational axis is at 90 degree regards to orbital plane.

 

[Knighty2112]

9 minutes ago, Mariusz said:

For me now your picture looks like the equator's plane is tilted 0 degree regards to orbital plane and the rotational axis is at 90 degree regards to orbital plane.

 

The axls is still tilted, but it lies parallel with the sun at this point. I could do another drawing to show the tilt, but all this would show at this angle is the south Pole 23.5 degrees higher up on the centre line between night and day, and the northern pole emerging at the back of the planet on the other side 23.5 degrees from the vertical. The axis is still tilted, but as we see it from this vantage point it looks vertical, even though we don't see the tilt in this rough drawing.

Clearly you can't visualise this at all, so I suggest you go through all the diagrams and videos posted in this thread until hopefully some day it all clicks. Good luck! 

[Mariusz]

1 hour ago, Mariusz said:

 

I understand what Marius is saying about the latitude affecting the sun's angular position in the sky and why this should also be the case (due to parallel sunrays) for the longitudinal aspect (I e. From above the north pole) but this is not the case because of the rotation of Earth on that axis. The terminator position is what represents sunrise and this is perpendicular to the sun's rays at equinox and therefore eyeryone sees the sun rise in the east and set in the West.

 

Yes, if so, imagine you are at the equator when Sun rise, you see it on the horizon on the east, let's stop all the motions Earth does. Exactly on the north you see Polaris on the horizon as well. Start to move your self towards Polaris you will see Polaris higher and higher in the sky until you will see it over head at the Pole. Sun should change the position during this move staying on the horizon I don't know how far it should go but at least south east when you get the pole. ( Also Sun should appear bit higher on the pole, than at the equator, because sun rise observer at the pole is a Relatively lower than sun rise observer at the equator). Probabbly Iam one of them who can't see things working on the spinning glob?

[Knighty2112]

Revised drawing to show the South Pole (marked as an asterisk) leading as it would appear (if we could see it) higher up on the vertical axis), and the North Pole behind the planet lower down but hidden on the vertical axis. The point of the equator for observer X at X1 is higher up on the vertical due to the 23.5 degree tilt, and due to the curvature of the earth it appears at X2 position at midday. The same goes for Y1 & Y2 position too. At all points on the vertical terminator of the earth the sun appears still to be due east. At midday at X2 the sun casts no shadow, but at Y2 it will cast a shadow as the sun is not at the observers local zenith point. Rem bet this is at the equinox point only.

If you are still not getting it at all then I am not sure what evidence, video or drawing is going to be able you to visualise this in your own head, so I suggest you review all the evidence in this thread again until eventually it will click with you.

 

 

[Knighty2112]

To try to visualise this hold a pencil up vertical, but tilt it slowly backwards so that point of the pencil is farther away from you, and the blunt end of the pencil is closer to you. The pencil still appears vertical to you even though the axis is now tilted away from you.