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Equinox


Mariusz

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Do I understand corectlly, that plane of the earth's equator sets to 0 degree with respect to ecliptic and we see that the arc passes through the zenith at the equator, resulting no shadow at high noon?

 

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Yes, at the equinoxes, the sun is directly over the equator but at the times stated at the top right of the wiki page I linked to. There will be times when this happens for other regions through the year but the equinoxes are when it happens at the equator and it points at/passes through the centre of the sun.

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Ok, thanks, sorry that my questions seems to be silly, but I really want to understand this subject so that no one can mislead me as we know great astronomical events are going to take place soon. 

My next question is: In the picture above I see four the same position of the plane of earth' S equator respect to the sun. How can I spot which position is related to equinox ( just considering earth's equator respect to the sun) when the picture is not labelled?

Thanks for your patience.

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The Earth orbits the sun so there's  only one orbital plane to consider but yes it is effectively bound to the centreline of the sun. The Earth is tilted at 23 ish degrees and rotates once per day.

This might help

15023103879542127677235.thumb.jpg.761240c2313a6b5ebccb38ba27c2a2d6.jpg

 

 

I think you need to buy a book or read more online.

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I suspect that the "problem" is that there is often an assumption that the earth tilts forward and back over a year but it doesn't. It is an apparent change in the tilt that is created by us simply orbitting the sun but the "fixed" tilt of the earth gives the annual appearance.

Catch is that it is not exactly easy to explain, I have seen a couple of attempts to explain it and it either clicks or you are left as perplexed as before asking. A similar one I noticed a few months back was someone trying to explain how the phases of the moon came about.

There are worse things to explain, at the eclipse there will be people standing there observing it and yet a percentage will if asked at any other time say the moon is never in the sky during the days time as in Sun = Day, Moon = Night.

Astronomy can be good fun, but occasionally it takes an odd insight to see things, especially some of the supposed simple ones.

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Thank you. As I see orbital plane is bound to the centreline of the earth as well. Does it say that position of the earth' S equator allways is the same respect to the Sun? If it is the same, what exactly is changing? Which angle is changing, where and when?

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Picture below shows spring equinox. We can see orbital plane passing by the centre of the earth and showing direction to the sun. The angle -23.5 degree- plane of earths equator respect to orbital plane is visible. The point, where observers see the sunrise is right below black handle, high noon on the line connecting two poles. 

Is everything ok with this picture to this point?

DSCN1187.JPG

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Many times people ask me this question during sessions. Although I am not that great with answering using technical terms and explanation.

I try to explain it using a torch (or a candle) and my hand. Just raise your thumb (Like you are giving a thumbs up) and extend your pinky finger. The tip of the Thumb is north pole and tip of your pinky finger is south pole. (You can do it using a ball...Just hold it in two fingers...upper one is your north pole and lower finger is south pole.) Then tilt it roughly to match the 23.5 degree. And now just go around  the torchlight like earth. (Or candle...light source is the sun and your hand is the earth)

(given that you used a ball held using two fingers) The point where your lower finger does not receive torchlight is summer solstice.(short nights in northern hemisphere)  When your upper finger is not receiving any light, it is winter solstice. (Longer nights in northern hemisphere) These two point will be 180 degrees apart and 90 degrees from each point, you will have your solstice. 

(Would have explained much better with photos if I were not travelling)

With reference to previous comments in this topic, you will get the idea.

I guess the main problem is that we can't directly think in 3D so it's tough to visualize in our mind.

For audience questions, you can do this right at the spot.

Hope this helped!

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  • 2 weeks later...
On 09/08/2017 at 22:09, Mariusz said:

Thank you. As I see orbital plane is bound to the centreline of the earth as well. Does it say that position of the earth' S equator allways is the same respect to the Sun? If it is the same, what exactly is changing? Which angle is changing, where and when?

The orbital plane is dictated by the centre of masses of the sun and Earth not their equators.  The angle of inclination or tilt of the Earth does not change other than by a tiny amount of 'wiggle' but as it orbits the sun obviously due to this tilt, the Earth's equator rises and falls in relation to the orbital plane (or solar equator if you like) hence the equinoxes.

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On 21/08/2017 at 12:05, Mariusz said:

In this video 2:55-3:10 one year of the Earth around the Sun. From this perspective (perspective of the Sun)we can see, what this move is all about. 

Precession??

 

That graphic is misleading. From the perspective of the Sun you would expect to see the Earth rotating on its axis. But the apparent "wobble" of the Earth's axis during that section of the video is in fact caused by the rotation of the "camera" - the axis still always points to the same place in the sky. The video is trying to explain some much longer-term motions that will just confuse you if you're trying to understand the equinox.

The video also contains a couple of mistakes: the biggest one is that the equinoxes are NOT at the crossing point of the analemma.

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On 23/08/2017 at 06:52, Mariusz said:

 

In this video 2:55-3:10 one year of the Earth around the Sun. From this perspective (perspective of the Sun)we can see, what this move is all about. 

Precession??

 

On 22/08/2017 at 13:51, harrym said:

But the apparent "wobble" of the Earth's axis during that section of the video is in fact caused by the rotation of the "camera" - the axis still always points to the same place in the sky

How can you prove it, that it is  caused by the rotation of the camera? Video below shows changing of the seasons and a wobble during a year, when camera is a fixed point. 

 

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3 hours ago, Mariusz said:

How can you prove it, that it is  caused by the rotation of the camera? Video below shows changing of the seasons and a wobble during a year, when camera is a fixed point. 

 

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. Of course they are not, so in fact the camera must be rotating. The Earth's axis always points in the same direction relative to the stars (well, it changes very slowly but over the course of one year we can ignore that). But the camera is rotating (and its rotational axis is aligned with the Earth's orbital plane, NOT its equator) so the stars appear to be rotating as well. The Earth's rotational axis points to Polaris, but Polaris appears to be tracing a circle in the sky because the camera is rotating. So the Earth's axis appears to wobble so that it still always points to Polaris. But in fact it is the camera that is rotating, not Polaris.

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2 hours ago, Moonshane said:

Maybe i am being dim but not sure what your gyroscope gif is showing?  Earth's southern pole is not in a fixed position? Maybe you could explain what you mean?

Earth is like gyroscope spinning and going around the sun. Gif below shows hipotetic situation when axis of the earth is fixed. It is  like if you take a globe, shift plane of the equator 23.5 degree respect to orbital plane and make a move around the sun with fixed elbow and wrist, standing in the one point. There will be the same season in the northern and southern hemisphere.

Make a note that axis in the south points all the time in the same place. Axis in the north points difrent places during the year

Gyroscope_precession.gif

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Ok I get that now but don't agree with you. However your gif illustrates the equinox point well. 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?

 I don't accept the Earth moves like your gyroscope as the poles of the Earth point to the same point during the orbit see the illustration I uploaded above. The equinoxes are caused by the fact that the Earth's equator relative to the sun, rises and falls due to tilt.

This page http://www.faculty.virginia.edu/rwoclass/astr1210/guide04.html explains it well under E  and this is where the following diagram is located 

Dec-of-Sun-v-Date.jpg.33e1eef564e39804cba148bd0bf661a5.jpg

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