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An astronomical model in New Scientist

Just for our stargazing readers

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Tim Stevens22/09/2019 15:14:57
1123 forum posts

New Scientist 21 Sept p 51 shows a simple model intended for youngsters to make to explain equinoxes etc. It goes on to explain precession of the earths axis, saying that this wobble goes round in 25,772 years. The picture shows the precession circle about 3 earth diameters north, and rather less than the equator diameter. BUT it goes on - 'in around 13,000 years time, summer in the northern hemisphere will happen when it currently experiences winter'.

I suggest that there are two things wrong with this statement-

1. the change over the next 13,000 years depends on where the precession angle is now. If it is half way to max lean, the effect of precession in 13,000 years will be back where is started from, for instance.

2. If the description and diagram of precession is correct, what will happen is that the seasons will slowly become slightly more (and less) distinct as we go through the 25,000 year cycle.

Surely if the seasons do wander in a 25,000 year cycle because of precession, then the term precession (of whatever varies) must involve a greater angle than the about 5 degrees shown? And it is not a simple wobble but a rotating one? And if so, why have I seen no references to a series of  seasonless periods in recent geological time?

Many thanks for any clarity you can offer.

Regards, Tim

Edited By Tim Stevens on 22/09/2019 15:17:32

Clive Foster22/09/2019 17:02:20
1992 forum posts
73 photos

Without actually having seen the article it sounds like a crap explanation. Whether inadvertently or deliberately I don't know.

Precession of the equinoxes has nothing to do with seasons. Basically it describes where the earths axis is pointing on any given calendar date. If the stars were fixed in relation to the sun (ye olde crystal sphere) the earths axis would point at the same star on the same date every year. Because the stars, sun and everything else is moving relative to each other this doesn't happen and the axis points a little sideways this year as compared to last year. Easiest way to think about it is the old "stars fixed to a crystal sphere" model and consider the sphere as rotating slowly with respect to the sun. So it takes 25,772 years before the axis once again points to the same star on the same date.

So far as the real universe in concerned its a spurious effect due to the colossal distances between stars, including the sun. So using the (physically) spurious crystal sphere model to show whats happening works just fine. Lies to readers and all that. Much clearer to explain what appears to be happening using easily grasped concepts then explain why the simple model doesn't fit reality.

In a practical world we rarely need scientifically correct models of things. Good thing really. Newtons Laws can be bad enough. Doing it properly with Quantum Mechanics, Wave Functions, String Theory et al doesn't bear thinking bout.


Neil Wyatt22/09/2019 18:43:44
17083 forum posts
690 photos
76 articles

I think NS is right, but explains it poorly. This diagram from Wikipedia is better:

Basically as the earth precesses over 23,000 years the seasons compared to the positions of the stars at any particular date (e.g. the equinoxes) gradually makes a complete rotation. So in 13,000 years the return of Orion to our skies will herald summer not winter.

Also, the pole star changes as the axis describes a circle around the heavens.:


Tim Stevens23/09/2019 13:34:34
1123 forum posts

Here is the article (I hope ...)

Equinox model from New Scientist

You will see the source of my confusion in the small black circle around the earth axis, marked 'precession'. 


Edited By Tim Stevens on 23/09/2019 13:37:12

Michael Gilligan23/09/2019 13:53:15
14780 forum posts
635 photos

< deleted >

Edited By Michael Gilligan on 23/09/2019 13:56:34

Neil Wyatt23/09/2019 13:59:17
17083 forum posts
690 photos
76 articles

Ah yes, the direction the earth leans at should remain constant as it orbits, well, aside from changing slowly as per the diagram, above.

Mike Poole23/09/2019 16:56:43
2326 forum posts
52 photos

I shall be keeping an eye on this season drift. Will this forum still be going for me to report backsmiley


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