MEASURE AXIAL PRECESSION

[MAGICSUN]

Is it possible to measure earth's axial procession with a good amateur telescope? It would require measuring the movement of a star in one year to the accuracy of a decimal point in seconds. I understand it is also vital to make the observations at precisely the same time. Any suggestions greatly appreciated.

[andrew s]

I don't think it is as simple as measuring the position of a star as you need to measure it relative to the earths axis..

You could try measuring the position of Polaris over a night in alt AZ coordinates and find where the centre of its  motion is. Then repeating a year or more late. You would have to watch out that measuring method did not already assume the rate as planiterium software often does calculation from position "now" to a given epoch generally 2000.

Regards Andrew 

Edited April 25, 2020 by andrew s

[MAGICSUN]

Thanks Andrew I think I understand that.

More specifically I was asking if a good amateur telescope could position with accuracy to 0.1 seconds?

Also can somebody explain if the stars have moved due to earth's axial precession at a rate of 1 degree every 71.6 years (or 20 minutes per year), why hasn't the date of the solstice also changed?

 

[andrew s]

Telescope pointing can be in the 10s of arc seconds so yes but you need a reference normally done by a catalogue of stars. The reference basis is a set of distant Quasars .

The day of the solstice does change. There are several causes, try googling.

Regards Andrew 

[MAGICSUN]

Tens of arc seconds is good, but I was hoping to be able to measure for instance 22.4 arc seconds.

The solstice date changes you refer to are adjustments for the inexact year length of 365.2425 days, and ongoing inaccuracies due to earth's speed of rotation gradually slowing down.  There has never been a calendar adjustment for axial precession. The position of the stars have moved 30 degrees over the past 2150 years, as evidenced by the shift in astrological signs in this period. Hence the relative date of solstice has shifted one day every 71.6 years. Yet the calendar has never been adjusted for this.

[andrew s]

Plate solving will tell you where the mount is pointing  to the accuracy you require but that is different to pointing to that accuracy. However this will be based on an epoch 2000 catalogue. 

I am not up enough on orbital mechanics to advise why the date has not changed. Certainly where on the horizon the sun rises on the day of the solstice has shifted but without further study I can't add any more.

Regards Andrew 

[andrew s]

Just had a quick look at "Axial precession " on Wiki it gives a good explanation of the shift but does not comment on why the date does not change. 

I would guess that it is corrected to a degree with the leap years of the Gergorian calendar. 

Again looking at Wiki that what it was aimed to do keep the calendar aligned with the equinoxes.

Regards Andrew 

Edited April 25, 2020 by andrew s

[MAGICSUN]

Thanks Andrew.

Yes the calendar corrected equinox drift due to earth rotation discrepancies, not Axial precession.

[andrew s]

1 hour ago, MAGICSUN said:

Thanks Andrew.

Yes the calendar corrected equinox drift due to earth rotation discrepancies, not Axial precession.

That's not what wiki says it is the axial procession it corrects for it does not correct for changes in the earths rotation rate.

Regards Andrew 

 

[miguel87]

If the change is 30 degrees in 2000 years then I would suggest no.

That's about 0.015 degrees a year.

What you gonna measure this movement relative to? Because it will happen equally to all stars.

[andrew s]

2 minutes ago, miguel87 said:

If the change is 30 degrees in 2000 years then I would suggest no.

That's about 0.015 degrees a year.

What you gonna measure this movement relative to? Because it will happen equally to all stars.

It's measured relative to the dates of the  highest and lowest points the Sun gets to  over the year and it was noticed hence the Gregorian Calendar. 

Regards Andrew 

[MAGICSUN]

Over the past two-and-a-half millennia, this wobble has caused the intersection point between the celestial equator and the ecliptic to move west along the ecliptic by 36 degrees, or almost exactly one-tenth of the way around, to the border of Pisces and Aquarius. This means that the signs have slipped one-tenth — or almost one whole month — of the way around the sky to the west, relative to the stars beyond.

https://www.livescience.com/4667-astrological-sign.html

This is reality today. It has never been adjusted for in any calendar.

Just because wikipedia mentions precession of the equinoxes, the Gregorian calendar does not include an adjustment of 1 day every 72 years. 

 

 

 

[andrew s]

15 minutes ago, MAGICSUN said:

Over the past two-and-a-half millennia, this wobble has caused the intersection point between the celestial equator and the ecliptic to move west along the ecliptic by 36 degrees, or almost exactly one-tenth of the way around, to the border of Pisces and Aquarius. This means that the signs have slipped one-tenth — or almost one whole month — of the way around the sky to the west, relative to the stars beyond.

https://www.livescience.com/4667-astrological-sign.html

This is reality today. It has never been adjusted for in any calendar.

Just because wikipedia mentions precession of the equinoxes, the Gregorian calendar does not include an adjustment of 1 day every 72 years. 

 

 

 

That is not how I understand things. There is a wobble superimposed on the axial procession.  I suspect we are using terms differently.  

To be honest I don't feel motivated to take this further. 

Regards Andrew 

[Mr Spock]

Astrology is complete nonsense. It has no place on any science forum.