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Right ascension and the siderial day

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#1 Steve Harris

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Posted 23 October 2020 - 09:01 PM

Silly question maybe, but the sidereal day being around 23h 56m long, why do the RA hours go all the way to 24?  Won't that make the "drift" time to measure RA differ a bit from actual (though not much)?



#2 KBHornblower

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Posted 23 October 2020 - 09:08 PM

Silly question maybe, but the sidereal day being around 23h 56m long, why do the RA hours go all the way to 24?  Won't that make the "drift" time to measure RA differ a bit from actual (though not much)?

It's only silly if you don't ask.  A clock that is adjusted to keep sidereal time shows 24 hours between transits of a star, and is the clock that is used for reckoning the RA.  I have seen solar and sidereal clocks side by side in observatories.


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#3 Steve Harris

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Posted 23 October 2020 - 09:16 PM

Aha, I wasn't aware that a siderial clock actually has a different rate! Thanks for clarifying.

#4 yosmithy

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Posted 23 October 2020 - 09:41 PM

I thought it was a great question, because I did not know how the day differed in a sidereal day. Good stuff



#5 Galaxyhunter

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Posted 24 October 2020 - 01:48 PM

If it wasn't a different  rate, you would  see the  same stars at the same time each night.  With the different  rate,  the stars slowly  drifts to the West at the same ( say midnight ) each night.


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#6 kathyastro

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Posted 24 October 2020 - 01:58 PM

I have an old wind-up travel alarm clock that was never any use for its intended purpose because it ran 4 minutes fast every day.  But it makes a perfect sidereal time clock!  I use it on multi-day star parties.  I can set it at home from a sidereal time app or website, and it will be close enough for the entire weekend.


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#7 Steve Harris

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Posted 24 October 2020 - 11:04 PM

If it wasn't a different  rate, you would  see the  same stars at the same time each night.  With the different  rate,  the stars slowly  drifts to the West at the same ( say midnight ) each night.

Well no, you could avoid having a different rate (ie. non-standards seconds/minutes/hours) by just using standard seconds but having fewer of them in a day, with RA having fewer as well to match, endng at 23h56(.x)m.



#8 catalogman

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Posted 25 October 2020 - 08:46 AM

The rotation of the Earth takes 23h56m, which is the sidereal day.
But the revolution of the Earth causes the Sun to move eastward,
so the time between solar transits is 24h, which is the solar
day.

 

Of course, the eccentricity of the Earth's orbit makes solar
time irregular, so the time definitions refer to a mean Sun.
On a 24-hour clock, the mean Sun transits at noon on a

standard time meridian every day.

 

-- catalogman


Edited by catalogman, 25 October 2020 - 09:20 AM.


#9 Andrekp

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Posted 30 October 2020 - 09:59 AM

Well no, you could avoid having a different rate (ie. non-standards seconds/minutes/hours) by just using standard seconds but having fewer of them in a day, with RA having fewer as well to match, endng at 23h56(.x)m.

Except that unnecessarily complicates other things.  It’s much easier to just apportion the sky by 360 degrees, call 15 degrees an hour, and go from there.  The sky is circular, so the measuring system should be as well.  Your way would put a 4 minute hiccup into that, for no real reason.  It’s be like the international date line: you couldn’t go across that line simply by moving, for example 36 degrees west, you’d have to account for those missing 4 minutes or time as well.  Etc.



#10 Steve Harris

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Posted 30 October 2020 - 07:55 PM

It’s much easier to just apportion the sky by 360 degrees, call 15 degrees an hour, and go from there. 

Sure it's easier, but it's not correct, because 15 degrees is not precisely a (standard) hour of rotation for the stars. And that was the original point, that it requires changing to time units that are 0.997... of the originals (ie. about 10 sec/hour off). Which I'd say is pretty confusing in itself.  Anyway, I wasn't proposing a new system or saying which one is better, just objecting to the claim that the rate of passage or secs/minutes/hours themselves has to be different.

 

Edit: Fixed ratio


Edited by Steve Harris, 30 October 2020 - 08:06 PM.


#11 Galaxyhunter

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Posted 30 October 2020 - 08:15 PM

Sure it's easier, but it's not correct, because 15 degrees is not precisely a (standard) hour of rotation for the stars. And that was the original point, that it requires changing to time units that are 0.997... of the originals (ie. about 10 sec/hour off). Which I'd say is pretty confusing in itself.  Anyway, I wasn't proposing a new system or saying which one is better, just objecting to the claim that the rate of passage or secs/minutes/hours themselves has to be different.

 

Edit: Fixed ratio

Doesn't  Sidereal & Solar time have to be different?



#12 KBHornblower

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Posted 30 October 2020 - 08:44 PM

Sure it's easier, but it's not correct, because 15 degrees is not precisely a (standard) hour of rotation for the stars. And that was the original point, that it requires changing to time units that are 0.997... of the originals (ie. about 10 sec/hour off). Which I'd say is pretty confusing in itself.  Anyway, I wasn't proposing a new system or saying which one is better, just objecting to the claim that the rate of passage or secs/minutes/hours themselves has to be different.

 

Edit: Fixed ratio

Neither one is "correct" to the exclusion of the other.  We use whichever one is useful for the task at hand, be it civil timekeeping or astronomical timekeeping.



#13 KBHornblower

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Posted 30 October 2020 - 10:41 PM

Addendum: The "fixed" stars mark an inertial frame of reference, and in that frame the Earth turns a complete rotation in the time we call a sidereal day.  In chasing the Sun, which appears from our point of view to be moving eastward relative to the stars, the Earth turns close to 361 degrees in that same inertial frame in what we call a mean solar day. 



#14 Andrekp

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Posted 31 October 2020 - 09:40 AM

Sure it's easier, but it's not correct, because 15 degrees is not precisely a (standard) hour of rotation for the stars. And that was the original point, that it requires changing to time units that are 0.997... of the originals (ie. about 10 sec/hour off). Which I'd say is pretty confusing in itself.  Anyway, I wasn't proposing a new system or saying which one is better, just objecting to the claim that the rate of passage or secs/minutes/hours themselves has to be different.

 

Edit: Fixed ratio

I think you are missing the point.  The subdivisions, whatever they are, need to be complete in one rotation around the sky.  360 degrees can divide into 15 degree subunits.  15 works well because it is roughly an hour.  But the important part is that each subdivision is complete.  The focus is on basic usability, not time.  Being a few seconds off does not hurt navigational use (which was the original main point), and it does no require having to compensate for a gap.

 

don’t get hung up on them being called “hours.”  It doesn’t mean that literally, so much as the next logical name up from minutes and seconds.  If they had called them Keplers, or something, you wouldn’t be complaining about them. 


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#15 gwd

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Posted 02 November 2020 - 08:20 PM

I have an old wind-up travel alarm clock that was never any use for its intended purpose because it ran 4 minutes fast every day.  But it makes a perfect sidereal time clock!  I use it on multi-day star parties.  I can set it at home from a sidereal time app or website, and it will be close enough for the entire weekend.

Years ago, I had a wind-up desktop alarm clock with an adjustment lever so you could force it to run fast.  If it allowed sufficient adjustment a clock like that would be useful for astronomy. 

 


 




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