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Calculating RA and decl of a star

izubiaurre

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izubiaurre

izubiaurre

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Hello,

I'm making some calculations about stars RA and decl and then getting azimuth and elevation for a given date.  I do get some calculated values but I don't know if they're correct or wrong (I'm making a mess with so many values). The values I calculated are for Theta Persei (Theta Per, θ Persei, θ Per) star. This is what I got so far:
  

VON ROEDERIK TEST

   2/22/2017 7:59:59 PM
   Variations of the proper motion: 2.7366353941186 right-ascension, 49.2305238246344 declination 
   Star's coordinates for the mean equinox: 2.75614796622705 right-ascension (alpha_0), 49.3023491068233 declination (declination_0) 
   Alpha:     2.75612677132799 , Delta:    49.300333849233


If I'm correct this Alpha and Delta values, are the corrected values due the small but continous movement of the star (Alpha:     2.75612677132799 , Delta:    49.300333849233). 

How I can test if these values are correct or not?

Thank you!

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izubiaurre

izubiaurre

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5 minutes ago, noah4x4 said:

Get SkySafari 5 Plus APP

Select star.

Select date (past or future)

Select "info"

It's all there. It might even be in the cut down SkyPortal which is a FREE APP

Thank you for your fast response, Noah.

SkySafari is pretty expensive for only testing purposes (50€), but I will take a look to SkyPortal if it can help me.

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noah4x4

noah4x4

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32 minutes ago, izubiaurre said:

Thank you for your fast response, Noah.

SkySafari is pretty expensive for only testing purposes (50€), but I will take a look to SkyPortal if it can help me.

Think you are looking at the 'Pro' version. SkySafari 'Plus' is much cheaper. 

But SkyPortal (for IOS or Android) or Stellarium are free. But my view is the APPs may be easier for the limited checks you require, as its navigation is so easy.

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furrysocks2

furrysocks2

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Example 21.b accounts for proper motion and precession to give a mean position - example 23.a then corrects for nutation and annual aberration to give the correct apparent position.

Running through the first only with J2000 RA/dec and proper motion values from SIMBAD, using your given final date, I get pretty close: 

2.7562984882	49.2998467253

Note that my calculations account only for proper motion and precession (mean position), but not for nutation or annual aberration (apparent position). I'm assuming that's the difference too between your mean and final numbers?

thetapersei.png.35b1ec64bd179d36ffb98c9234434fe8.png

 

I'd assumed you started from J2000 coordinates but I may be wrong - what values are you using? We start RA within 1.5 arcseconds of each other, but declinations differ by 7.5 arcseconds?

 

(Edit: I should declare now that this is all new to me... I saw your question, found the chapter in the book and gave it a crack for fun, and to perhaps give you another sample to validate against...)

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izubiaurre

izubiaurre

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18 hours ago, furrysocks2 said:

Have you got a copy of Astronomical Algorithms by Jean Meeus? There's an example reduction for Theta Persei in example 21.b.

I have a copy of this book, that's why I was working with Theta Persei example, but because it has only one example and I have a minimal error I asked how I could test other dates to check my algorithms.

17 hours ago, furrysocks2 said:

Example 21.b accounts for proper motion and precession to give a mean position - example 23.a then corrects for nutation and annual aberration to give the correct apparent position.

Running through the first only with J2000 RA/dec and proper motion values from SIMBAD, using your given final date, I get pretty close: 


2.7562984882	49.2998467253

Note that my calculations account only for proper motion and precession (mean position), but not for nutation or annual aberration (apparent position). I'm assuming that's the difference too between your mean and final numbers?

thetapersei.png.35b1ec64bd179d36ffb98c9234434fe8.png

 

I'd assumed you started from J2000 coordinates but I may be wrong - what values are you using? We start RA within 1.5 arcseconds of each other, but declinations differ by 7.5 arcseconds?

 

(Edit: I should declare now that this is all new to me... I saw your question, found the chapter in the book and gave it a crack for fun, and to perhaps give you another sample to validate against...)


Yes, I started with a J2000 RA and decl and it's proper motion (if I remember correctly, taken from Wikipedia). My initial values are: 

RA: 2.73666277777778 Decl: 49.2284666666667

And proper motion: 

Proper motion RA: 0.03425, Decl: -0.0895

Also, I'm using aberration, precession, obliquity and nutation. According to the book, at the date of 11/13/2028 3:33:35 AM 

	My results: Alpha:     2.77066427945211 , Delta:    49.352066560726 
    Books results: AlphaTest: 2.77066444444444 , DeltaTest 49.3520694444444
   
Topocentric results of my code:
     Theta Persei azimuth: 301.123947514799º - elevation 47.5819979838398º
Topocentric results of the book:
     Theta Persei azimuth: 301.123950954249º - elevation 47.5820003359397º
Topocentric results of stellarium:
     Theta Persei azimuth: 300.9337º - elevation 47.5708º

 

 

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furrysocks2

furrysocks2

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I'll have another look tomorrow or so... see if I can reproduce the book's example... note that I think the RA proper motion in the book might be a misprint... looks off by one decimal place compared with what I find elsewhere... Wikipedia looks to have the same values as SIMBAD.

Have you written your own code to calculate, using a library, etc...?

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izubiaurre

izubiaurre

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Yes, I'm using a combination of a library and my own code. The library has a lot of functions but it's impossible to understand if you don't have a complete book of astronomical calculations because there's no help nor comments on the functions. 

At the moment I'm trying to calculate a star topocentric position of a given date and it's rise/set/transit times.

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furrysocks2

furrysocks2

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31 minutes ago, izubiaurre said:

At the moment I'm trying to calculate a star topocentric position of a given date and it's rise/set/transit times.

My kind of fun! ;) Is this for studies or just personal interest?

What library are you using? I've played with aaplus only, though have come across scipy and astropy through playing with other data.

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furrysocks2

furrysocks2

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2 hours ago, furrysocks2 said:

I think the RA proper motion in the book might be a misprint... looks off by one decimal place

Ignore - it's quoted in seconds, not arcseconds... still, SIMBAD equivalent is approx 0.02231 seconds where Meeus has 0.03425.

Worth noting rounding errors as well - I started my calcs again using astropy and had to round the effects of proper motion (as he did - 3 and 2 decimal places, respectively) to achieve the same α0 and δ0. Without rounding, RA was nearly 1s (~14.8") out, and Dec ~2.6" out (note units). That's quite a lot, but understandable given too many decimal places would clutter an example, and to not then use the rounded numbers would be confusing.

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izubiaurre

izubiaurre

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Double data type can handle a lot of decimals so, rounding should not be the origin of the deviation. Maybe three are some errors in the functions, but normally that gives +/-min error in hour calculations (acceptable if you're waiting for an special light moment light golden hour, twilights or sunrise/sunsets). 

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furrysocks2

furrysocks2

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5 minutes ago, izubiaurre said:

rounding should not be the origin of the deviation

If you're comparing his example with your reproduction of the example, note that Meeus rounds where you may not. Unless you need the accuracy to be sub arc-minute, I'd be tempted not to worry too much. If you understand the process and implement it well enough that Stellarium or similar agrees (within some acceptable tolerance), you'll probably be fine.

Good luck with it all.

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furrysocks2

furrysocks2

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3 hours ago, izubiaurre said:

  


Topocentric results of my code:
     Theta Persei azimuth: 301.123947514799º - elevation 47.5819979838398º
Topocentric results of the book:
     Theta Persei azimuth: 301.123950954249º - elevation 47.5820003359397º
Topocentric results of stellarium:
     Theta Persei azimuth: 300.9337º - elevation 47.5708º

 

Check your proper motion... try 0.02231 instead of 0.03425 and -0.08999 instead of -0.0895.

Differences between your code and book look like (lack of) rounding, whereas I reckon Stellarium will have different values for the proper motion as Meeus and SIMBAD/Wikipedia disagree.

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izubiaurre

izubiaurre

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VON ROEDERIK TEST

   11/13/2028 3:33:35 AM
   Initial RA: 2.73666277777778 Decl: 49.2284666666667
   Proper motion RA: 0.02231 Decl: -0.08999
   Variations of the proper motion: 2.73739654148241 right-ascension, 49.2296179172985 declination 
   Star's coordinates for the mean equinox: 2.77027483549342 right-ascension (alpha_0), 49.3503377818603 declination (declination_0) 
   Alpha:     2.77056829538027 , Delta:    49.3520654104295 
   
Topocentric results:
     Theta Persei azimuth: 301.12433345807º - elevation 47.5810969653502º

This is what I got after I used the new proper motion values. Minor alterations from the old ones, so it has to be something related to the formulas that are in the code functions...

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furrysocks2

furrysocks2

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13 hours ago, izubiaurre said: 

VON ROEDERIK TEST

   11/13/2028 3:33:35 AM
   Initial RA: 2.73666277777778 Decl: 49.2284666666667
   Proper motion RA: 0.02231 Decl: -0.08999
   Variations of the proper motion: 2.73739654148241 right-ascension, 49.2296179172985 declination 
   Star's coordinates for the mean equinox: 2.77027483549342 right-ascension (alpha_0), 49.3503377818603 declination (declination_0) 
   Alpha:     2.77056829538027 , Delta:    49.3520654104295 
   
Topocentric results:
     Theta Persei azimuth: 301.12433345807º - elevation 47.5810969653502º

This is what I got after I used the new proper motion values.

 

I'm getting confused at the first step. I don't know how far I'll get but basically trying to go independently from SIMBAD to Stellarium, ignoring the values in the book. I'm using astropy (SkyCoord, Angle, etc...). I'm also outputting error wrt the values in your test run above. 

Initial coords...
02h44m11.987s +49d13m42.4111s
2.73666306667, 49.2284475278

Error in RA:  0.0155999999095 arcsec
Error in Dec: -0.0689000000989 arcsec

Proper motion per year: 0.33466arcsec, -0.08999arcsec

Final epoch: 2028-11-13T03:33:35.000
t = 0.28866935857 (28.866935857 Julian years)

Effect of proper motion: 9.66061arcsec, -2.59774arcsec

Taking into account proper motion...
02h44m12.6311s +49d13m39.8134s
2.73684196683, 49.2277259346

Error in RA:  -29.9470312962 arcsec
Error in Dec: -6.81113783235 arcsec


Converted to final mean equinox...
02h46m10.9898s +49d20m54.5153s
2.7697193958, 49.3484764704

Error in RA:  -29.9937433338 arcsec
Error in Dec: -6.70072111622 arcsec

 

Can you explain exactly how you reach this line: 

Variations of the proper motion: 2.73739654148241 right-ascension, 49.2296179172985 declination

Unless I've misunderstood something, your value for dec appears to be greater than the starting dec by approx 4.14" though proper motion is -ve in dec, and your change in RA is 4x greater than I'd expect.

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izubiaurre

izubiaurre

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Have you taken a look at what I posted, furrysocks2?

Despite of the errors, I'm currently trying to calculate rise/set/transit times for this stars but no correct results yet (it tells me that it doesn't rise, nor set, neither transits for 2017 year).

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