J2000 to Now - Rotation_EQJ_EQD

[mikasmart]

Hi !

Thank you for this wonderful module!

I am looking for a way to get the position of a star on today's date. I have the coordinates of the star but in J2000 format (ra/dec).
I view a Rotation_EQJ_EQD function but I can't figure out how to use it and there is no demo about it.

Someone can help me ? My code is in javascript.

[cosinekitty]

Hi @mikasmart ! That is a great question. You were on the right track looking at Rotation_EQJ_EQD. This is not a trivial conversion, so I have added a demo to illustrate it for you. Take a look at equator_of_date.js. Here is a sample of how to use it for the equator at the current date and time:

$ node equator_of_date.js 12 45
time = 2021-07-07T00:56:06.051Z
Equator-of-date coordinates: RA=12.0181, DEC=44.8819

Or you can optionally specify a particular date and time to use:

$ node equator_of_date.js 12 45 1972-02-14T14:35:00Z
time = 1972-02-14T14:35:00.000Z
Equator-of-date coordinates: RA=11.9765, DEC=45.1535

[mikasmart]

Thanks ! It's exactly what i mean.

[mikasmart]

I did a test with M57 and using data from Stelarium. The results are very close but incorrect.
Maybe the calculation lacks precision?

With M57 (data from Stelarium)
Date / Degrees / Decimal conversion
J2000 / 18h53m35.08 - 33°01'45.0" / 18.89308 - 33.02917
NOW / 18h54m23.72 - 33°03'24.4" / 18.90659 - 33.05678

Result from equator_of_date (Must be equal to M57 & NOW)

NOW / 18h54m22.32 - 33°03'21.6" / 18.90621 - 33.05597

Test started with command node equator_of_date 18.89308 33.02917

[cosinekitty]

Oh no, this is embarrassing! I realize I made a mistake. There is still a large discrepancy. I need to investigate more. Please stand by...

[cosinekitty]

I can see now that Stellarium does perform both precession and nutation calculations. I made a silly mistake in my spreadsheet that fooled me into thinking that it didn't. I have been looking at their source code, both for the web version and their native C++ version. I'm still trying to figure out the discrepancy. Here are some of things I'm looking at:

• The simple example I provided does not correct for aberration. I'm not sure what your goals are, so aberration may or may not be a helpful correction. This can account for a position difference of more than 20 arcminutes.
• Nearby stars (like Procyon) can have significant proper motion over the time span from 2000 to 2021. I doubt a distant object like M57 would be affected much though. But for Procyon, this changes its position by -0.25 arcminutes in RA and -0.37 arcminutes in DEC. In general, one needs to adjust for the movement of stars over time.
• I intend to add more rigorous testing of precession and nutation calculations. Perhaps I will use Stellarium source code to generate some test data and verify it against Astronomy Engine calculations. I'm pretty sure I would have noticed such a large error by now, but I want to make sure there is nothing wrong with this part of the code.

@mikasmart, can you tell me more about what you are trying to do? It would help me figure out whether aberration corrections are a good idea for your project.

[mikasmart]

@mikasmart, can you tell me more about what you are trying to do? It would help me figure out whether aberration corrections are a good idea for your project.

With pleasure! I would very much like to discuss my project but I don't want to talk about it here. My project uses Arduino & smartphone.
My email address: (redacted) - Contact me for more information

[cosinekitty]

@mikasmart, I am working on new functions in Astronomy Engine to help correct for stellar aberration, which can cause errors of up to 20.5 arcseconds (about 1/3 of an arcminute). I believe this is part of the discrepancy you discovered between Stellarium and Astronomy Engine.

I estimate this work will take about a week to finish. Then I will be in a better position to repeat tests with Stellarium and see if I'm getting closer to agreement with it.

I will update here when I have something interesting to report.

[cosinekitty]

@mikasmart I just updated the equator_of_date.js to correct for aberration, and the results are very encouraging. Before, for the M57 Ring Nebula, the discrepancy between Stellarium and Astronomy Engine was about 0.4 arcminutes. After correcting for aberration the disagreement is less than 0.01 arcminutes. I hope you can try the latest version and confirm that it is more in agreement with Stellarium.

[laheller]

Hi @cosinekitty

First of all, let me congratulate you on your astronomy engine library! I just found it and still wondering, how well it is implemented and how easy is to use it! I am coming from the C#.NET world and I also tried other libraries, like for example Joe Sauve's AASharp which is very good too but has somewhat poor or almost zero documentation.

Nevemind, I had the same question as @mikasmart and meanwhile I am thinking about, whether is it possible to do it a different way, using the Astonomy.Equator( ) method, like this:

I have one star from the Henry Draper catalog, having its _RA.icrs and _DE.icrs coordinates, both are J2000 in decimal degrees.

My C# code is then:

using CosineKitty;

// J2000 equatorial coordinates of the HD11150 star
var RA = 28.9986; // according to VizieR catalog desc it's in decimal degrees, later we need to convert to sidereal hours by dividing 15
var DE = 80.9102; // decimal degrees

var obs = new Observer(lat, lon, height);
var now = DateTime.UtcNow; // the date we want to convert to
var astro_now = new AstroTime(now);

// do the conversion
// first let's define a star, but switch RA coordinate in decimal degrees to sidereal hours, because the method expects it
Astronomy.DefineStar(Body.Star1, RA / 15.0, DE, 1000.0);
var coord = Astronomy.Equator(Body.Star1, astro_now, obs, EquatorEpoch.OfDate, Aberration.None);
// do something with the coord, e. g. display it

My question is, whether is the above calculation correct for the input star? Does the Equator method convert J2000 to JNow coordinates this whay?

BR,

Ladislav

[cosinekitty]

Yes, using EquatorEpoch.OfDate will report the star's position using the Earth's equator at the time astro_now, correcting for precession and nutation.

[laheller]

Thank you!