I have no background in astronomy. I have been wanting to write code to make star charts for a given location and time, which led to this question.

I figured out that I would need to convert the coordinates of stars from the equatorial coordinate system to a horizontal coordinate system defined by a time and place of interest.

I decided to use the Yale Bright Star Catalog for this purpose.

From what I could gather, the Yale Bright Star Catalog does not explicitly mention the coordinate system it uses, it mentions the epoch to be J2000 frequently though and reports RA, DEC. So, it is my understanding that the coordinate system should be an equatorial one with J2000 as the equinox.

As a novice, I could understand the horizontal, equatorial and ecliptic coordinate systems. However, when I started using an astronomy library to write the code, I realised that in practice, there are equatorial coordinate systems such as FK5 and ICRS with minute differences; and that ICRS is the coordinate system adopted by the IAU.

So, which coordinate system is the YBSC catalog in, exactly?

I have noticed that mentioning the epoch, but not the exact coordinate system is the somewhat confusing, but standard way to report coordinates. Example: coordinates in M33's Wikipedia Page, or CDS name resolver.

So, in general, which equatorial coordinate system should be considered 'default' when just the epoch is mentioned as in the examples above? Should it be FK5, ICRS, or something else?

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    $\begingroup$ As the "Other online documentation" link on that page explains, the J2000 coordinates are in the FK5 system. For all but the most exquisitely careful astrometry, the difference between FK5 and ICRS is usually not worth worrying about. $\endgroup$ Oct 30, 2019 at 8:43

2 Answers 2


The questions confuses things: J2000 is the epoch / equinox in which the catalogue is in. This defines the reference point in time for Right Ascension and Declination. See e.g. here https://en.wikipedia.org/wiki/Right_ascension and here https://en.wikipedia.org/wiki/Epoch_(astronomy).

Now FK5 and ICRS are different reference systems, but the difference between both is small (less than 80 micro arseconds). Nowadays, we work mostly with the ICRS, which replaced the FK5 at the end of the last century. Transformation between the reference frames and epochs are given in the specialised literature (e.g. "The Explanatory Supplement to the Astronomical Almanac" or see the references here: https://www.iers.org/IERS/EN/Science/ICRS/ICRS.html ). If you do not want to dive deeply into this, I suggest to use implementations that are readily available, e.g. https://docs.astropy.org/en/stable/coordinates/index.html )

The Yale Bright Star Catalogue (in the revised edition) contains of its entries in J2000 and it is tied to the FK5 (ICRS wasn't there when it was released).


FK5 is based on optical observations. ICRS is based on radio data. Radio observations (ironically) are much higher accuracy than optical through the application of long baseline interferometry.

Other than extremely accurate astrometry (fractions of an arcsec), they are essentially identical.

  • $\begingroup$ Welcome to Astronomy SE! Is it possible to add some links that support your answer in a similar way to what I did here? Thanks! (for example GRAVITY 3 mas vs EHT 25 uas, or just compare $\lambda/L$, but note that some crazy-big optical interferometers are on the "horizon") $\endgroup$
    – uhoh
    May 27, 2021 at 17:21

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