JNow and conversion to J2000

Just a general question wrt the conversion of JNow to J2000. I understand the differences but am curious as to some possible nuances to the conversion process. My mount, GTD G53F, reports in JNow and a reported position will exactly match that in CDC. C2A is off slightly as is Stellarium. Stellarium reports both JNow and J2000 but when their J2000 coordinates are compared to SGP they are close in RA but off in Dec. Nothing that a plate solve won’t cure but it has me wondering what would account for the differences? Would using different catalogues account for minor variances? Is there perhaps a difference in the formulae or catalogue references used in the conversion process?

Not a big deal but just curious.

As a part deux to this question, Is there a rationale for SGP only reporting J2000 and converting rather than having a JNow option?

I believe that I have solved one of my pointing issues today as my gotos have always been off a bit. I seem to have been inputting JNow coordinates into my sequences. Presumably, if SGP converts my mount from JNow to J2000 the goto coordinates should be J2000.



I can’t think of how. Really it should just be a simple conversion between the coordinates. Maybe check that your location and time are the same between all devices including your mount?

SGP uses J2000 as those coordinates do not move. A JNow from 2018 vs a JNow for the same object today will be different. Essentially JNow doesn’t give you a complete picture of where the object is as you also have to know the Date (Now in this case). So to store a JNow location we’d also need to capture the Date/Time that it occurs at and basically create a micro epoch like J2021.01.05.1750. J2000 gives us the other half of that puzzle so from a storage perspective J2000 makes sense. From a “I need to slew my mount now” perspective JNow makes sense. There isn’t much (any?) loss going from J2000 to JNow and back.


Hi Jared,
Thanks, Loc etc., are consistent between programmes. I see what you mean about how JNow can create logistical issues as it presents a moving target rather than a fixed point in time. I guess the mystery for me is how different programmes can interpret JNow with ever so slightly different results. Perhaps there is something in various conversion formulae that accounts for small differences. As I mentioned it isn’t a big deal as plate solving corrects positioning but I was just curious.

Thanks again,


It depends on where those applications are getting your Location and Time from. Some might use the mount, some might use windows, some might use user input options. SGP will use the user profile if available and then query the mount for Lat/Lon.

I’m assuming by “plate solving corrects positioning” that the applications are still off with one another or is everything magically synced after a solve?


Hi Jared. Thanks, that gives me more insight. You are right about plate solving magically correcting things. :slight_smile: Where the mount ends up after a first goto is close to the target coordinates. Once plate solved later gotos are pretty well bang on target.