Real basic question-Stellarium accronym

[Narayan]

Hey,

Total beginner at this stuff. In Stellarium, what does the DE stand for in 'Hour Angle/DE' of a celestial object, and what is it a measure of?

Thanks

[hgjevans]

It's Declination - it's like a latitude coordinate for points in the sky, as measured in degrees from the celestial equator. So, up at the north celestial pole (near Polaris) it's 90°, 0° for anything at the equator (Mintaka, in Orion's belt is pretty close), and -90° at the southern celestial pole.

[Narayan]

Thanks!

So what is the difference between the J2000 declination and this one. Are they not both calculated from the celestial equator?

Also, is there any quick guides to understanding the terms that follow beneath?

[damnut]

They will be the same, because these points can move they are measured from one particular spot at a particular time. These are then declared such as the J2000.0 or a Besselian which as a prefix B1950.0 for example. The julian prefixes have been used since 1984. Now I'm confused

Good description here :-- http://en.wikipedia.org/wiki/Equinox_(celestial_coordinates)

[JamesF]

I think there are (at least) two effects at play here.  The first is the Earth's change of axial tilt over time causing the apparent declination of stars to change.  The second is that there's also an axial "wobble" (there's a proper word for it, but I can't recall what it is right now) making a smaller contribution.

So, if you're going to give the declination of a particular object (which is relative to the celestial equator), then you also have to (effectively) define the plane of the celestial equator when that declination was correct.

James

[hgjevans]

Indeed - the Earth's axis 'precesses' over the aeons, so that the coordinates are only precisely correct at a given time. I'm not sure at what point the difference really matters - maybe if you're into things like measuring 'proper motion', or some other such technical stuff, then you'd have to worry about it. But I guess most of us can probably ignore the difference.

[Narayan]

Thanks