Declination, why?

[wrinkly]

Used to be involved in Radar and raced bicycles, so aware of nutation and inclines as they were harder work.

Realise that not holding a Doctorate in English is a disadvantage but 'Right Ascension' is understandable as it follows the natural East-West arc progression of the stars. Why 'Declination', which suggests below the horizon to illiterates like me? Surely it should be 'Inclination'.

Kindly forgive my ignorance.

[FraserClarke]

Hmmm. interesting question -- never thought of it like that

according to OED, the root is from the latin declinare, which means to 'turn away'. So if you take the equator as good global reference line, then the declination is how far away from that line an object is. Seems to make sense in that context? Similarly I guess for magnetic declination -- i.e. "how far is the needle from the line"...

[ollypenrice]

I'd agree with the above. Since the celestial equator is at '0 degrees declination' it seems reasonable to interpret that as 'no declination' so confirming that the celestial equator is indeed the reference plane. I suppose we could have used the polar axis for reference in which case 0 deg would have been at the poles and not the equator? Clearly the celestial equator is derived from the position of the polar axis since any great circle other than one passing through the poles is arbitrary. This being so it is, maybe, a little eccentric of us to call the equator the zero line? No doubt I'm missing something, as usual.

Historically I suppose we were just following the latitude convention of zero for the equator. Would this be because of our use of the sun at local noon to define our position?

Ho hum...

Olly

[obscura]

Azimuth and Elevation in the days of radar. Alt Az, RA Dec, Az -El, Long Lat? Throw in Zulu, GMT, LST, RA and it all becomes clear. ??

[FraserClarke]

I suppose we could have used the polar axis for reference in which case 0 deg would have been at the poles and not the equator? Clearly the celestial equator is derived from the position of the polar axis since any great circle other than one passing through the poles is arbitrary. This being so it is, maybe, a little eccentric of us to call the equator the zero line? No doubt I'm missing something, as usual.

Yes, it comes from the polar axis of the Earth, but that makes the celestial equator, defining declination (c.f. latitude), anything but abritrary. It is the only great circle which is at 90 degrees to the polar axis of the Earth.

Of more historical significance, when the Sun is on the equator (the equinoxes), you have exactly 12 hours of day and 12 of night (at all points on the Earth). The Sun is also exactly half way between it's maximum and minimum altitudes through the year -- a nice thing for ye olde druids to measure, for example.

Would this be because of our use of the sun at local noon to define our position?

Yes, that's how you get longitude (c.f. Right Ascension), which is totally abritrary/political because there is nothing to constrain a great circle passing through the poles.