Info needed on celestial sphere make-up

I need some info on the make up of the celestial sphere. I am ok with regards using Declination and Right-ascension etc, but I would like to know what defined the 00.00hrs line on the celestial sphere? From looking at charts, I know it goes from Polaris (just about!) and passes to the left of Cassiopeia, down past the equator. But why start there?

Seanie.

I need some info on the make up of the celestial sphere. I am ok with regards using Declination and Right-ascension etc, but I would like to know what defined the 00.00hrs line on the celestial sphere? From looking at charts, I know it goes from Polaris (just about!) and passes to the left of Cassiopeia, down past the equator. But why start there?

Seanie.

seanie,
because it starts from the vernal equinox, i.e. the point where the sun crosses into the north hemisphere of the celestial sphere.
stephen.

Hi Seanie,

Basically the 0 position for right ascension is a line which joins up the 'first point in aries and the north (and south) celestial poles. The first point in aries is the point where the celestial equator crosses the ecliptic. This is also the position where the Sun lies at the vernal (spring) equinox. The first point in aries is actually now in pisces. This is because of the wobble of the earths axis called precession. This results in the first point in aries shofting over time. In 23,000 years time the first point in aries will actually be back in aries again.

I normally recommend people interested in celestial mechanics to read this;

www.astronomynotes.com/nakedeye/s1.htm

its a great read and worth sticking with.

Thanks for the heads-up Stephen and Dave. I knew about the first point of Aries and all that, but wondered due to the fact that it does indeed go through Pices on a star chart as the only ecliptical constellation.

And thanks for the link too Dave, very informative!



Seanie.