<HTML>Dave Moore wrote:
>
> I still don't understand where you're getting 148 degrees
> from.
I think I know what he's doing wrong – I think that he's using a planispheric projection and not accounting for the distortion of the proportions of the ecliptic by a projection that is based on the celestial pole. He has also, I think, not fully comprehended the nature of the consequences of precession on the imaginary lines on the celestial sphere, but this is probably less of a problem for his 'dilemma'.
Bent, if my first assumption above is correct, you need to either visualise or make a 3D model of the celestial sphere. The projection used in standard planispheres distorts things greatly (look at the distortion of shapes of constellations at low southerly declinations). In 'real life' the lines of RA would converge towards the SCP; in a NH planisphere they continue to diverge S of the celestial equator. In 'real life' there is the same length of ecliptic north of the celestial equator as there is south of it; there is also the same angle (180 deg) of ecliptic N&S of the equator – there must be, since the plane of the ecliptic and the plane of the equator will <b>always</b> intersect on a line through the centre of the Earth.
If you draw yourself a planisphere with a projection based on the pole of the ecliptic, you will find that for the period in question and <i>measured on the planisphere</i>, the vernal equinox has moved 177 deg along the ecliptic and more than this (about 212 deg) along the celestial equator.
In short, you can only measure arcs of circles on a planisphere if they have their centres on the (projected) pole of the projection. For a standard planisphere, these are the circles of declination.
If you have trouble visualising this, make a spherical physical model. A tennis ball is ideal. Mark two points diametrically opposite each other. Draw a great circle through these points (yes, folks, I know that's tautologous [g]). Draw another great circle (it helps if it's a different colour) through the points, inclined to the first circle. One circle is the ecliptic, the other is the celestial equator. Voila!
It's a great little tool for helping to visualise the celestial sphere – you should be able to find a mug or glass that you can use as a horizon if you want to play with such things.
HTH
S</HTML>
