Right ascension

[GOURDHEAD]

:unsure: If a star were directly overhead with respect to any observer on the night of the vernal equinox, would its right ascension be 0:0 (hours, minutes)? Can anyone name (or provide a link to) highly visible "skymarks" with right ascensions very near whole hour numbers. How fixed is the vernal equinox as seen against the celestial sphere? Since the precession of the poles cause the direction to celestial north to move about the celestial sphere, I assume the direction to right ascension 0.0 also moves.

Are guesses at how long it takes the sun to circumnavigate the galaxy made using extremely small micro-changes in the direction of 0.0 right ascension after compensating for the proper motion of the reference stellar objects? Values given for the period of the sun's galactic orbit seem to vary over tens of millions of years; have there been recent improvements in accuracy? :unsure: :unsure:

[Dave Mitsky]

A star that is transiting the central meridian (not necessarily at the zenith) at midnight on the vernal equinox has a right ascension of 0 hours 0 minutes.

The mean orbit (i.e., the ecliptic) of the Earth and the plane perpendicular to its mean spin axis (i.e., the mean equator) are moving due to gravitational perturbation by the other planets (planetary precession) and and the torque on the Earth's equatorial bulge caused by the Sun and the Moon (luni-solar precession). The intersection of these two planes defines the mean equinox of date.

On the autumnal equinox sidereal (star) time equals local mean solar time.

Dave Mitsky

[GOURDHEAD]

Thanks Dave