Subject: Parallactic angle (was Re: Is there a book to teach practical astronomy?)

From: [email-masked] (Steve Allen)
Date: 6 Jun 1996 14:56:42 GMT

In article <[email-masked]>,
Andrew Cooke <[email-masked]> wrote:
>       - What is parallactic angle?

The most common current usage of "parallactic angle" appears to be
a misnomer.  I've been trying to find out whence this originated.

As near as I can find, the origin of parallactic angle lies in the
hand application of annual parallax to the catalog coordinates of a
star.  It is the angle between the direction to ecliptic north and the
direction to equatorial north.

Looking at Smart's Spherical Astronomy, however, the term is applied
to the angle between the local vertical and equatorial north.  In the
spherical triangles both of these have the same geometry, but the
context is very different.

In most current cases the angle should be called the "vertical angle".
I don't know if Smart is the first instance of this misnomer.  But the
result is that it has propagated into subroutine libraries and may be
too entrenched ever to be undone.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
[email-masked]      Voice: +1 408 459 3046     FAX: +1 408 454 9863
WWW: http://www.ucolick.org/~sla                PGP public keys:  see WWW