[BAA Comets] Azimuth

[rmiles at baa.u-net.com]

One further point concerning Position Angle.

There is sometimes confusion between which way is East and which West as
seen in the field of view of a telescope. An easy way to distinguish East
is to consider where in the field an object/star Enters (normal to the
edge of the field) if the drive were to be switched off.

Re. Azimuth - N.B. this is a terrestrial-based positional reference point.

Richard Miles


> When measuring the position angle of a comet tail, north is 0, east 90,
> south 180 and west 270.  Meteorologically the same convention is used for
> wind direction.  A compass azimuth has east has 90 and west as 270 (or at
> least my mapping compass does).  I would suggest giving west as azimuth
> 270 and east as 90.  I'm not sure where the -90 convention comes from,
> possibly a mis-interpretation.  The astronomical almanac is quite explicit
> that azimuth is measured clockwise from north through east.  Geomagnetic
> declination is measured as +90 or -90, and it may come from this.
>
> Jonathan Shanklin