NavList:

   

Reply

Alexandre Eremenko wrote:

> Here is a question to the Lunars experts:
> Why do we need a sextant at all??

We don't.

>
> Just watch for the moment when the moon
> "collides with a star" on its normal way
> (there are so many stars around!)
> and notice this moment on your watch.
> It is easy to modify your lunar reduction programs
> then to find GMT of this "collisoon".

Is it really that easy? You need to compute the contact angle to make
sense of the timing. That angle can make a difference of 1/2 an hour in
the timing. Or you use the semidiameter of the moon as given topocentric
distance in an iteration towards it. This is how I would do it on a
computer. But with logarithm tables on a ship? Would it help to measure
the cusp angle, if this were possible with sufficient accuracy?

> (The "true distance" at this moment equals to
> the corrected "semidiameter". Then take Moon's parallax
> into account and that's it!)

Parallax is up to twice the diameter of the moon. How do you know in
advance whether you have an occultation at all, as seen from your place?
You might need two permanent lookouts: One for other vessels and a
second one for the moon.

> So you have the whole night to look for these collisions.

It may be a long wait, though, until one happens. I you are lucky, you
will see 5 per year. Presently, the Nautical Almanac tabulates ca. 170
stars down to magn 3.5. Roughly 70 of these are situated in the 57 deg
band in which the Moon resides. Let these all be aligned on the same
meridian of RA. Then the mean distance between any neighbouring stars is
1.6 moon diameters.  The moon will have less than 2 "collisions" in
three months.

Half of the occultations that happen you will miss because you are on
the wrong side of the Earth and half of the remaining events happen on
the unfavourable  (i.e. illuminated) limb of the moon.

> Then why was not this practiced at least as much as
> the Lunars were?

In addition to the above, the accuracy of general star catalogs improved
only slowly. But astronomers (e.g. Mayer) used occultations of selected
stars (e.g. Aldebaran, Regulus) the position of which had been carefully
measured for the purpose of verifying their lunar tables.

*****

On land, it's a different story. If you have access to a good telescope,
tomorrow (to be precise, 2004-10-28, after 1 UT) is a VERY good time to
look for occultations. Within three hours, you might see up to 60 events
(30 disappearances and 30 reappearances) involving stars mostly between
magnitude 11 and 12. At the very same time, you will also be able to
observe one lunar distance between moon and sun with the naked eye. You
are right: We don't need a sextant at all. Couldn't even use it for such
a big distance.

Herbert Prinz

   

Reply