Joe wrote:
You have to be able to ID stars to shot them.

Of course my favourite fix was equal altitudes at lunch. Anyone know
what that is and why navigators like it?


Gaz


The sun near local apparent noon appears not to change altitude,
sometimes for three minutes or more. The length of time of ambiguity is
related to the zenith distance between the observer and the sun---the
greater the distance, the longer the episode of ambiguity. During this
period it is not possible to know the exact moment of LAN, which means
that it is not possible to know the longitude of the boat, it being
dependent on accurate GMT.

If, however, you take a sextant shot well before noon, when the sun is
clearly increasing in altitude, then after noon reset your sextant to
precisely the same altitude and wait for the sun to come to the horizon
in your sextant, the time of local apparent noon will be precisely half
the time between the two sights of equal altitutude, added to the time
of the first sight. If several sets of equal altitudes are taken, an
average of the estimates of local apparent noon will be obtained.

The Admiralty Manual of Navigation devotes a chapter to equal
altitudes. I have used this technique over the years in long zenith
distances (46 degrees) and short (12 degrees), with excellent results.

Joe

I used two sextants, one for the equal altitudes to give longitude and
the other for merpass and latitude. The easiest fix of the day!