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I still do not understand several things.

On Wed, 15 Dec 2004, Ken Muldrew wrote:

> If the star is rising in the East or setting in the West,
> then the time
> sight and altitude could be done at the same time

I don't understand several things in this sentence.
1) What is the meaning of "If".
2) Altitude of ANY star gives you local time at the
moment it is measured, is not it?
What is special about "time sight"? Why not to use the star altitude
that is needed for the lunar anyway, as a time sight?

I tried to repeat Thompson altitude computation.
I use the formula
sin h=sin L sin Dec +cos L cos Dec cos LHA,
where L is the latitude (I use Thompson's value 50d47'24")
Dec is the star's declination (I use Thompson's value 8d22'25" N
for Altair)
LHA=(RAstar-RAsun-LocTime)
I use Thompson's values RAstar=19h41m3sec for Altair
                        RAsun =16h11m13sec
                       LocTime=9h3m45sec.

I obtain h=10d35'40".
How did Thompson obtain a different value of 10d54'19"?
What does his DR long have to do with this at all?
Or he used some different method to compute altitudes?

By the way, what exactly was his watch supposed to show?
I mean what is 9h3min45sec, exactly?
The time elapsed from his Local Noon (=sun culmination)?
Or the time corrected for the equation of time?
(In any case my altitude calculation does not agree with his one).

> Even when Thompson uses the sun for a lunar,
> I think he still calculates
> the altitude even though he uses the sun for his time sight

The reason of this totally escapes me.
What is a typical interval between his Sun-Lunar and
time sight?

> For the star altitudes this is true but for the lunar
> altitude the right
> ascension and declination have to come out of the almanac,
> so DR longitude
> is needed to get those values.

But did not you con jecture that his mistake of 2 degree
in STAR altitude is due to an error in his DR longitude?
It seems this cannot be so: the star altitude depends on
the local time only, you don't need to know GMT and your Lat.

Because this 2 degree mistake in the star altitude easily explains
his error in the final longitude, I am trying to understand
why did it happen.
I tried to repeat his alt calculation (see above) and I don't
understand where the difference comes from.

Alex.

   

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