Re: VMs: Identifying VMS stars, and the longitude problem

[elvogt@xxxxxxxxxxx]

Just a bit of nitpicking here...

Zitat von Pamela Richards <spirlhelix@xxxxxxxxx>:

> 
> Lunar eclipses occur every twelve months and are
> easier to observe than Solar ones, the Earth casting a
> larger shadow on the Moon than does the Moon on the
> Sun.

Of course, the Moon casts its shadow on Earth... I take this to be a slip of 
the pen...? ;-)

>  So you have to be in a certain location to see
> them.  Solar eclipses also occur every twelve months,
> alternating each six months with eclipses of the Moon.
>  They are seen when the Moon is New.

Not quite true, both solar and lunar eclipse can happen twice a year each (two 
nodes of the ecliptic...), and it's conceivable to have a full moon lunar 
eclipse, immediately followed by a new moon solar eclipse.

Actually, solar eclipses are slightly more common than lunar eclipses. It just 
appears different, because a solar eclipse can only be seen within the eclipse 
region on Earth, whereas a lunar eclipse may be observed from anywhere. (As 
long as it's night there.)

> 
> As for the accuracy of timekeeping methods at night, I
> have heard that water clocks were quite accurate and
> were in use in ancient times.  

That may be good enough for one night, but if you want to justify Ptolemy's 
claims ("The time of rise and setting of celestial bodies varies proportional 
to your longitude"), you'd need something better. Mind you that traveling 
around 150km will only shift the rise of a star by one measly minute, so you 
have to keep your watches synchronized while you're travelling for several days 
at least!

Cheers,

   Elmar


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