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

Hi, Elmar!

Nitpick away, let's try to clear up as much as we can.

--- elvogt@xxxxxxxxxxx wrote:

> 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...? ;-)

When there is a Lunar eclipse, the Earth casts its
shadow on the Moon.  Remember, each Lunar eclipse is a
sygygy; three planets (luminaries are considered
planets in astrology) are aligned.  A Lunar eclipse is
a syzygy with the Sun on the outside, the Earth
between, and the Moon on the other end.  It's the
Earth you see a shadow of, cast by the Sun onto the
Moon.  This is another reason Aristotle and the
ancients took the Earth to be a sphere: at no time is
the shadow of the earth on the Moon a straight line
rather than a round form.

When there is a Solar eclipse, the Moon may be said to
cast its shadow on the Earth.  Perhaps to be more
clear, I needed to say that the Moon covers less of
the Sun in a Solar eclipse compared to the size of the
Earth's shadow on the Moon in a Lunar eclipse. 
> >  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.)

Yes, I was referering to solar eclipses when I said
you had to be in a certain location to see them.  And
I did check my ephemerides; yes, two eclipses lunar
eclipses in 2004; May, and October (coming up).  We
also have a partial solar soon.

> > 
> > 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!

The best way to keep accurate time is by the stars. 
You don't even have to synchonize them.  What do you
know about the use of an astrolabe?

I won't trouble you with the information if you are
already familiar.



> Cheers,
>    Elmar
> -------------------------------------------------
> debitel.net Webmail
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"I'd rather learn from one bird how to sing, than to teach ten thousand stars how not to dance."

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