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Gary LaPook adds:
A little background on "magnitudes." The ancients realized that stars
were of different brightness levels. Starting with the brightest they
could see they called first magnitude. Then the stars that they could
discern were not quite as bright as the first group they called second
magnitude. Then the ones they could discern as dimmer than second they
called third, etc. they ended up with sixth magnitude as the dimmest
group of stars that could be seen. This was popularized by Ptolemy in
his Almagest.

When modern equipment was developed to actually measure the intensity
of the star light they measured that the intensity of the first
magnitude stars wase about 100 times brighter than 6th magnitude, five
increases in magnitude. Taking the 5th root of 100, you find that the
actual ratio of intnsity from one magnitude to the next is about 2.5
to 1.

This shows that the human senses, in this case sight but also hearing,
have  non linear response curves. It takes about a doubling in
brightness for you to perceive any increase at all. The same is true
in hearing where the sound intensity must double for you to hear a
difference. This type of sensory perception allows us to hear sounds
one trillion times louder than the faintest sound we can hear. That is
why the decibel scale is used to discribe sound levels since it is a
logarithmic scale. A db is 10 times the log of the power ratio. Log of
1 trillion is 12, times 10 is 120db which is loudest sound you can
hear without damage to your ears.

The moon is -12 magnitude so is 18 magnitudes brighter than the
faintest stars you can see which means the moon is about 15 million
times brighter than a 6th magnitude star. The sun is at -26 magnitude
making it about 400,000 times brighter than the moon and 5,400,000,000
(5.4 billion) times brighter than the faintest star you cn see.

On Mar 9, 1:17 pm, "Bill Noyce"  wrote:
> Just a guess off the top of the head -- are those the magnitudes (i.e.
> brightness) of the bodies?  More negative = brighter ...
>     -- Bill
>
> On 3/9/07, Gary  wrote:
>
>
>
> > Having just started celestial I have greatly enjoyed what I have
> > learned so far.  It is extremely enjoyable to take sights, reduce them
> > and actually end up where you know you are.  This summer I will
> > actually get to use this tool on blue water which should be a lot of
> > fun.
>
> > In the nautical almanac there are numbers in the column headings next
> > to Venus, Mars, Jupiter and Saturn.  For instance for today they are
> > -3.9, +1.2, -2.1, +0.1 respectively.  I have not found an explanation
> > for what these numbers are for.  Can anyone help me out?  ThaAks


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