Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Name or NavList Code:
    Re: Nautical Almanac clarification
    From: Gary LaPook
    Date: 2007 Mar 09, 16:44 -0800

    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
    To post to this group, send email to NavList@fer3.com
    To , send email to NavList-@fer3.com

    Browse Files

    Drop Files


    What is NavList?

    Get a NavList ID Code

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

    Enter the email address associated with your NavList messages. Your NavList code will be emailed to you immediately.

    Email Settings

    NavList ID Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site