Welcome to the NavList Message Boards.

NavList:

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

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    March 22 Lunar Observations
    From: Arthur Pearson
    Date: 2002 Mar 23, 21:10 -0500

    Gentlemen,
    
    Here are the numbers from a set of lunar distances off the sun on March
    22, 2002.  I would welcome any comments on my procedures and the
    problems I encountered, and if anyone is inclined to work the
    calculations on their own, I would be curious how they come out.
    
    All times reflect GMT from my GPS unit.
    Latitude and longitude are also from GPS.
    Angles are expressed XX* YY.Z for degrees and minutes or YY.Z' for
    minutes alone.
    Index Error = 2.63' ON (determined by Bruce's averaging method)
    Latitude:       N 42* 21.7
    Longitude:      W 71* 15.5
    
    I took eleven distances as follows:
    GMT             Ds
    18:40:46        98* 29.9
    18:43:41        98* 32.2
    18:44:47        98* 32.4
    18:46:10        98* 31.0
    18:47:16        98* 31.2
    18:48:16        98* 34.6
    18:49:28        98* 34.8
    18:50:45        98* 39.8
    18:52:25        98* 41.4
    18:53:37        98* 34.8
    18:54:51        98* 36.2
    
    As you can see, I had considerable trouble getting consistent distances.
    I may have misread one or more of these, or I may have bumped the knob
    inadvertently. Bruce's book emphasizes the sensitivity of long angles to
    parallelism of the telescope, so I tried to make these contacts in the
    center of the scope. Needless to say, I have lots of room for
    improvement.
    
    My next step was to graph these distances against time and then plot a
    reference line that shows the slope of the change in comparing distances
    from the hour before to the hour after the observations. Here are the
    Nautical Almanac data I used:
    
    Sun GHA 18:00   88* 17.4
    Sun GHA 19:00   102* 17.6
    Sun Dec 18:00   0* 46.1 North
    Sun Dec 19:00   0* 47.1 North
    Sun SD          16.1'
    Moon GHA 18:00  349* 11.9
    Moon GHA 19:00  3* 37.4
    Moon Dec 18:00  24* 40.0 North
    Moon Dec 19:00  24* 40.2 North
    Moon HP         57.8'
    
    From this I calculated:
    D1 =    97* 55.9 (Comparing distance at 18:00)
    D2 =    98* 26.9 (Comparing distance at 19:00)
    Slope = +5.16' every 10 minutes during this hour (for ease of plotting
    the reference line)
    
    I plotted this reference line alongside the graphed distances. I ignored
    the highest two distances based on my belief that they were spurious.  I
    visually established a "best fit" line parallel to the reference line
    through the 9 remaining valid distances. A lot of judgment here, this is
    a pseudo-scientific process, but it allowed me to select a point on the
    best fit line as follows:
    
    Selected GMT of observation:            18:50:00
    Ds corresponding to selected time:      98* 34.8
    
    As I am landlocked, my altitudes were first calculated based on my
    selected time and my latitude and longitude. From the calculated
    altitudes, I work backwards to get apparent altitudes (altitudes correct
    for all but parallax and refraction). Bruce has tables for this, I have
    to fiddle around with a sight reduction spreadsheet to come up with the
    corresponding values.
    
    Sc =    40* 41.7
    Sa =    40* 42.8
    Mc =    70* 27.7
    Ma =    70* 8.2
    
    With all the above in hand, I can clear the distance and determine GMT
    of the observation. Here are the intermediate values and final results:
    
    Moon's refraction and parallax =        19.53'
    Sun's refraction and parallax   =       1.06' (I use Sun HP = 0.0024*)
    Moon's augmented semi diameter =        16.0'
    Sun's semi diameter =                   16.1'
    Ds =    98* 34.8
    Da =    99* 4.3
    D  =    98* 19.2 (the cleared distance)
    D1 = 97* 55.9
    D2 = 98* 26.9
    GMT per observation =   18:45:07, March 22, 2002
    
    This would suggest that my "watch" is fast by 4 minutes and 43 seconds.
    However, as I assume time by GPS is quite accurate, the error is in my
    observations and calculations. Presuming the calculations are accurate,
    I measured the angle as smaller than it actually was. Bruce has tables
    to calculate the error in the distance, I have to fiddle with my
    spreadsheet to determine that I should have measured Ds = 98* 37.3, an
    error of about 2.5'. When I go back to how I placed my "best fit" line
    on the graph of my sights, I might have gotten closer to the mark if I
    had included the two distances I discarded as spurious, but they were
    clear outliers in the series, so I would not have used them in practice
    at sea.
    
    So it's back to the sextant to shoot some more. If anyone finds errors
    in my calculations or problems with my procedure, please let me know.
    
    Regards,
    Arthur
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

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

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

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

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    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