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
    Re: Sun altitude limits for natural horizon?
    From: Stan K
    Date: 2016 Feb 21, 20:16 -0500
    In the 1990s, I'm pretty sure that the USPS courses used excerpts from the 1972 Nautical Almanac.  I am not home to verify this, but give it a shot and see if it makes sense.


    -----Original Message-----
    From: Frank Reed <NoReply_FrankReed@fer3.com>
    To: slk1000 <slk1000@aol.com>
    Sent: Sat, Feb 20, 2016 9:05 pm
    Subject: [NavList] Re: Sun altitude limits for natural horizon?

    Thanks, Ron! That's nearly as useful as having the altitudes. The data includes the azimuths of the bodies. While this can't pin things down quite as exactly as altitude data, it does let us test out most of the details. First of all, if we go to the USNO web site and enter the time as 20:47 UT, the date as January 29, 1996 (just to get started since that's the publication year of the example), and use the DR position, we get back data on the stars that matches the listed azimuths, nearly enough but with some interesting small differences. So the time as given in the problem is consistent with the data. Zone time of 19:47 in that longitude corresponds to 20:47 UT. I'll take a moment to add that navigators were crazy back then: listing sights in zone time was always nuts.
    What year is consistent with the data? All we have to do is start in 1996 and go back a year at a time until Jupiter falls into line. And that happens in 1991 -- here's the USNO web site output for 1991, everthing else the same. Now since Jupiter orbits the Sun in slightly less than twelve years, there is another near match in 1979. But the form also shows a GPS position so we can rule that out. How about going ahead twelve years to 2003? That, too, is a close match, but you said you were looking at a "very old version" of the document. That argues in favor of 1991. It's a solid bet that the data was generated for sights in 1991 or 2003, and of the two, 1991 is a closer match on the Jupiter azimuth and the context of your description.
    Also notice that the Moon was very close to Jupiter in the sky at this time on January 29, 1991, and to make it even better this was the day before Full Moon. It would have provided a lot of illumination of the horizon in late twilight. Four of the objects used for sights are within 60° azimuth of the bright horizon directly below the Moon. Only Canopus is outside that range.
    It is certainly possible that these sights could have been taken at this location, date, and time, and with the Moon in the sky, it's even reasonable. I'm a little suspicious of one other item on the plot. For the "Fix" we see the same latitude and longitude as listed for "GPS" lat and lon. Given that the crossing "tangle" of the lines of position is about ten nautical miles across, the one sigma error ellipse is probably a dozen miles across or more. A fix that exactly matches the known GPS position to the nearest mile would be quite unlikely. The position shown as the "Fix" is certainly consistent with the GPS position, and maybe that's all that was intended by this plot.
    Frank Reed
    PS: We should confirm the Sun's altitude. There are many ways to do this with various tools. For amusement, there is a hidden way to do this using the same USNO web tool. On the output page, you edit the URL (the address of the page). It contains an item "qqo=all". If you change that to "qqo=sun" leaving everything else unchanged, you get the corresponding data for the Sun at that time even though it's well below the horizon and not normally listed.
    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting 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