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    Re: Nautical Almanac and JPL ephemerides
    From: Frank Reed
    Date: 2015 Jun 5, 11:33 -0700

    Bob Goethe, you wrote:
    "What was the character of the Nautical Almanac from the late 1800s through to 1934?"

    Your question really begs for a history of celestial navigation. But I'll just say that for most of the 19th century, and changing slowly into the early decades of the 20th century, nautical astronomy (the earlier name for celestial navigation) was primarily a daytime activity: noon Sun for latitude and morning or afternoon time sights for longitude. The almanac data that was primarily required by a navigator was the Sun's daily declination and equation of time. The rest was a luxury, and many navigators did not even purchase an annual almanac. A table of Sun data was good for a dozen years, with a few limitations.

    And you asked:
    "If GHA for sun and SHA for stars were not being provided (and after 1912, data required for the lunar method was not provided), what WAS in that volume?"

    Trying to understand the history of celestial navigation by looking at what's listed in the almanac will lead you astray. For example, although the lunar distance tables were dropped from (some) almanacs in 1912, lunars had been thoroughly obsolete at sea for over sixty years. The primary data in the almanacs from the early 19th century through the early 20th century included the Sun's Dec and EqT, as noted above, and for the other objects they used the same data as astronomers, namely right ascension and declination.

    You can find many, many copies of the various nautical almanacs (there wasn't just one!) online today. There is a representative sample available directly through the NavList message boards main page (under the "Resources" menu, select "Bowditch and More"). Here's a link to it, an index that I assembled some years ago: http://fer3.com/arc/navbooks2.html. Scroll down nearly to the bottom of the page and look for "NA 1767" etc.

    By the way, the modern "Nautical Almanac" did not exist until 1958. Before then there were several distinct publications. The volume with the apparent title "Nautical Almanac" was actually an astronomers' almanac at that date. I did an hour-long presentation on the history of the Nautical Almanac at our gathering at Mystic Seaport in 2008 celebrating the fiftieth anniversary of the modern Nautical Almanac. There are all sorts of surprising ins and outs in that history.

    Frank Reed

       
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