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    Re: 9th Century Lunar problem...
    From: Frank Reed
    Date: 2020 Sep 7, 12:01 -0700

    Dale, you wrote:
    "From a monastery somewhere in England in the 9th century, the attached observations were made. Can anyone (I'm thinking Bruce Stark) determine the lat lon and perhaps the name of the monastery?"

    Latitude? Yes, within some limits. I'm sure this has been done, probably quite a few times, historically, but there's no harm in trying again today. What would be the best estimate for latitude based on those shadow lengths? I would suggest using the "middage" or "mid-day" shadow lengths. Those have a fairly unambiguous meaning, and they are easy to observe and measure. 

    We should also consider that these may not be observations at all. It's quite possible that the numbers here were calculated (or measured from a carefully-drawn diagram). In this pre-scientific era, theory and ritual calculation could take priority over observation.

    Could we get Longitude? Not a chance. Not even close! The information about the Moon's visibility in this document is not even remotely useful for determining longitude. Just ask yourself this: suppose you had notes from an analogous observer in, let's say, British Columbia (same latitude as the observer in England, but radically different longitude) made in the same era and describing the same details about sundial shadow lengths and moon visibility. Can you see that they would not differ in any way from the notes in this document? There's nothing longitude-dependent here. 

    So what astronomical information would you need to determine longitude from a historical document?? You need an event that is seen at the same instant of time by observers across a large portion of the globe, like a lunar eclipse or better yet an explosion on the lunar surface (O, if only we could  be so lucky, but no, this never happened in any known records...). If we have specific events in a lunar eclipse, like the time when it fully disappears in the Earth's shadow, given in terms of accurate local time (this is often the tricky part!), then we are able to compare absolute time against local time, and that is longitude. Alternatively, if have observations of an event which occurs at different times in different places that can be "reduced" unambiguously to a common standard, like an occultation of a star by the Moon or a carefully measured lunar distance, then in the same way, if we also have corresponding local time, we can get longitude.

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

    PS: Best means of estimating longitude in this document: it's written in Anglo-Saxon (Old English)! This long-dead language had a relatively small geographic range of longitude in the 9th/10th centuries --just England, narrowly defined! While it's conceivable that a monk/scholar might travel to a distant land, like Rome, in this period, it's likely that the author would then have switched to Latin as the language of learning. The astronomical evidence is irrelevant to longitude in this document, but the linguistic evidence is strong. :)

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