[The Mekon (John Briggs)]

For some time I have been interested in how astronomers, surveyors, and mariners determined longitude prior to the development of the chronometer in the late 1700’s.
One of the main methods to determine the longitude difference between two places was by observation of the Jovian satellite events. These could be determined well before hand and the results tabulated for future reference. Mariners could use these events to determine the longitude of their landfall by use of a good telescope from a stable land platform. Later chronometers could be “rated” at remote islands also by observation of Jovian satellite events.
I was keen to know just how good a result could be obtained using the instrumentation available at the time. Last week on the 26th June I was in Cooma, and with no almanac (or internet access) I decided to get out my Vixen Custom 60L and find out if any events were occurring around Jupiter. Now I consider that this 60mm F15 achromat might be similar in performance to telescopes available in the 1700’s, which perhaps would have been a 3” glass with a focal length of around 4 feet. I was using a power of 130x.
Just after 19:00 I focused on Jupiter and saw one moon very close to the limb. I watched this closely over the next ten minutes and determined it was about to be occulted. Seeing was quite good and as the minutes ticked by I saw the moon close with Jupiter’s limb, form a “nipple”, then a “pimple” and finally a small bump. One habit I have developed over the years is to keep an accurate wrist watch, and I finally determined that I could no longer see any variation in Jupiter’s limb at 19:19:30 EST.
On arriving back home I looked up the Astronomy 2018 Yearbook and found that at 19:20 EST Europa was scheduled to be occulted by Jupiter.
I must say I am surprised by the accuracy of my observation. This timing would give an error of longitude of a mere 7 minutes, or around 5.8 miles – any mariner of the 1700’s would have been very pleased with such accuracy.
A few questions remain - The yearbook only tabulates to the nearest minute – mariners are used to working to the nearest second, so the accuracy could be less than that stated above.
Also I am assuming that the timing in the yearbook is for the end of the event (i.e. the completed occultation) and not the beginning (first contact)
Any comments are most welcome. John
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