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  #1  
Old 30-06-2018, 08:15 PM
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The Mekon (John B)
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Longitude by Jovian satellite events

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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  #2  
Old 01-07-2018, 09:41 AM
geolindon (Lindon)
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Thanks John for your interesting post, a nice replication of old technology. My new awareness of Jupiter's moons' usefulness will increase my enjoyment of them
Regards, L
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  #3  
Old 02-07-2018, 11:50 AM
Wavytone (Nick)
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Yes it was a perfectly good method provided you had a good scope and some land to put it on.

The great challenge was determining longitude at sea, on a rolling ship.
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Old 02-07-2018, 04:44 PM
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Thank you Linden, Wavytone. I am nor sure if too many are interested in this aspect of astronomy. I am particularly interested to find out how a modern instrument like a 60mm F15 refractor would have compared to those in use at the time. Observing with my 132mm CFF would make the task too easy, I reckon I could get the timing down to around 5 seconds with the CFF.
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Old 02-07-2018, 05:39 PM
geolindon (Lindon)
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i dunno John,

i imagine the real old articles are still around, as you would know better than me
unless this is fake facts even GG's original is preserved.
https://catalogue.museogalileo.it/ob...Telescope.html

they would have had less polluted skies.

regards, L
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  #6  
Old 02-07-2018, 08:33 PM
Wavytone (Nick)
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John,

By the way, this would make a great high-school experiment for a physics class.

IMHO some of the old refractors should not be underestimated for resolution. While there were a lot of junk ones, some makers really did know how to produce a diffraction-limited lens.

In this respect is notable that quite a lot of the good quality 18th century refractors are about 3" aperture and I suspect this is the reason - smaller than this was not good enough to determine longitude to a useful accuracy.

While it's possible to observe the galilean moons in a 50mm - 60mm refractor IMHO the accuracy of the timings would be questionable as at say maximum 120X it really lacks the resolution to show the phenomena of the satellites well enough. However it is something worth trying and I'll see if I can rig up a small one around f/15...f/20.

A 75mm is a more likely candidate in terms of resolution, and in this respect I have a modern 70mm APO that shows Uranus nicely, so will try it for timing Jupiters moons next time I am setting up.

A 105mm is a definite yes, from personal experience with a beautiful working antique example. At school we had an 1880 Thomas Cooke 4.25" f/16 refractor and I spent many happy evenings behind this, Jupiters moons and Saturn were both fine sights and it was easily a match for modern scopes in terms of resolution. This scope made astronomy a lifelong passion for me.

What's more in year 12 I made a Ramsden eyepiece (using a motor driven spindle a 15mm one was quite easy to make) which showed that even modest eyepieces of that era performed quite well, despite the limited field of view (30 degrees).

Pre 1900 most objectives would have been air-spaced (as was the Cooke). The area where the old scopes suffer badly is transmission - with 8 air-glass surfaces (4 in the objective and 4 in the eyepiece) transmission was probably 50%. The old speculum mirrors don't seem so bad in comparison ! Using a modern (MgF coated) Kellner made a significant and visible improvement in transmission compared to my DIY Ramsden.

In this respect I can understand why some 18th century observers resorted to seemingly potty ideas like a singlet objective at f/30 followed by a singlet lens for an eyepiece, which would have had 75% transmission; the whole thing supported on a small crane.

Last edited by Wavytone; 02-07-2018 at 10:33 PM.
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  #7  
Old 15-07-2018, 10:09 AM
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DaveGee (Dave Gault)
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Quote:
Originally Posted by The Mekon View Post
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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Hi John,

I assume your observation was on the 26th June.

Occult (by Dave Herald) lists this event as occurring at 9:18.7 UT (= 19:18:42 AEST) See attached file. I've marked the event with a <

I'm not sure how the yearbook times the event but Occult gives times for mid-event.

It's my understanding that navigators used the lunar distance method to determine longitude at sea before Harrison and his clocks solved the puzzle. Navigators could use a sextant to measure the various angles from the deck of a moving ship. Usually, the Master, the Captain and several lieutenants made the observations and an average was taken. Cook was highly proficient at reducing the observations to a longitude value, even then the task took several hours of hard work.

Now we have GPS. Thanks US taxpayers.
Attached Files
File Type: txt 201806-Satellite events of Jupiter.txt (4.9 KB, 3 views)
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