NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fix by Occultations
From: George Huxtable
Date: 2009 Feb 5, 21:06 -0000
From: George Huxtable
Date: 2009 Feb 5, 21:06 -0000
Bruce Hamilton wrote- "I have attempted to see the occultations of Jupiter's moons, but that is a long term method. I don't mind watching the moons of Jupiter for long periods of time, but it's a bit of a guessing game. Did Io disappear or did I blink? Whereas the moon occulting a star seems to be a much more definite event." I don't understand Bruce's problem here. Yes, the Moon occulting a star is a more "definite" event in that it happens instantaneously, whereas Io, disappearing into Jupiter's shadow, takes a few seconds to extinguish. And the better the light collection from your telescope, the later will be your timing of its final twinkle. But what does "Did Io disappear or did I blink?" imply? If Io returned, then he blinked. If it didn't, then it immersed. It isn't necessary to watch Jupiter for a long time, unless he is calibrating a clock with a large unknown error, because the immersions are closely predicted. Why does he call it "a long-term method?" True, there can be long gaps between immersions, especially near Sun conjunction when Jupiter is invisible for more than a month. But over much of the year several such timeable Jupiter events occur each week. The big problem about the Jupiter satellites is that they can't be seen from on board ship, because you need quite a lot of magnification, necessitating some sort of tripod. But with a simple tripod stand, and preferably an equatorial mount, then a cheapo hobby reflector, say 3-inch with magnification of 50, is more than adequate. In [7265], Bruce asked- "Has anyone had any luck putting the concept into practice?" That was how the longitudes of France were mapped, by surveyors setting their watches from Jupiter satellite events, long before the days when such events were predictable in time. What surveyors then had to do was to compare, after the event, with the local time the same event had been observed at the Paris observatory. Those were the observations that radically reduced the perceived size of France, causing King Louis XIV to complain that he had lost more territory to his astronomers than he ever had lost to his enemies. It later became a routine method for establishing longitudes of harbours and headlands worldwide, to put them on accurate charts. The book, "Spherical and Practical Astronomy", that Bruce described as "an amazing book", is what others may know better by the name of its author, Chauvenet. It is, indeed, amazing. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---