As a reasonably experienced occultation observer (and the very reason I got into being a time-nut - so I could time these observations), the main problem is that the number of binocular-observable occultations is actually quite rare. When the star appears or disappears behind the bright limb it is actually hard to see - even if the star is very bright. When the moon is nearly full, even disappearances behind the dark limb are hard.
So ideally you want bright star disappearences on a dark limb with a moon before first quarter. (Last quarter as well - but then it's a reappearance and you don't quite know where to look). This limits the number of bright stars quite drastically. And then you have clouds... Jim On 27 January 2012 07:11, Jim Lux <jim...@earthlink.net> wrote: > On 1/26/12 10:14 AM, Chris Albertson wrote: > >> On Wed, Jan 25, 2012 at 8:38 PM, Jim Lux<jim...@earthlink.net> wrote: >> >>> OK.. without getting into celestial navigation, the whole thing of >>> telling >>> time with the moon is intriguing. And with some forethought and data >>> available today, we could fairly easily do what folks back in the 18th >>> century could not. >>> >>> Let's say you run a suitable celestial model and identify all the >>> reasonably >>> bright and identifiable star that the moon occults in a given day. The >>> moon >>> moves about 1/2-1 degree per hour against the star field, so the question >>> is, could you find, say, a star every couple hours. >>> >> >> If you have a telescope and you can measure where it is pointing >> relative to the local meridian, then you don't need the moon. You can >> use a fine wre in the optical path an watch for when a star crosses >> the wire. The advantage of this is the telescope does not need a >> tracking motorized mount. It can be fixed to a concrete pier. Even >> a modest scope in the city can see hundreds of stars per hour. >> > > > I was thinking of something that works anywhere in the world (pretty much) > with things that you can hold in your hand (the table and your low power > scope/binoculars). > > In theory, if you knew approximate time (say to a minute or so), then you > wouldn't even need to find the star.. Look for the moon, the star will be > right next to the limb, and wait til occultation occurs. > > > > > >> Using the Moon is only useful if you can't measure where the scope is >> pointed. The Moon provides a good, well known reference. >> > > And easy to find in the field. > > > > So for a > >> portable setup it could work best but there is a built-in problem with >> the Moon, you may not have good data on the shape of the limb. >> Mountain ranges and valleys between peaks are different depending on >> your location on Earth. If you move even a mile your star might hit a >> different place. In fact people have used Lunar occulations to map >> the height of lunar mountains. Another effect is diffraction. The >> stars don't just "wink out" because they do have a finite diameter >> People have actually used the moon to measure the diameter of stars by >> accuratly measuring the defraction effects. But the project had >> problem because of large boulders and mountains on the moon made it >> hard to know the orientation of the "knife edge" and worse, this would >> chane if you move just a few feet, some different boulder might be >> there. >> > > This is a very good point.. what sort of effect are we talking about. The > moon subtends roughly 1/2 degree, 30 min of arc. What fraction of the > lunar diameter are these mountains? Say, 10km high out of 3400 km > diameter, so one part in 340, or roughly 1/10th minute of arc > > 1 degree = 4 minutes of time, so 1 minute of arc is 4 seconds of time. > > Those hills and rocks are on the order of the 1 second time measurement > uncertainty. > > > > > Another idea that maybe is even better is to use radio observations >> with two antenna that have a very long east/west baseline. You watch >> the difference in phase to a distant radio source. As the phase >> different passes zero you know it just went overhead and then the time >> would have to equal the R.A. of the radio source. Problem is the >> physical length of the cables you'd need to lay out and the lack of >> really bright radio sources. In theory one could get arbitrary time >> accuracy this way. A few radio source are "easy" to detect with >> affordable surplus/ebay equipment. >> > > > ______________________________**_________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/** > mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.