On 1/26/12 10:14 AM, Chris Albertson wrote:
On Wed, Jan 25, 2012 at 8:38 PM, Jim Lux<[email protected]> 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.
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