[time-nuts] telling time without a clock

Jim Lux jimlux at earthlink.net
Thu Jan 26 15:11:40 EST 2012


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





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