[time-nuts] telling time without a clock

Jim Palfreyman jim77742 at gmail.com
Thu Jan 26 17:55:53 EST 2012


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 <jimlux at earthlink.net> wrote:

> 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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