NavList:

   

Reply

Fred:

I'm neither an astronomer nor a surveyor, but I worry that none of these
methods would align a transit scope with sufficient precision that
Delta-T could be measured accurately.  I'm sure they give an alignment
good enough to measure time to a fraction of a minute, but that's one or
two orders of magnitude shy of what's needed to measure Delta-T,
Geoffrey Kolbe's goal.

Lu Abel

Fred Hebard wrote:
> I'm sure George will jump in here, but my wife's "Elementary
> Surveying" text lists 4 basic methods:
>
> 1) shadow method with the sun;
> 2) equal altitudes of the sun;
> 3) direct observation of the sun --requires Greenwich time;
> 4) circumpolar star observations at culmination, elongation or any
> hour angle --requires Greenwich time.
>
> The shadow method uses a plumbed staff and a piece of string.  Mark
> off intervals of more-or-less equal time, say 20 minutes, at the tip
> of the shadow, with noon more-or-less at the center.  Draw a smooth
> curve through the marks.  Draw a circle from the pole, intersecting
> the previously drawn arc at two places.  The midpoint of the line
> running between the two intersections is due north from the pole.
>
> Equal altitudes are the same principle as the shadow method, but
> using a transit.
>
> Both non-Greenwich-time methods suffer a bit from changing
> declination of the sun; the error claimed is max 30'.  One could
> probably go into an almanac to correct for this, or run the shadow
> method at a solstice or a few days around the solstice.  Stonehenge?
>
> A stupid method: since a transit telescope is permanently fixed once
> aligned, align it to the maximum elevation attained by various objects.
>
> Fred Hebard
>
>
> On Apr 21, 2008, at 3:30 PM, Geoffrey Kolbe wrote:
>
>
>> With the recent discussion on the abolition of the leap-second and the
>> problems of Delta T, I have been pondering the possibility of
>> measuring
>> Delta T myself.
>>
>> The transit telescope was invented by that Danish polymath genius Ole
>> Roemer in about 1675 and quickly adopted by Greenwich and then by
>> all the
>> world's observatories as a means to determine time. So, this seems
>> a good
>> way to go. But the main problem would be setting the telescope up
>> so that
>> it was aligned to the meridian. With a transit theodolite - which is
>> essentially a portable alt-az telescope - one can easily time the
>> moment a
>> star or the sun transits the vertical cross wire in the telescope.
>> Using
>> the calculated azimuth of the sun or star for that moment, it is
>> easy to
>> correct the plate azimuth of the theodolite and swing it around to the
>> meridian. But, since I want to use the telescope to measure time, I
>> would
>> prefer to find some other way to set it up which did not involve
>> the use of
>> absolute time.
>>
>> It seems that in England, there was a flurry of interest in small
>> transit
>> telescopes in the late 19th century as country gentlemen and the
>> newly rich
>> industrialists needed some way to determine the time in their country
>> estates. To this end, a book called "A Treatise on the Transit
>> Instrument
>> as Applied to the Determination of Time" was written in 1882 by
>> Latimer
>> Clark. Unfortunately, although Google books tantalizingly lists the
>> contents of the book, it does not seem to be available.
>>
>> There are a couple of ways that I can think of. First would be use
>> Polaris,
>> of course. But given that a transit telescope looks South, it may be
>> inconvenient to use Polaris - especially at these high latitudes
>> (Scotland). Second would be to time the transits of two stars of
>> preferably
>> similar SHA but greatly differing declination. Only if the
>> telescope is
>> aligned to the meridian will the difference in transit times be
>> correct.
>>
>> Can anyone come up with any other ways to align a transit telescope
>> to the
>> meridian, which does not involve the use of absolute time?
>>
>> Geoffrey Kolbe
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
>
>
> >
>

--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply