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    Re: Aligning a transit telescope to the meridian
    From: Fred Hebard
    Date: 2008 Apr 21, 16:53 -0400

    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
    >
    >
    >
    >
    >
    >
    >
    >
    > >
    
    
    
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