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    Aligning a transit telescope to the meridian
    From: Geoffrey Kolbe
    Date: 2008 Apr 21, 20:30 +0100

    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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