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    Re: Aligning a transit telescope to the meridian
    From: Lu Abel
    Date: 2008 Apr 21, 14:07 -0700

    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
    > >
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