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    Re: clock synchronization
    From: George Huxtable
    Date: 2004 Feb 5, 15:48 +0000

    Patrick Stanistreet asked-
    >Based upon a rather hasty bit of research concerning how it
    >was possible to synchronize clocks in different parts
    >of the world it seems that galileo, cassini and others
    >that I cannot credit due to ignorance came up with the
    >idea of creating an astronomical clock based on the
    >eclipses of jupiters moons.  By creating accurate
    >tables of the times of the eclipses it was possible
    >to set clocks to both local time and greenwich time
    >to perhaps a few seconds at locations quite distant
    >from each other.  Although not a feasible method
    >for ships on the ocean, they could at least compare
    >their clocks in ports that had observatories monitoring
    >jupiters moons.
    >The next question is say at greenwich, how was it decided
    >to set the local time to a particular clock time say 12:00
    >noon to the second?  Was it a convention or was their some
    >method useful to any nearby observatory?
    Reply from George,
    Accurate timekeepers were a vital component of any observatory.
    At Greenwich there were two accurate clocks made by the famous Thomas
    Tompion, each with a pendulum 13 feet long, so it beat every 2 seconds
    (i.e, four seconds for a complete oscillation cycle). They were both set to
    keep Greenwich Mean Time. A telescope, dedicated to that job alone, was set
    in a forever-fixed position, pointing at the altitude of the star Sirius as
    it crossed the meridian. With a telescope, Sirius is bright enough to be
    seen even in daytime. So summer or winter, whenever it wasn't cloudy, the
    moment of transit of Sirius across that telescope's cross-hairs was noted
    on the clock at intervals of exactly 1 sidereal day.
    The ratio between the length of the sidereal day and the length of the mean
    solar day was precisely known, as N/(N+1), where N is the number of days in
    a year (365.2422). So that was easy, once the initial time-offset had been
    By the way, the original Tompion clocks still exist, one still at the
    observatory, the other at the British Museum. They were designed so as to
    need rewinding only once per year. The two clocks that tick slowly away
    behind the wainscoting in the old Octagon Room are replicas.
    The moment of passage of the apparent Sun across the Greenwich meridian was
    also determined, with a transit telescope, against the same clock, which
    allowed the Equation of Time to be determined with precision. The equation
    of Time varies by about ?15 minutes. Greenwich Mean Time was defined in
    terms of the average value of the Equation of Time being zero (hence its
    name, mean time). This was a convention that quickly came to be agreed by
    astronomers. So, after a year's observations, first by Huygens, later
    refined by Flamsteed using the Greenwich clocks, the difference between
    Mean time and apparent Sun time on any day was accurately known. This was
    important, especially as for its first 70 years, the almanac related all
    observations to apparent time, not mean time. Knowing the equation of time,
    the clock difference between sidereal time (from Sirius) and apparent time
    could be calculated precisely for each day.
    The times (Sidereal, Apparent Sun, Mean Sun) were, of course, all local
    times at Greenwich. There was nothing special about Greenwich to make it
    different from any other observatory, except that the first nautical tables
    to be published were by Maskelyne at Greenwich, and related to times
    measured at Greenwich. As a result charts then tended to measure longitudes
    from Greenwich, which came to be accepted worldwide, and so observers at
    other locations had to relate, precisely, their own local times to local
    times at Greenwich. This involved measuring the difference in longitude.
    A similar setting-up operation could be carried out at another observatory
    (Cape Town, say). Because the Equation of Time applies worldwide, and the
    hour-angle difference between Sirius and the Sun is everywhere the same at
    the same moment, there's no need to repeat those parts of the measurement.
    Again, though, this provided local times, if longitude difference with
    Greenwich was unknown.
    How could this longitude (time) difference be found, in the days before
    radio and telegraph, in the days when shipping dozens of chronometers to
    and fro was impractical?
    One method was by lunar distances. On land, lunars could be measured more
    precisely than at sea. Hundreds of observations, from the same spot, could
    be averaged. Even when Moon position predictions were still a bit
    uncertain, a geographer could observe a lunar and then enquire of another
    observatory (Greenwich, say) to what extent the position of the Moon around
    that time corresponded with published predictions. Such a leisurely
    approach wasn't useful to a mariner, who needed to know his position there
    and then.
    On land, at a local observatory with a good clock, a geographer could set
    up a powerful telescope to time the passage of Jupiter's inner moons, into
    and out of the shadow of the planet itself; an observation that was quite
    impossible at sea. By comparing these events with predictions, times to a
    few seconds could be determined. Even in the days before accurate
    predictions, observed times could be compared, long after the event, with a
    similar observation at the other observatory. If however, it happened to be
    cloudy there at the time, or if the satellite-eclipse had been timed from a
    very distant longitude and was not observable at the home-observatory, that
    didn't work.
    One difficulty about Jupiter satellite immersions related to the finite
    size of the moons themseves, and the fact that it took many seconds for the
    light to fade as they passed into shadow. Greenwich predictions were
    presumably based on the geometrical moment when the light completely
    extinguished, or perhaps when it could no longer be seen in the telescope
    used at Greenwich. However, when the final twinkle disappeared depended on
    the light-gathering power of the telescope used, and the skill of the
    observer. So there was an inherent uncertainty in the Jupiter-moon
    measurements of a few seconds.
    Pressure to observe and predict these Jupiter-moon occultations led to the
    discovery that they occurred about 8 minutes later than normal when Jupiter
    was near the Sun, and about 8 minutes earlier when it was at opposition.
    This led the astronomer Romer to correctly deduce that this was due to the
    finite velocity of light, and when the size of the Solar System became
    well-estimated in the mid 1700s, the velocity of light was rather precisely
    Derek Howse's "Greenwich Time and the Longitude", 1997, pub Philip Nelson.
    This is a good source About Greenwich and its clocks.
    "The Quest for Longitude", ed William J H Andrewes, Harvard, 1996. This has
    a good article about longitudes by Jupiter moons.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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