  The Clementine Gnomon is a solar meridian telescope dedicated to solar
astrometry operating as a giant pinhole dark camera, being the basilica of
Santa Maria degli Angeli the dark room. This instrument built in 1701-1702 by
the will of pope Clement XI by Francesco Bianchini (1662-1729) gives solar
images free from distortions, excepted atmospheric refraction, because the
pinhole is opticsless. Similar historical instruments are in Florence (Duomo,
by Toscanelli and Ximenes), Bologna (San Petronio, by Cassini), Milan (Duomo,
by De Cesaris) and Palermo (Cathedral, by Piazzi). The azimut of the Clementine
Gnomon has been recently referenced with respect to the celestial North pole,
and it is 4'28.8"\pm0.6", a comparison with similar coeval instruments is
presented. Also the local deviations from a perfect line are known with an
accuracy better than 0.5 mm. With these calibration data we used the Gnomon to
measure the delay of the solar meridian transit with respect to the time
calculated by the ephemerides (DUT1). The growth of this astronomical parameter
is compensated by the insertion of a leap second ad the end of the year in
order to keep the Universal Time close to astronomical phenomena within less
than a whole second. On December 31, 2008 at 23:59:59 there is one of those
leap seconds leading to 23:59:60 before the new year's midnight 00:00:00, being
DUT1\approx0.7 s at that date. DUT1 has been measured with an accuracy of
\pm0.3 s.
