1891-92. ] CIRCULAR-LETTER. ahs 
knowledge, resulted from the existence of two different modes of counting time. 
(5) For spectroscopic and photographic observations of the sun, it is now recognized 
that the day should be reckoned from midnight, and the same reckoning would natur- 
ally be used by the observer when he takes spectroscopic and photographic observa- 
tions at night, and also in determinations of the places of comets, stars, etc., which he 
may make in connection with his spectroscopic observations. It seems absurd to 
expect the same observer to change his system of reckoning mean solar time according 
to the class of observations he is making at the moment. (6) The proposal to include 
in the routine work of an observatory, photography of the stars, as well as of 
the sun, will further increase the difficulty of maintaining a distinction as regards time- 
reckoning between the various classes of astronomical observations. (7) At many 
observatories, magnetical and meteorological observations are carried on concurrently 
with astronomical observations, and it is admitted that for the two former classes the 
day commencing at midnight should be used. (s) For the distribution of the time to 
the public, a work which is undertaken by many observatories, the civil day would be 
used. (9) Thus civil reckoning commencing at midnight must be used for solar, mag- 
netical, and meteorological observations, and also for the distribution of time to the 
public, so that the retention of astronomical reckoning would involve the use of two 
different systems of mean solar clocks, differing by 12 hours, in the same observatory— 
a circumstance likely to lead to intolerable contusion. (10.) As regards the supposed 
discontinuity which would arise from the change in the Nautical Almanac, the differ- 
ence of time-reckoning is precisely similar to that which would have to be taken into 
account in the comparison of Greenwich observations with those made at any other 
observatory. The astronomical calculator is in the habit under the present system of 
allowing for the difference in time-reckoning between different observatories, and his 
task would be greatly simplified if he had only to deal with universal time.—/efort to 
the Trustees of Greenwich Observatory, by W. H. M. Christie, M.A., LL.D., Astron- 
omer Royal of England. 
IV. The first of these recommendations proposes a change in the method of counting 
astronomical time which has come down to us from antiquity, and which is now uni- 
versal among astronomers. The practice of taking noon as the moment from which 
the hours were to be counted originated with Ptolemy. This practice is not, as some 
distinguished members of the Conference seem to have supposed, based solely upon 
the inconvenience ‘to the astronomer of changing his day at midnight, but was adopted 
because it was the most natural method of measuring solar time. At any one place 
solar time is measured by the motion of the sun, and is expressed by the sun’s hour 
angle. By uniform custom, hour anyles are reckoned from the meridian of the place, 
and thus by a natural process the solar day is counted from the moment at which the 
sun passes over the meridian of the place or over the standard meridian. . . . A 
change in the system of reckoning astronomical time is not merely a change of habit, 
such as a new method of counting time in civil life would be, but a change in the 
whole literature and teaching of the subject. The existing system permeates all the 
volumes of ephemerides and observations which fill the library of the astronomer. All 
his text-books, all his teachings, his tables, his formulz, and his habits of calculation 
are based on this system. To change the system will involve a change in many of the 
