of Edinburgh, Session 1879 - 80 . 
547 
with the mean time clock of the Royal Observatory, and again 
rated after nine days. The comparison of the time may be held 
as true to within a quarter of a second. The observation may be 
recorded thus : — 
North Latitude, . . . 55° 55' 42" 
West Longitude, ... Oh. 12m. 42s. 
Greenwich mean solar time of disappearance — 
1880, March, 17d. 12h. 23m. 55-8a., 
as by the mean time clock of the Royal Observatory of Edinburgh. 
In order to compare this with the predicted motions, the moon’s 
right ascension and declination were interpolated strictly from the 
hourly table in the “ Nautical Almanac,” and the star’s place was 
taken from the “Elements of Occupations,” while the parallax was 
computed on the supposition that the earth’s equatorial is to the 
polar axis as 300 to 299. In this way the expected time was com- 
puted to be 17d. 12h. 23m. 46 - 3s., or 9 '5s. earlier than the observed 
time. 
There is no astronomical phenomenon more definite as to time 
than the occultation of a star, nor any perhaps more easily observed 
when the disappearance is against the dark edge of the moon. 
Provided that the telescope be sufficiently powerful to show the 
star, it is of little or no moment whether the definition be good, or 
even whether the instrument have been well adjusted to focus. In 
all cases the disappearance is instantaneous. 
But, although the observation be thus satisfactory, there are 
various difficulties in the way of the calculations. In the first 
place, there is the error to which our lunar tables are liable ; these 
tables, all founded on previous observation, are brought forward by 
an estimate of the laws and rate of change, and thus are unavoid- 
ably subject to a gradually increasing uncertainty. Wonderfully 
exact as these tables are, it would always be necessary, before 
drawing any exceedingly minute conclusion, to study the tabular 
error as obtainable from nearly contemporaneous observations on the 
moon. Next we have the possible error in the tabulated place of 
the star; an error, in the case of small stars, which is not to be 
