6 THE ANTARCTIC MANUAL. 
PHOTOMETRIC OBSERVATIONS OF THE ECLIPSES OF 
JUPITER'S SATELLITES. 
It is suggested that observations of the eclipses of J upiter’s 
satellites should be included in the programme of work to be under- 
taken for the determination of longitudes. The mode of observation 
which can be most readily utilised for such a purpose appears to be 
that in which the reading of a wedge photometer is recorded, corre- 
sponding to the extinction of a satellite at different times during the 
progress of its eclipse. Professor Sampson, of Durham, who is en- 
gaged in the discussion of the photometric observations of these 
eclipses made at the Harvard College Observatory, strongly recom- 
mends this course, and has been so kind as to furnish suggestions as 
to the method of observation to be followed, which his familiarity 
with the subject renders of great value. 
The following will be the method of observing a disappearance : 
before the brightness begins to diminish make 6-10 measures of it, 
noting also the time; when it begins to fall off, set the photometer so 
that the satellite is just seen, read the photometer, wait and note the 
time of disappearance ; set the photometer again so that the satellite 
is just seen, and repeat. This is to be done as often and as rapidly 
as allows of steady work. But uniformity rather than great numbers 
of observations gives the light curve clearly. 
Mutatis mutandis. The same will apply to the observation of a 
reappearance. 
In recording the observations all that is required is the chrono- 
meter time at each extinction, and the corresponding reading of the 
photometer. In the subsequent reductions the former must, of course, 
be corrected for adopted error on local time, and the latter for error 
of scale reading. It will be well to secure observation of the eclipses 
of all four of the bright satellites ; obtaining, when possible, observa- 
tion of the disappearances and the corresponding reappearances of 
II. arid IV. 
It is most important that the observation should in each case 
include a number of readings corresponding to the extinction of the 
fully illuminated satellite. There is appended, as a specimen, a copy 
of the record of an eclipse of each of the satellites, as recorded at the 
Harvard College Observatory. 
