181 
iiwtb to tk a Velum 
slowly forward until the sun’s reflected image makes a rough Contact 
with the moon, seen by direct vision through the unsilvered part of the 
horizon glass; clamp the index, screw in the telescope, and make the 
contact perfect in the centre of the field with the tangent screw, moving 
the sextant slowly round the axis of the telescope, by which means the 
reflected image of the sun will appear to pass the moon, and the accuracy 
of the contact can be tested. 
.Between the Moon and Star or Planet .—The angular distance between 
a star or planet and the moon is always measured to the moon’s en¬ 
lightened limb, which is often the farthest from the star or planet. 
When this is the case, the moon must be brought by reflection past the 
star or planet before the contact can be made: in other respects the 
observation is precisely similar to that already described, when the 
angular distance of the sun is taken. 
In observations of this class, the utmost attention must be paid to 
accuracy, and a faulty habit of observation in making contacts of the 
moon’s limb with a star is not necessarily eliminated, as is very generally 
supposed, and frequently stated, by taking distances east and west of the 
M V 
moon. For example, if it is an observer’s habit, in making a contact, to 
place the star within the moon’s disc, M, as at S', the distance S" S' is too 
small, and the distance S'" S' too great; but supposing the moon to be moving 
in the direction from S' to S'", each distance will give too early a Greenwich 
time, for each will give the time when the moon’s limb was actually at S'. 
When, however, the sun is the object observed east and west of the moon, 
errors of this sort in observation, if constant, will be eliminated, since, as 
the moon’s enlightened limb is always turned towards the sun, such errors 
would increase both distances and produce errors of an opposite descrip¬ 
tion in the Greenwich time.* A single observation is of little value; 
* For further information on this subject, read the article on Lunar Distances 
in 4 Chauvenefs Spherical Astronomy? 
