436 
NEW OBSERVATIONS OF THE TLANET MERCURY. 
at Flao'stafF, and 207 drawings and 7 sketches at Mexico. 
markings appear depicted on the planet's surface. 
In all of the drawings, 
These markings were seen and 
drawn by every member of the Observatory, six observers in all, and through two 
different telescopes, the 24-inch and the 6-inch. 
8. Phase of Observation. — The planet was watched from as slender a crescent 
as it was possible to pick up to almost a full disk. Near inferior conjunction it is 
greater angular diameter of the disk being more than 
Ne 
perior 
difficult to find the planet, the 
offset by the phase and the fainter illumination of the surface sh 
conjunction, on the other hand, the detection of the planet is much easier than one 
would suppose. In Mexico I observed it to within 4° of right ascension of the Sun, 
and only stopped there because of removing the observatory to Flagstaff. All that is 
necessary to render it readily visible is to screen the object-glass as much as possible 
from the direct rays of the Sun. 
9 
Rotation 
As the markings thus revealed proved : (1) dis- 
tinct; (2) permanent; and as the drawings, in all 311 of which they appear, were 
made at intervals of time ranging from a few minutes to many months apart, the rota- 
tion period of the planet was patent, and with it the position of the axis of rotation. 
Tliey showed that the planet rotates (1) in a period synchronous with its orbital 
revolution ; (2) round an axis substantially perpendicular to the plane of its orbit. 
To make this evident at a glance I have, in Plates XXX. and XXXI., reproduced two 
sets of three triads of drawings each, the one set by Miss Leonard, the other by me. 
It 
b 
seen that there is no perceptible shift in the positions of 
m 
ark- 
ings in any of the several six triads. Yet the intervals between the drawings 
Plate XXX. are : 
Between 1 and 2 
(( 
« 
u 
2 
3 
4 
U 
u 
it 
3 
4 
5 
h. 
0.7 
3.1 
20.1 
1.2 
Between 5 and 6 
u 
II 
7 " 8 
8 " 9 
h. 
2.2 
3.9 
0.6 
Or between the extremes of row 1, an interval of 
it 
it 
it 
<t 
2, 
tt 
it 
ii 
u 
u 
and between the extremes of row 3, 
(( 
it 
Similarly in Plate 
we 
have 
h. 
3.8 
3.4 
rows 1 and 2, an interval of 27.2 
4.5 
Between 1 and 2 
« 
<€ 
ti 
2 " 3 
3 « 4 
4 « 6 
h. 
0.7 
5.0 
65.3 
3.4 
Between 5 and 6 
(( 
tt 
It 
6 
7 
8 
ii 
(6 
c: 
7 
8 
9 
h. 
2.0 
G8.1 
1.4 
21.5 
