MAIIS IN THE AUTUMN OF 1877. 
241 
the aspect shown in No. 1, six days less 22 min., or seven days 
joins 1 min. after he had presented the aspect shown in 
No. 2. 
It may, however, be convenient to the observer to introduce 
the correction for Mars’s angular motion round the earth, which, 
indeed, though small for the motion of Mars during six or seven 
days (even) when he is in opposition, necessarily becomes appre- 
ciable in the course of several weeks, during which the planet is 
favourably placed for observation before and after opposition. 
The correction can readily be made as follows. From the 
<4 Nautical Almanac ” mark in the position of Mars at inter- 
vals of ten days (say) in any atlas showing longitude and lati- 
tude. (In my 44 School Atlas ” the longitude and latitude lines 
are indicated by their points of intersection to every 30° ; but 
it will be found easy to fill in, on a tracing taken from the 
proper map, the intermediate longitude and latitude lines to 
every o° or 10°). Thus the geocentric motion of the planet in 
longitude is indicated. Direct motion in geocentric longi- 
tude delays pro tanto the coming of a Martian meridian 
to the centre of the disc of Mars, while retrograde motion in 
geocentric longitude hastens pro tanto the arrival of a Martian 
meridian at the centre. For instance, suppose that on a given 
day soon after opposition Mars has retrograded a° in longitude 
from his opposition place on the chart and that the epoch t is 
calculated for a given view of the six formulas numbered 1, 2, &c., 
without taking into account the change of Mars’s position rela- 
tively to the earth. Then that view will be presented at the 
time t — (4 m. 6^s.)z. 
The above data will be sufficient for determining the aspect 
of the planet at any time during the approaching opposition. 
Account will, of course, have to be taken of the gibbosity of 
Mars, as affecting the apparent position of his central meridian 
at any time ; but the 44 Nautical Almanac ” supplies the neces- 
sary information for this purpose. 
The points to which I would direct the special attention of 
observers are three, — first, the position of the south polar snow- 
cap ; secondly, the rotation-period of the planet ; and thirdly, 
the determination of the configuration of various lands and seas 
presently to be mentioned. 
If on every good observing night the angle of position of the 
centre of the snow-cap with reference to the centre of the disc 
could be determined in the same manner as in the case of a 
double star — the centre of the snow-cap corresponding to the 
companion and the centre of the disc to the primary — the 
observations could not fail to be of value, as showing whether 
the snow-cap occupies the true pole, or if not, how far from the 
true pole its centre lies, and also showing where the true south 
