353 
on the Atmosphere of the Moon. 
its twilight 67 geogr. miles into the dark hemisphere, whilst 
the denser part of the lunar atmosphere, whose height does 
not exceed 1356 feet, produces a faint twilight not above 
io-§- geogr. miles in breadth. Thus, as my successful obser- 
vations on the twilight of Venus led me to the discovery of 
that of the moon, so did these latter reciprocally confirm the 
former : and thus, which ever way we contemplate the sub- 
ject, must we be struck with the coincidence that prevails 
throughout. 
4. But if the lunar atmosphere be comparatively so rare, it 
follows that the inflection of light produced by it cannot be 
very considerable ; and hence does the computation of M. du 
Sejour, according to which, the inflection of the solar rays 
which touch the moon, amounts to no more than receive 
an additional degree of authenticity * Besides which, 
5. As the true extent of the brightest lunar twilight amounts 
to 2 0 34/, the obliquity of the ecliptic in the moon only to 
i° 29' ; the inclination of the orbit of the moon, on the con- 
trary, to 5 0 15', and its synodic period, during which it per- 
forms a revolution round its axis is = 2 g d 12'’ ; it follows, that 
its brightest twilight, to where it loses itself in the light reflected 
by the almost fully illuminated disk of our earth, must, at 
least at its nodes, last f 3', and that it will be still longer at 
other parts of the orbit, according to the situation of the 
nodes. 
6 . And lastly, it being a well known fact* that the fixed 
stars, as they approach the moon, diminish in splendour at the 
most only a very few seconds before their occultations, it was 
* See De la Lande’s Astron. §. 1992 — 1994. 
+ See Selenot. Fragm. §. 531, with its note. 
