For very distant objects the effect of
relief vanishes because such objects look
the same no matter by which eye they are
viewed. It would, therefore, seem Impos
sible to utilize the stereoscope in astron
omy. Rut a gigantic observer with eyes
thousands of miles apart would doubtless
see the moon and the sun as actual spheres
instead of flat <11; k?. If views of tfce?e
bodies taken far enough apart can be com
bined stereoscopically, the efTect would be
the same. This has actually been accom
plished.
For a long time astronomers have tried
to adapt the stereoscope to astronomy,
and satisfactory relief photographs of the
moon have been obtained by taking views
at sufficiently long intervals and utilizing
the libratlon, that is. the slight swinging
of the moon to and fro in space.
This phenomenon, which shows us our
satellite from different points, produces
the same perspective efTect as if we should
suppose the observer transported succes
sively, but on the same date, to two points
of space such that by joining them to the
moon's center, the lines so determined,
should form an angle equal to the ampli
tude of the libratlon. In the case of maxi
mum libration we have an angle of four
teen degrees.
The distance of these two imaginary sta
tions would then be about one-quarter of
the distance from the earth to the moon.
The stereoscopic effect Is more than satis
factory. for the moon appears in exag
gerated relief, and seems to be egg shaped
instead of spherical. The sun's disk pho
tographed at an interval of several days
looks like a sphere.
