is a tiny lunar Impact crater
on the eastern lunar limb, in the western Mare Marginis
. To the southeast is the prominent Neper crater
. Due to its location, Theiler crater is subject to the libration
of the Moon
and is not always visible to observers on the Earth
. It is a circular, bowl-shaped formation of no particular significance. To the south-southeast is the Neper walled-plain
Note: The official IAU selenographic coordinates place this crater at 83.3° E. However its visual location on lunar photographic atlases is actually closer to 82.3° E. The reason for this minor discrepancy is unclear.