Theiler 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.

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