Abstract
The Orion Nebula cluster has been investigated using 44 plates with a time span of about 42 years from a 26-in. refractor. Plate-constant reductions indicate a mean scale change of about -0.000116 over the interval, but differential atmospheric refraction reduces the scale change to approximately -0.000105. There is a 13.4-F average temperature difference between the two epochs. Thermal effects account for an observed scale difference of 0.000011 per deg F. The resultant scale change is about 0.000045; thus the present data show weak evidence for a small expansion among the Orion stars examined.
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