PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 5 JULY 15, 1919 Number 7 
EVIDENCE OF STREAM MOTION AFFORDED BY THE FAINT 
STARS NEAR THE ORION NEBULA 
By Adriaan van Maanen 
Mount Wilson Observatory, Carnegie Institution of Washington 
Communicated by G. E. Hale. Read before the Academy, April 28, 1919 
The increased accuracy which can be obtained in the determination of 
proper motions of the stars by long-focus instruments makes it necessary to 
take into account quantities which otherwise could be neglected. When the 
probable error of the motion in a coordinate becomes as small as 0''003 or 
0''004, we may no longer neglect systematic errors of this size or larger, if they 
can possibly be determined. 
In a discussion of the proper motions of 85 stars in the region of the 
Pleiades/ photographed with the 80-foot focus of the 60-inch Mount Wilson 
reflector it was found that in the reductions the quadratic terms of the coordi- 
nates could not be neglected, notwithstanding the fact that the size of the field 
was only 24 by 30 minutes of arc. For this and other reasons it was sus- 
pected that by neglecting such terms in a former discussion of the proper mo- 
tions of 162 stars near the Orion nebula,^ photographed with the 40-inch Yerkes 
refractor, we had not attained the best possible results. A new solution has 
therefore been made including the quadratic terms; the necessary corrections 
are of the same order as those for the Pleiades field and may not be neglected. 
The comparison reveals the relative efficiency for this kind of work of the 
long-focus reflector, which seems easily able to withstand a comparison with 
the refractor. 
The fact that we are here dealing with a field in which many of the stars 
must belong to the nebula, especially in the center of the plates, has its dis- 
advantages, as well as its advantages, for, the motion of the nebula being small, 
we cannot separate by their proper motion alone the individual stars belonging 
to the nebular system. This necessitates the exclusion for reference purposes 
of most of the central stars, whereby the weight of the reductions is con- 
siderably lessened. 
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