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Page:Popular Science Monthly Volume 57.djvu/526

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POPULAR SCIENCE MONTHLY.

only about 60 stars whose parallaxes can be said to be determined; and one-half of these are too near the apex, or have too small a parallax, to permit of any conclusion being drawn from them.

A second method is based on the spectroscopic measures of the motion of stars in the line of sight, or the line from the earth to the star. A star at rest in the direction of the solar apex would be apparently moving toward us with a velocity equal to that of the solar motion. Assuming the center of mass of all the stars observed to be at rest, we should get the solar motion from the mean of all.

Thus far, however, there are only about 50 stars whose motions in the line of sight have been used for the determination, so that the data are yet more meagre than in the case of the proper motions. From them, however, using a statistical method Kapteyn has derived results which seem to show that the actual velocity of the solar system through space is about 16 kilometres, or 10 statute miles, per second.