Abstract
DURING last summer and autumn Mr. Douglass made at this observatory 341 micrometric measures of the diameters of Mars. In addition to their general value as micrometric measurements, these turn out to be of a peculiarly interesting character. For on reducing them I find that beside furnishing, from their great number, relatively accurate values of the equatorial and polar diameters and of the polar flattening, they yield a by-product as unexpected as it is important. Their discussion reveals, in short, what appears to be unmistakable evidence of a twilight upon the planet, sufficiently pronounced to be visible from the earth, and actually to have been measured unconsciously by Mr. Douglass. That Mars possessed an atmosphere, we had what amounted to proof positive before; but that the fact should again be brought to light in this literal manner, as a silver lining to a cloud of figures, is a point of some curiosity. The measures had no such end in view; indeed, to detect the presence of an atmosphere by measures of the diameters had not suggested itself to any of the most adventurous of observers. Yet, as will be seen, the quantities upon which the evidence rests are so large as to be quite without the pale of accidental error, being ten times as great as the probable errors of observation, and twice as large as those that disclose the polar flattening. That they have hitherto escaped detection is due to their having been masked by another factor affecting the size of the polar diameter, as will appear in the course of this paper. To the unsuspected presence of these two causes, at times nearly offsetting each other, so far as relative values go, is attributable in all probability much of the discrepancy in the determinations of the polar flattening hitherto made.
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LOWELL, P. Evidence of a Twilight Arc upon the Planet Mars. Nature 52, 401–405 (1895). https://doi.org/10.1038/052401b0
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DOI: https://doi.org/10.1038/052401b0