420 
SCIENTIFIC SUMMAKY. 
ASTRONOMY. 
A HITHERTO Unnoticed Inequality in the Moon's Motion in Longitude . 
— Professor S. Newcomb, of the Washington Observatory, has noticed 
an irregularity of the moon’s motion which at present has received no ex- 
planation. After applying all the known corrections to the observations of 
the moon at Greenwich and Washington, from 1862 to 1874, “ I was sur- 
prised,” he writes, 11 to find systematic errors outstanding which could not 
be corrected in the lunar elements. Of their reality there could be no 
serious doubt, because the Greenwich and Washington observations agreed 
in showing them. At first I was disposed to attribute them to inequalities 
in the surface of the moon, but a more careful observation showed that they 
were periodic, and developed on the moon’s longitude, being at first positive 
when the longitude was between 180° and 360°, and negative in the first 
semi-circumference. But this was found to be subject to a sensible alteration, 
the point of maximum positive error moving forward to about 0°, and that 
of negative error to 180° in the course of a few years’ observations.” The 
maximum amount of this inequality is 1"*5, and its period is 27*4304 days 
+ 0*0040 d. “ There is a large preponderance of probabilities,” says Pro- 
fessor Newcomb, u against the real period being either less than 27*42 days, 
or more than 27*44 days. No known term in the moon’s longitude falls 
between these limits. The moon’s sidereal period is 27*32 days, and her 
anomalistic period is 27*55 days, so that the new term falls half way between 
these two. The non-accordance of this period with that of any term here- 
tofore sought for is the reason why this term has not before been noticed. 
A term of unknown period would not be remarked unless its magnitude was 
such as to visibly affect the individual comparisons of theory with observa- 
tions, and Hansen’s Tables, as corrected, are the first ones of which the 
residual errors are so small that a term of 1 "*5 would be remarked in the 
comparison with observations.” 
The Atmosphere of Venus. — In 1849 Madler and Clausen published 
some observations of Venus when near inferior conjunction, which showed 
that the sun’s rays underwent a considerable refraction from the atmosphere 
of the planet. Madler used the following formula : — 
Let V = the angular distance between the centres of the sun and Venus ; 
x y the prolongation of the cusps of the planet S, the semi-diameter of the 
