[Footnote 11: Mercury is best seen when in quadrature to the sun, but _not_ (as I have seen stated) at those quadratures in which he attains his maximum elongation from the sun. This will appear singular, because the maximum elongation is about 27°, the minimum only about 18°. But it happens that in our northern latitudes Mercury is always _south_ of the sun when he attains his maximum elongation, and this fact exercises a more important effect than the mere amount of elongation.]
[Footnote 12: It does not seem to me that the difficulty of detecting Mercury is due to the difficulty "of identifying it amongst the surrounding stars, during the short time that it can be seen" (Hind's 'Introduction to Astronomy'). There are few stars which are comparable with Mercury in brilliancy, when seen under the same light.]
[Footnote 13: I may notice another error sometimes made. It is said that the shadow of a satellite _appears_ elliptical when near the edge of the disc. The shadow is _in reality_ elliptical when thus situated, but _appears_ circular. A moment's consideration will show that this should be so. The part of the disc concealed by a _satellite_ near the limb is also elliptical, but of course appears round.]
[Footnote 14: From a paper by Mr. Breen, in the 'Popular Science Review,' October, 1864.]
[Footnote 15: 'Intellectual Observer' for July, 1867, to which magazine the reader is referred for full details of Mr. Howlett's method of observation, and for illustrations of the appliances he made use of, and of some of his results.]
[Footnote 16: As the sun does not attain such an altitude as 18° during two months in the year, it is well to notice that the true length of the sun's apparent solar diameter is determinable even immediately after sun-rise, if the line of graduation is made to coincide with the _horizontal_ diameter of the picture on the screen--for refraction does not affect the length of this diameter.]
[Footnote 17: The words "Left" and "Right" indicate which end of the sun's ecliptical diameter is uppermost at the dates in upper or lower row respectively.]
PRINTED BY W. CLOWES AND SONS, DUKE STREET, STAMFORD STREET, AND CHARING CROSS.
End of the Project Gutenberg EBook of Half-hours with the Telescope by Richard A. Proctor