ORIGIN AND OPERATIONS 



U. S. NAVAL ASTRONOMICAL EXPEDITION. 



During the summer of 1847 a letter reached me from Dr. C. L. Gerling, a distinguished 

 mathematician of the Marburg University, in which he says : " Since the date of my last, I 

 have been occupied with the volume of astronomical observations you had the kindness to send 

 me, and it has occurred to me that it might be acceptable to you to receive by letter, in advance 

 of its publication, the contents of a brief treatise which I shall transmit to M. Schumacher in a 

 few days for publication in the ' Astronomiche Nachrichten.' Should you find my views correct, 

 this will enable you to commence observations in America this year, which, I fear, could not 

 be the case if you awaited a printed copy of the paper. The subject is this : I am of opinion 

 that astronomers act unwisely in considering the solar parallax deduced from the transits of 

 Venus in 1761 and 1769 sufficiently correct, and do not avail themselves of more modern 

 methods of observation for the purpose of gradually acquiring more accurate knowledge of it. 

 It is true, indeed, that the oppositions of Mars were long ago proposed for this purpose ; but I 

 am not aware that any effective use has been made of them since 1751, although the Nautical 

 Almanac has regularly furnished an ephemeris. There is, however, a third method, which 

 presented itself to me some time ago, and I cannot comprehend why it should have been so 

 entirely neglected I mean by observations of Venus during the period of its retrograde motion, 

 and more especially when the planet is stationary. 



" The delicate and faint crescent form of Venus at the conjunctions offers excellent opportu- 

 nities for observation ; and from what I have been able to accomplish with my small instrument, 

 I have every reason to believe that most excellent results are obtainable with meridian instru- 

 ments at observatories in opposite hemispheres, but lying nearly under the same meridian. 

 Furthermore, at that time Venus is almost twice as near to the earth as is Mars when in oppo- 

 sition, and observations upon it have the very important advantage that it is not absolutely 

 essential they should be simultaneous, or nearly simultaneous. Again, when the planet is 

 stationary the observations of one meridian may be readily referred to another by interpolation, 

 without risk of error, and at this time it is much nearer to the earth than Mars can be in the 

 most favorable case. Finally, the distance of the planet from the sun being about 29 micro- 

 metrical, may be combined with meridional observations. In my opinion, then, it should be 

 our object to multiply meridian observations of Venus about the periods when it is stationary, 

 and endeavor to obtain micrometrical measurements from all parts of the earth, more especially 

 from voyagers. Let us suppose a traveller at a place, A, of the southern hemisphere, to observe 

 the planet in the evening at a time T referring it to a projected point of the heavens A, and 

 that it is also observed on the meridian during adjacent days at an observalory B of the north- 

 ern hemisphere : the meridian of the earth, in whose plane projected, Venus was found to be at 

 the time T, will be readily ascertained from its known right ascension, (corrected by the 

 observations at B). This terrestrial meridian will also be intersected by the parallel of the 

 observatory at B in a point C, where a simultaneous observation on the meridian has not, 



