XU 
Introduction 
Spoerer’s observations, as published (1874) in his first extensive memoir, “ Beobachtungen der Sonnenflecken 
zu Anclam,” cover the period from January 1, 1861, to September 7, 1867. He first used a “3^-foot” telescope 
and ring micrometer; later he employed a “seven-foot” telescope, with a network of ruled lines in the focal plane. 
It is thus evident that the instrument employed by Peters was superior to those of his contemporaries, and his 
method of reduction was not less rigorous. A comparison of his positions of spots with those obtained elsewhere 
on the same dates is therefore of interest. Inasmuch as Spoerer used quite different elements of the sun’s axis, 
and a different period of rotation from those employed by the other observers here involved, a new reduction of his 
positions would be necessary before any comparison could be made. This comparison is therefore not feasible here. 
In the case of Carrington’s measures the conversion table given above serves to make the positions of 
Peters conform in longitude with those of Carrington, except for the difference in the value of the longitude of 
the node. For the purpose of comparison, 108 spots observed on fourteen days in i860 by both Carrington and 
Peters were collected, only such spots being employed as could be regarded as certainly identified. The results 
are as follows: 
MEAN OF DIFFERENCES, PETERS-CARRINGTON 
In longitude, with respect to sign.+ 39 .'2 
In longitude, without respect to sign.45.2 
In latitude, with respect to sign.+ 13.4 
In latitude, without respect to sign.28.8 
Inasmuch as Carrington actually used a value for N smaller by 47' than he subsequently derived from his whole 
series of observations, his values in longitude should be increased by that amount. Hence the outstanding mean 
difference in longitude, P.-C., reduced to the same longitude of node, becomes —8'. Carrington’s use of 1 = 7° io' 
instead of 1 = 7° 15' introduces a slight difference in the individual latitudes, but this is practically balanced in the 
series of observations compared. 
We therefore conclude that there is no systematic difference between the positions of Carrington and Peters 
amounting to more than o?2—a satisfactory agreement. Accidental differences averaging from o?5 to o?75 occur, 
however. These differences are doubtless due, in large measure, to the fact that two observers can hardly agree 
in setting upon the nucleus of so indefinite an object as a sun-spot. The actual changes in the spots between the 
time of their observation in England and in America are also involved. 
The observations at Kew and Clinton are strictly comparable, after the latter are corrected in longitude for 
direction and rotation-period by our conversion table. For the purpose, 114 spots measured at both stations on 
21 days between February 7, 1862, and July 2, 1863, were used, with these results: 
MEAN OF DIFFERENCES, PETERS-KEW 
In longitude, with respect to sign.+ 7.'6 
In longitude, without respect to sign.19.0 
In latitude, with respect to sign.— 7.0 
In latitude, without respect to sign.17.4 
We may thus infer that differences of a systematic nature amount to scarcely more that o?i in either co-ordinate, 
while the accidental differences average o?3- The better accordance than that in case of Carrington is presumably 
due to the superiority of the Kew positions. Very considerable differences, however, are seen in the Kew measure¬ 
ments of the same spot when two photographs were made on the same day. We find that on ten of the twenty- 
one days used in the comparison of Peters and Kew such duplicate observations were made at Kew, the average 
interval between the two plates being 0^064. A comparison for 61 spots gave the following results, expressed in 
the sense first measure minus second measure: 
In longitude, with respect to sign.—11 '3 
In longitude, without respect to sign. 17.2 
In latitude, wdth respect to sign.— 0.3 
In latitude, without respect to sign.9.2 
The accidental differences in the longitudes of the two Kew plates are therefore practically of the same magnitude 
as those of Peters-Kew; while those in latitude are only one-half smaller. This clearly tends to confirm the high 
opinion as to the accuracy of the positions of Dr. Peters which a careful examination is certain to give to anyone. 
