It is impossible for two reasons, to compute a precise geocentric ephemeris of this comet in the ordinary way during the time of its closest approach to the Earth. The first reason is that the solar ephemeris is only given to 7 aecimals, and this is not sufficient when the comet's coordinates are nearly equal numerically and differ in sign; thus on 1927 June 2 7 we have
Y -0.95725@ : 0.3992784
x -x , + 0.0281324 y -y,-o.o272851 z -2 3 + 0.0040937 with I/e=25.4. An incertainty of I unit in the 7 t h decimal will in declination amount to f 0'15 and may in right ascension be of the same amount'). The second reason is that the Sun's place may be I # or even 2 " in error, and this will also be multiplied 25.4 times. In other words, if the comet's place is deduced from its heliocentric coordinates in the usual way, an error approaching a minute of arc may be introduced. It is better therefore to compute a purely geocentric ephemeris which will be free from the errors of the Sun's place. It will be seen that this can be done. ' 1 1 ' =Lu" + s" --,I]\ . 2 ' " I 2 240 1. B. : J1v.~.' is insensible will be found in Table 4. T a b l c 4. S e c o n d D i f f e r e n c e s of t h e C o m e t ' s Gco-(.en t r i c R c c t ;i n g u la r C o o r d i n a t e s ( I 927.0). unit: 9th decimal 1927
. I".V -I"y . /"-.