546 Proceedings of the Royal Society 
The following Communications were read : — 
X. On the Occultation of the Star 103 Tauri. (b. a. c, 1572.) 
By Edward Sang. 
An occultation of a star, though not appealing to ordinary ob- 
servation with the same force, is intrinsically an event as striking 
as an eclipse of the sun. It establishes the fact of the moon’s 
proximity. Were it not that the moon’s brightness overpowers the 
light of the small stars, occultations would he commonplace pheno- 
mena. As things are, we can watch, with the eye unaided, the 
eclipses of the planets and larger stars, not down, perhaps, to below 
the third magnitude; and the rarity of such conspicuous objects 
makes the occultations correspondingly rare. 
By help of a telescope of two or three feet in focal length, we are 
able to examine stars even so small as of the sixth magnitude, and 
thus greatly to increase the number of observations, so much so that 
as many as 150 occultations may he visible from one place in the 
course of the year. 
The particular case to which I would draw attention is thus one 
of many : it derives its interest from the proximity of the planet 
Mars, whose occultations will have been carefully observed from 
many places. 
The three objects — the moon, Mars, and the star — were all within 
the field of the telescope ; their 
relative positions at the instant 
of the star’s disappearance being 
as shown in the accompanying 
figure. The observation was 
made with a telescope having 
an aperture of 1*9 in., a focal 
length of 23*5 in., and a magni- 
fying power of 26. The moon’s 
dark edge was distinctly visible, 
the atmospheric tremor was 
slight, so that, notwithstanding 
the moon’s proximity to the horizon, the disappearance was watched 
under very favourable circumstances. The time was noted by an 
excellent chronometer, which was compared, twelve hours thereafter, 
