Ecdtnd 
IV. Memoir on the Diminution of the Obliguity of jhe 
Ecliptic, as resulting from ancient Observations. By 
M, Lapiace.’ Translated from the « Connoissance des 
Tems for 1811” by Tuomas Firmincer, Esq. 
{Concluded from vol. xxxvi. p. 434,] 
Greek Observations. 
Peedi observation at Marseilles was made between 
the two epochas of the preceding observations. In the se- 
cond. book of his Geography, and the 4th chapter, Strabo 
says: ‘* According to Hyppacus, at Byzantium, the pro- 
portion of the gnomon’s shadow is the same as Py.beas pre- 
tends to have observed at Marseilles ;” and in the 5th chapter 
of the same book he adds, * at Byzantium, in the summer 
solstice, the proportion of the gnomon’s shadow is 421 to 
120.” ; 
It was no doubt from that observation that Ptolemy, in 
his Almagesies (b. xii. ch. vi.), draws through Marseilles 
the 14th parallel, in which the shadow’s length at the 
summer solstice is 20% parts, the gnomon’s being 60 parts. 
Pytheas was, at latest, contemporary with Aristotle ; there- 
fore his observation may, without any sensible error, be 
referred tu the year 350 before our cera. After correcting 
it with the refraction and parallax, it gives 19? 98" gg" 
for the distance of the sun’s centre at the solstice, to the 
zenith of Marseilles. The latitude of that town’s observa- 
tory is 43° 17’ 49”, from which if the preceding distance ig 
subtracted, we shall have 23? 49’ 20” for the ecliptic’s obli- 
quity in Pytheas’ time. 
The new Solar Tables, published by the Board of Longi+ 
tude, which are founded on the formulas on book vi. chap. 
xil. of Méc. Cél. give 23° 46’ 7” for the ecliptic’s obliquity, 
corresponding to the year 350 before our zra; the difference 
3’ 13” is within the limits of errors of which Pytheas’ ob- 
servation is susceptible. 
About a century later than Pytheas’ observation, Erato- 
sthenes undertook to measure the earth, and founded that 
measurement on solstitial observations of the gnomon made 
at Syena and Alexandria. (Cleomedes, b.i. On .contem- 
lation of Celestial Bodies, ch. x. OF the Earth’s Magnitude.) 
Bocas made use of a vertical gnomon raised in a 
spherical segment. The summit cf the gnomon being in 
the cextre of the segment, he found the distance. between 
the zeniths of Syena and Alexandria to be equal to a 50th 
part 
