Obliquity of the Ectiptic. 28 
exact, more independent of the false parallax he attributed 
to the sun’s refraction, errors from division of the in- 
struments, and of those of the observations; the latitude 
30° 3’ 20" added to §3° 31’ 6", gives 113° 34’ 26"; from 
which if we subtract 90°, we shall have 23° 34' 26" for 
the obliquity of the ecliptic towards the year 1000. The 
formulas of Méc. Cél. give 23° 34' 50", which agrees, as 
nearly as can be desired, “with Ebn- Jouris? observations. 
The Persian astronomy presents a detailed observation 
of the obliquity of the ecliptic, made by Ulugbey, in 1437, 
with a great instrument, which probably was a gnomon of 
avery great length. This great observer found at Samar- 
kand, the capital of his dominion, the sun’s altitude at beth 
‘solstices, corrected by the parallax which he supposed the 
sun to have, equal to 73? 52 54” for the summer solstice, 
26° 52' 20" for the winter’s. He made the parallax of the 
sun 2’ 99,4: the altitudes, as he has observed them, were 
-then'?e° 52''19”,5, and 26° 56'6",7. Correcting res by 
refraction and true parallax, they become 73° 51’ "58" »4 and 
26° 48 29",6, which gives 23° 31’ 48" for the obliquity of 
‘the ecliptic in 1437, and 39° 39 49" for the latitude of 
Samarkand. According to the formulas of M/éc. Cél., the 
obliquity of the ecliptic, at that epoch, should have been 
23° 31’ 5”, which only differs 43” from the result of 
Uluebey’s observations. 
' Let us now collect the results we have j just found : 
OBSERVATIONS ANTERIOR TO OUR RA; 
Ollig. of Ecl-ptic | Olliquity ty Excess of the 1st 
ly Observation. Formulas. over the last. 
1100, Cheou-King 23° 54'22”,0.. 23°51'58" .. 2’ 4”, 
BOs TYINCSE Tio .g MOOR WO i OR Aer ae ae 
850, Eratosthenes 23 4539 .. 23°45 19 .. 0 12 
50, Lieou-hiang 23 4539 ... 23 44 3,4 .. 1 34,6 
OBSERVATIONS POSTERIOR TO OUR JERA. 
173, ‘Obser. Chinese 23°41’ 33”... 23° 42' 197" .. AA" 1 
461, Tsou-chonge’.. 23 38 52,3 .. 23 30°52... 10,7 
629, Litchoufoung.. 23 40 4,1°.. 23 38 17... 1°47, 
$80, Albatenius’ .. 23:35 41 .. 23 3513 .. 28 
1000, Ebn-Jounis .. 23.34 26 .. 23 34'50 .. 24 
1279, Cocheou-King QP SD. 1 DALY QS eBD (WBS 20 
1437, Ulugbey SP dake t Cee: 1: MOS gp tae ys on Tae pr: 
The whole of this observation established in an incon- 
testable manner the successive diminution of the ecliptic. 
Their agreement with the formulas of Mfléc. Cél. leaves no 
room to doubt that this diminution is entirely owing to the 
; B4 attraction 
