[ 537 3 
hour, it will move over 23 ".7 in 5': 55". And 
confequently, from the inftant of the real beginning 
of a tranfit, 5' : 55" mud elapfe before it can begin 
• apparently. 
It may, I know, be objected here, that the aber- 
ration of the Sun ought not to be taken into confe- 
deration, becaufe the calculations from the folar tables 
give the apparent places of. the Sun, or its longitude 
with the effe<ft of aberration included, and therefore 
always about 20 y too little. But from this obfervation a 
conclufion will follow very different from that which 
the objection fuppofes. The retardation I have 
mentioned is properly the time that the calculated 
phafes of a tranfit of Venus will precede the apparent 
phafes, fuppofing the tables from which the calculation 
is made to give the true places of the Sun. 
If they give the apparent places of the Sun, this 
retardation, inftead of being leffened, will be con- 
fiderably increafed. In order to prove this, I muft 
defire it may be remembered, that in deducing by 
trigonometrical operations the geocentric places of 
a planet from the heliocentric, the Earth is fuppofed 
to be in that point of the ecliptic which is exadtly 
oppofite to, or 180 0 from the place of the fun, and 
that this fuppofition is juft only when the fun’s true 
place is taken. In reality, the Earth is always about 2o' / 
more forward in its orbit than the point oppofite to 
the Sun’s apparent place ; and in confequence of this 
it will happen, that in calculating a tranfit of Venus 
from tables which give the Sun’s apparent places, a 
greater difference will arife between the calculated and 
the obferved times than if the tables had given the 
Sun’s true places. 
Vol, LX. 
Zzz 
For 
