* 
2 ARISTARCHUS OF SAMOS 
stamping the air with impressions like themselves, as it were 
that ‘ colours in darkness have no colouring ’, and that ‘ light 
is the colour impinging on a substratum ’. 
Two facts enable us to fix: Aristarchus’s date approximately. 
In 281/280 B.C. he made an observation of the summer 
solstice; and a book of his, presently to be mentioned, was 
published before the date of Archimedes’s Psammites or Sand- 
reckoner, a work written before 216 b. c. Aristarchus, there 
fore, probably lived circa 310-230 b.c., that is, he was older 
than Archimedes by about 25 years. 
To Aristarchus belongs the high honour of having been 
the first to formulate the Copernican hypothesis, which was 
then abandoned again until it was revived by Copernicus 
himself. His claim to the title of ‘ the ancient Copernicus ’ is 
still, in my opinion, quite unshaken, notwithstanding the in 
genious and elaborate arguments brought forward by Schia 
parelli to prove that it was Heraclides of Pontus who first 
conceived the heliocentric idea. Heraclides is (along with one 
Ecphantus, a Pythagorean) credited with having been the first 
to hold that the earth revolves about its own axis every 24 
hours, and he was the first to discover that Mercury and Venus 
revolve, like satellites, about the sun. But though this proves 
that Heraclides came near, if he did not actually reach, the 
hypothesis of Tycho Brahe, according to which the earth was 
in the centre and the rest of the system, the sun with the 
planets revolving round it,, revolved round the earth, it does 
not suggest that he moved the earth away from the centre. 
The contrary is indeed stated by Aetius, who says that ‘ Hera 
clides and Ecphantus make the earth move, not in the sense of 
translation, but by way of turning on an axle, like a wheel, 
from west to east, about its own centre’. 1 None of the 
champions of Heraclides have been able to meet this positive 
statement. But we have conclusive evidence in favour of the 
claim of Aristarchus; indeed, ancient testimony is unanimous 
on the point. Not only does Plutarch tell us that Cleanthes 
held that Aristarchus ought to be indicted for the impiety of 
‘ putting the Hearth of the Universe in motion ’ 2 ; we have the 
best possible testimony in the precise statement of a great 
1 Act. hi. 13. 3, Vors. i 3 , p. 341. 8. 
2 Plutarch, Be facie in orbelunae, c. 6, pp. 922 f-923 a. 
contemporary 
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