142 | The Sky 
q 
chunks of rock a few miles in diameter, others being almost 
the size of the Earth. 
The order of the planets, from the nearest to the sun out- 
ward into space, is: : 
Mercury 
Venus 
Earth 
Mars 
Jupiter 
Saturn 
Uranus 
Neptune 
Pluto 
LITTLE MERCURY 
Mercury, the planet nearest the sun, has the shortest period | 
of rotation of all the planets and is the smallest. It keeps one 
face toward the sun, the other turned away from the sun in- 
variably. As a result one hemisphere must be very much 
hotter than the other. Because Mercury always appears very 
close to the sun in the sky and is therefore always more or 
less in its glare, little is known about the telescopic appear- _ 
ance of its surface. In other respects Mercury is a very fas- | 
cinating planet because in its motion it exhibits some anoma- 
lous motion (precession of the perihelion) which cannot be 
accounted for according to classical mechanical theories. An 
attempt to rectify this deplorable situation was made by 
Leverrier, the famous theoretical astronomer of the Paris 
Observatory. He postulated the existence of another planet 
even nearer the sun than Mercury. Immediately he received © 
