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THE MATCH OF LUNAR
AND EARTH MATERIAL
IS TOO PERFECT
THE ORIGIN OF THE MOON
IN CONTEXT
KEY ASTRONOMER
Reginald Daly (1871–1957)BEFORE
1913 British geologist Arthur
Holmes produces the first
modern geologic timescale,
which proposes that Earth
is at least 1.5 billion years old.AFTER
1969–72 The Apollo missions
bring back moon rock to be
analyzed on Earth.1975 Following analysis of
moon rock, US astronomer
William Hartmann and
others return to the giant
impact theory to explain
the new evidence.2011 Norwegian−American
planetary scientist Erik
Asphaug and Swiss
astrophysicist Martin Jutzi
suggest that the moon formed
with a tiny companion moon
and the two later collided.G
eologists had broadly put
together a story for Earth’s
billions of years’ existence
by the early 20th century. But the
origin of the moon remained open
to speculation. Up until the 1940s,
most astronomers subscribed to
a theory put forward by George
Darwin, son of the naturalist
Charles. In 1898, Darwin proposed
that the moon formed when a hot
and fast-spinning Earth threw out
molten rock that coalesced into anAstronomers have found evidence
that two small planets orbiting the star
HD 172555 collided a few thousand
years ago. A similar collision involving
Earth probably formed the moon.orbiting satellite. He suggested
that the moon had once been
much closer to Earth and was
slowly drifting away. This has since
been confirmed by measurements
showing that the moon is moving
away by about 1½ in (3.5 cm) a year.