The Astronomy Book

74

I DARE VENTURE

TO FORETELL THAT

THE COMET WILL

RETURN AGAIN

IN THE YEAR 1758

HALLEY’S COMET

I

n the 16th century and for

much of the 17th, advances

were made in understanding

the motions of planets, but the

nature of comets remained a

mystery. Up until at least 1500,

comets had been feared as

harbingers of doom in Europe.

Astronomers were familiar with

these bright blotches of light and

their long, beautiful tails moving

slowly across the sky over periods

of a few weeks or months, but had

no idea where they came from,

nor where they disappeared to.

Things changed, however,

in 1577, when an exceptionally

bright comet lit up the night sky

for several months. By studying

IN CONTEXT

KEY ASTRONOMER

Edmond Halley (16 5 6 –174 2)

BEFORE

c.350 bce Aristotle declares

that comets are weather

phenomena in Earth’s

upper atmosphere.

1577 Tycho Brahe calculates

that a comet he has observed

must exist far outside

Earth’s atmosphere.

AFTER

1758 The comet that Halley

predicted duly reappears, 76

years after its last sighting.

1819 German astronomer

Johann Encke discovers a

second periodic comet, which

reappears every 3.3 years.

1950 Dutch astronomer Jan

Oort proposes that the solar

system is surrounded by a

huge cloud of comets, and that

stars may perturb their orbits.