87
See also: Ole Rømer 58–59 ■ Isaac Newton 62–69 ■ Nevil Maskelyne 102–03 ■ Geoffrey Marcy 327
EXPANDING HORIZONS
Assisted by his sister Caroline,
Herschel systematically quartered
the sky, recording curiosities such
as the unexpectedly large number
of double and multiple stars. He
even attempted to compile a map
of the Milky Way galaxy based on
the number of stars he counted
in different directions.
On March 13, 1781, Herschel
was scanning the constellation
Gemini when he spotted a faint
green disk that he suspected might
be a comet. He returned to it a few
nights later, and found that it had
moved, confirming that it was not
a star. Upon looking at Herschel’s
discovery, Nevil Maskelyne realized
that the new object was moving far
too slowly to be a comet, and might
in fact be a planet in a distant orbit.
Swedish-Russian Anders Johan
Lexell and German Johann Elert
Bode independently computed
the orbit for Herschel’s discovery,
confirming that it was indeed a
planet, roughly twice as far away as
Saturn. Bode suggested naming it
after Saturn’s mythological father,
the ancient Greek sky god Uranus.Irregular orbit
In 1821, French astronomer Alexis
Bouvard published a detailed table
describing the orbit of Uranus as it
should be according to Newton’s
laws. However, his observations of
the planet soon showed substantial
discrepancies with his table’s
predictions. The irregularities of its
orbit suggested a gravitational pull
from an eighth, more distant planet.I looked for the Comet or
Nebulous Star and found
that it is a Comet, for it
has changed its place.
William HerschelBy 1845, two astronomers—
Frenchman Urbaine Le Verrier and
Briton John Couch Adams—were
independently using Bouvard’s data
to calculate where in the sky to look
for the eighth planet. Telescopes
were trained on the predicted area,
and on September 23, 1846,
Neptune was discovered within just
one degree of where Le Verrier had
predicted it would be. Its existence
confirmed Bouvard’s theory and
provided powerful evidence of
the universality of Newton’s laws. ■William Herschel Born in Hanover, Germany,
Frederick William Herschel
emigrated to Britain at 19 to
make a career in music. His
studies of harmonics and
mathematics led to an interest in
optics and astronomy, and he set
out to make his own telescopes.
Following his discovery of
Uranus, Herschel discovered two
new moons of Saturn and the
largest two moons of Uranus. He
also proved that the solar system
is in motion relative to the rest of
the galaxy. While studying the
Sun in 1800, Herschel discovered
a new form of radiation. Heperformed an experiment using
a prism and a thermometer to
measure the temperatures of
different colors of sunlight,
and found that the temperature
continued to rise in the region
beyond visible red light. He
concluded that the Sun emitted
an invisible form of light, which
he termed “calorific rays” and
which today we call infrared.Key works1781 Account of a Comet
1786 Catalogue of 1,000 New
Nebulae and Clusters of StarsIn the 1780s, Herschel built his
“40-foot” telescope with a 47 in (1.2 m)
wide primary mirror and a 40 ft (12 m)
focal length. It remained the largest
telescope in the world for 50 years.