DIRECTORY 335
IBN AL-HAYTHAM
c.965–1040
Also known by his Latinized name
Alhazen, Ibn al-Haytham worked at
the court of the Fatimid Caliphate
in Cairo. A pioneer of the scientific
method, whereby hypotheses are
tested by experiment, al-Haytham
wrote a work popularizing
Ptolemy’s Almagest and, later, a
book casting doubts on aspects
of Ptolemy’s system.
See also: Consolidating
knowledge 24–25
ROBERT GROSSETESTE
c.1175–1253
English bishop Robert Grosseteste
wrote treatises concerning optics,
mathematics, and astronomy. He
translated Greek and Arabic texts
into Latin, introducing the ideas of
Aristotle and Ptolemy into medieval
European thought. In his work De
luce (On light), Grosseteste made
an early attempt to describe the
entire universe using a single set of
mathematical laws. He called light
the first form of existence, which, he
said, allowed the universe to spread
out in all directions, in a description
reminiscent of the Big Bang theory.
See also: The geocentric model
20 ■ Consolidating knowledge 24–25
JOHANNES HEVELIUS
1611–1687
ELISABETHA HEVELIUS
1647–1693
From an observatory he built on
top of his house, Polish astronomer
Johannes Hevelius made detailed
maps of the surface of the moon.
Although he made and used
telescopes, he preferred to map
star positions with just a sextant
and the naked eye, making him
the last major astronomer to do so.
Hevelius’s second wife, Elisabetha,
whom he married in 1663, helped
him to compile a catalog of
more than 1,500 stars, which she
completed and published following
his death. A tireless and skilled
observer in her own right,
Elisabetha was one of the first
notable female astronomers.
See also: The Tychonic model
44–47CHRISTIAAN HUYGENS
1629–1695Dutch mathematician and
astronomer Christiaan Huygens
was fascinated by Saturn and the
strange “handles” that telescopes
revealed to protrude from either side
of it. With his brother Constantijn,
he constructed a powerful telescope
with improved lenses through which
to study the planet. Huygens was
the first to describe the true shape
of Saturn’s rings, explaining that
they were thin and flat, and tilted
at an angle of 20 degrees to the
plane of the planet’s orbit. He
published his findings in 1659 in
the book Systema Saturnium. Four
years earlier, he had discovered
Titan, Saturn’s largest moon.
See also: Observing Saturn’s
rings 65OLE RØMER
1644–1710Working at the Paris Observatory,
Danish astronomer Ole Rømer
demonstrated that light has a finite
speed. Rømer was working on aproject to calculate the time of day
using the eclipses of the moons
of Jupiter, a method first proposed
by Galileo to solve the problem of
measuring longitude at sea. Over
a number of years, Rømer carefully
timed the eclipses of the moon Io
and found that their duration varied
depending on whether Earth was
moving toward Jupiter or away from
it. He reasoned that this variation
was due to a difference in the time it
took the light from Io to reach Earth,
and estimated that light takes
22 minutes to travel a distance equal
to the diameter of Earth’s orbit of
the sun. This gave the speed of light
as 140,000 miles/s (220,000 km/s),
about 75 percent of its true value.
Rømer’s finding that light has a
finite speed was confirmed in 1726,
when James Bradley explained the
phenomenon of stellar aberration
in terms of light speed.
See also: Stellar aberration 78JOHN MICHELL
1724–1793English clergyman John Michell
studied a wide range of scientific
fields, including seismology,
magnetism, and gravity. He designed
the torsion balance, which his
friend Henry Cavendish later used
to measure the strength of gravity.
Michell was also the first person to
propose that an object might be so
massive that light would be unable
to escape its gravitational pull. He
calculated that a star 500 times the
size of the sun would be such an
object, which he called a “dark star.”
Michell’s idea was largely forgotten
until the 20th century, when
astronomers started to take the
concept of black holes seriously.
See also: Curves in spacetime
15 4 –55 ■ Hawking radiation 255