Nicolaus Copernicus (1473-1543, Polish) began a new era of astronomy when he concluded
that the Sun was the center of the universe instead of the Earth.
Galileo Galilei (1564-1642, Italian) is the father of observational astronomy. In 1609, he heard
about the Dutch invention of the telescope, and built one for himself. Even though his telescope
was not very powerful compared to the amateur equipment available today, he was able to
make a number of stunning discoveries which changed the face of astronomy. He saw the
craters, mountains, and valleys of the Moon, noticed the huge number of stars making up the
Milky Way, kept precise records of sunspot activity and the phases of Venus, and discovered
four moons orbiting Jupiter. These moons are still called the Galilean Moons today, in honor of
the earth-shattering scientific effects of the discovery. During a time when the Earth was still
considered to be at the center of the universe, he publicized the fact that other astronomical
bodies, such as Jupiter"s moons, were clearly revolving around something other than the Earth.
Galileo"s support of the Copernican model of the universe frightened the Church, which put
Galileo on trial in 1633. He was forced to renounce his Copernican views and was held under
house arrest for the rest of his life.
Johannes Kepler (1571-1630, German) was Tycho Brahe"s assistant and student. He inherited
his teacher"s extensive collection of astronomical records, and used them to develop three laws
of planetary motion. He believed in the Copernican model of the universe, although he found it
difficult to fit Tycho"s observations of Mars into the model with a circular orbit. He therefore used
the idea of elliptical orbits to describe the motions of the planets, which became known as
Kepler"s first law. His second law states that a line from the Sun to a planet sweeps out equal
areas in equal amounts of time. The third law was a masterpiece of simplicity: the square of the
number of years of a planet"s orbital period is equal to the cube of that planet"s average distance
from the Sun.
Isaac Newton (1643-1727, British) was a mathematician who developed extensive
mathematics to describe the astronomical models of Copernicus and Kepler. His Theory of
Universal Gravitation was the foundation of Kepler"s laws of planetary motion, but it also went
further: Newton showed that the laws governing astronomical bodies were the same laws
governing motion on the surface of the Earth. Newton"s scientific ideas are so complete that
they still offer an accurate description of physics today, except for certain cases in which 20th
century physics must be used.
Edmond Halley (1656-1742, British) became famous for predicting the 1682 appearance of a
comet called Halley"s Comet. He proved that the orbit of comets is periodic.
William Herschel (1738-1822, British) was discovered Uranus and two of its moons. He also
discovered two more moons of Saturn and several asteroids, and made a catalog of 2,500
astronomical objects. He found the polar ice caps on Mars, which are today being studied by
several satellites in the hopes of shedding light on the existence of water on Mars.
Joseph von Fraunhofer (1787-1826, German) discovered dark lines in the spectrum coming
from the Sun. He carefully measured the positions of over 300 of these lines, creating a
wavelength standard that is still in use today.