The modern age in astronomy was initiated
by the Polish astronomer Nicolaus Copernicus (1473-1543) whose precedent-shattering
book De Revolutionibus Orbium Coelestium ("On the Revolutions of
the Celestial Spheres") was published in 1543. According to the Copernican
hypothesis, the center of the universe was occupied by the sun; round it
revolved the planets and among them the earth, which also rotated daily
on its axis (see Fig. 1).
At the time of its appearance there were no physical grounds for preferring
this hypothesis to the traditional geocentric scheme; but it simplified
the construction of planetary tables and thus commended itself to practical
astronomers. Copernicus' theory was looked upon with great disfavor by
the Church, which preferred to follow a geocentric philosophy.

The principal complication in a planet's
motion was the apparent loops in the planet's path. Copernicus showed that
these could be explained if the earth was considered to revolve annually
about the sun. He was thus able to eliminate the large epicycle which Ptolemy
and other astronomers had been compelled to introduce into each planet's
scheme of motion. However, in representing the inequalities now known to
arise because a planet's orbit is elliptical, Copernicus still employed
the concepts of the epicycle and deferent used by the ancients. From observation
he was able to determine the relative sizes of the planetary orbits with
reasonable accuracy. He also explained the slow precessional motion of
the equinoctial points round the ecliptic by supposing that the earth's
axis of rotation described a cone in space in a period of about 26,000
years; this explanation of the precession opened the way for Newton's dynamical
interpretation of the phenomenon.

Tycho Brahe

The planetary tables continued to be based
upon observations inadequate in both number and accuracy. This deficiency
was remedied by the other outstanding 16th-century astronomer Tycho Brahe
(1546-1601). Laboring for more than 20 years at his island observatory
in the Danish Sound, Tycho observed the moon and planets through all the
vicissitudes of their orbits using instruments of his own ingenious design
and construction. He discovered two characteristic inequalities in the
moon's motion: the variation and the annual equation. Variation arises
from the fact that, since the earth and the moon are generally at different
distances from the sun, they are accelerated toward it by different amounts,
the moon being alternately accelerated and retarded in its orbit. This
phenomenon exhibits an annual periodicity (the annual equation) because
the distance of the earth-moon system from the sun varies according to
the time of year.

Tycho also established that the temporary
star of 1572 (now recognized to have been a supernova) was not an atmospheric
explosion, but a cataclysm occurring deep in the heavens where the possibility
of such changes was denied by the followers of Aristotle. From his observations
of the great comet of 1577 and of later comets, Tycho found that they were
at much greater distances than the moon. These discoveries all contributed
to the overthrow of the Aristotelian cosmology. Tycho, however, rejected
the Copernican theory in favor of a system of his own in which the earth
constituted the stationary center of revolution of the sun and moon while
the other planets revolved round the sun.

Kepler's Law

Indirectly, Tycho Brahe nevertheless contributed
materially to the triumph of the Copernican theory, for at his death his
observations came into the possession of his German assistant, Johann Kepler
(1571-1630), who employed them to set the planetary motions in an entirely
new light. After many months of fruitless attempts to fit an old-time epicyclic
system to the motion of Mars, he discovered that each planet revolves in
an ellipse with the sun in one of the foci; that the radius vector joining
the sun to the planet sweeps out equal areas in equal times; and that the
squares of the various planets' periods of revolution are proportional
to the cubes of their mean distances from the sun.

Physical Explanation of Kepler's Laws

Kepler's publication of his laws (1609-1619),
and his calculation of planetary tables based upon them (1627), did much
to establish the Copernican theory. However, his efforts to formulate a
physical explanation of his planetary laws reached no satisfactory conclusion;
accepting Aristotelian mechanics, he supposed that force was required to
maintain the planets in motion, not to retain them in closed orbits.

Galileo

A revolution in mechanical principles was
called for, and this was initiated by Kepler's great contemporary Galileo
Galilei (1564-1642). Although hardly attaining, in all its generality,
to the principle of inertia (which was to be Newton's first law), Galileo
was led by his mechanical experiments to recognize that no force was needed
to maintain a planet in circulation about the sun. It was Galileo, too,
who from 1610 onward achieved the most spectacular results through application
of the newly invented telescope to astronomy. With instruments of his own
construction he discovered the mountains on the moon, the four largest
satellites of Jupiter, the phases of the planet Venus, and anomalous appendages
to Saturn. He was also one of the earliest observers to detect sunspots.

Such discoveries weighed against traditional
views of the universe in favor of the Copernican theory, which Galileo
brilliantly defended in his great Dialogue Concerning the Two Chief World
Systems . The publication of this book in 1632 precipitated a conflict
between Galileo and the Roman Inquisition, which compelled Galileo to renounce
the Copernican doctrine.

Newton

The principle of inertia seems first to have
been clearly stated by the French scientist and philosopher René
Descartes in 1644. Its application to planetary theory was grasped by the
ingenious experimenter Robert Hooke in about 1666; and it was placed first
among the laws of motion by Isaac Newton (1642-1727) in his Philosophiae
Naturalis Principia Mathematica (1687; "Mathematical Principles of
Natural Philosophy"). Newton proved that the moon's central acceleration
could be explained by reference to the familiar force of gravity, diminished
conformably to the moon's distance in accordance with the inverse square
law. The concept of gravity, generalized into a force of attraction between
material particles, accounted for the elliptic orbits of the planets and
their satellites and for the precession of the equinoxes. Newton also went
some way toward accounting for the tides, for the spheroidal shape of the
earth, and for the principal inequalities of motion of the moon. (See also
Celestial Mechanics.)

Gravitational astronomy, established thus
by Newton, was greatly advanced in the eighteenth century through the application
of mathematical techniques more effective and general than his pure geometry.
A group of continental analysts applied the calculus to the problem
of determining the motions of three mutually gravitating bodies, which
makes possible approximate solutions for the systems of the earth, moon,
and sun, and of the sun and two planets. These developments in lunar and
planetary theory were summed up by Pierre Simon de Laplace (1749-1827)
in his Traité de Mécanique Céleste ("Treatise on Celestial
Mechanics"), published in parts from 1799 to 1825.

Royal Observatory at Greenwich

In 1675, King Charles II established the Royal Observatory
at Greenwich, near London, largely in the interests of navigation. At the
same time, he created the post of astronomer royal.

Flamsteed

The first astronomer royal, John Flamsteed (1646-1719),
compiled a catalog of nearly 3,000 telescopically determined star locations,
and he improved the tables of the moon's motion. At that period it was
proposed to utilize the moon's rapidly changing position among the constellations
as an index of standard time, comparison of which with the local time would
determine the observer's longitude. This method was eventually superseded
through the invention of marine chronometers, which were unaffected by
the motion of a ship. The marine chronometers constructed by John Harrison
in the mid-eighteenth century were the most celebrated. (See also Time.)

Halley

Flamsteed was succeeded as astronomer royal by Edmond
Halley (1656-1742), best known for his association with the historic comet
that bears his name. Newton had shown that comets, like planets, describe
orbits under their gravitation toward the sun. Halley suspected that the
comets seen in 1531, 1607, and 1682 were in fact the same comet, traveling
in an elongated ellipse and making a return to our skies every seventy-six
years or so. He predicted that the comet would return about the end of
1758. Its appearance on Christmas Day of that year was a triumphant vindication
of Halley's prediction and a proof that comets, like the planets, move
in obedience to definite laws. This greatly helped to bring about general
acceptance of Newton's hypothesis of universal gravitation, and at the
same time was a deathblow to the superstitious belief that the appearance
of a comet was the herald of some disaster. Halley also announced in 1718
that, with the lapse of time, certain bright stars had changed their relative
positions in the sky; all the stars were later found to be subject to such
"proper motion." (See also Comet.)

Bradley

Halley's successor at Greenwich was James Bradley (c.
1693-1762); he traced certain periodic fluctuations in the apparent positions
of the stars to the optical phenomenon of aberration (1728) and to a nutation
which the moon's attraction imposes upon the mean precessional motion of
the earth's axis. (See also Earth.)

Bradley's Greenwich observations long remained unsurpassed
in accuracy. Classified by Friedrich Wilhem Bessel (1784-1846) in 1818,
and later by Arthur Auwers, they served for the accurate determination
of the proper motions of stars. Bessel's own observations provided the
basis for the Bonner Durchmusterung (1859-1862; "Astronomical Observations"),
a catalog of the positions and magnitudes of some 320,000 stars.