Astronomy is perhaps the oldest science. The ancients saw cosmic meanings in the stars, and they organized their lives around lunar and solar cycles (i.e. the month and year). They also observed the solstices, the equinoxes, and of course the four seasons. Over many centuries the “precession of the equinoxes” corrupted Julius Caesar’s ancient calendar (the Julian calendar); in 1586 it was replaced with the Gregorian calendar, which features the system of leap years we know today. Aristotle’s earth-centered (“geocentric”) system of crystalline spheres dominated astronomy for 2000 years. Aristarchus in 270 B.C. was ahead of his time in suggesting that the sun is at the center of the universe, and that the earth spins like a top. Eratosthenes (ca. 200 B.C.) calculated the size of the earth; Hipparchus (2nd century B.C.) calculated the distance to the moon and established a system of latitudes and longitudes. Ptolemy (1st century A.D.) published the great compilation of astronomical knowledge (the Almagest), and he offered the epicycle theory to explain new observations not explained by the geocentric theory of the universe. In 1543, the Polish astronomer Nicolaus Copernicus overturned the geocentric theory by publishing a book on his heliocentric (sun-centered) theory. Johannes Kepler soon joined the great astronomer Tycho Brahe, systematizing Tycho’s observations with calculations proving that orbits are elliptical. Kepler also established his three great laws of celestial motion. Galileo improved the telescope and discovered many new astronomical features; his work publicly discredited the geocentric doctrine, leading to the famous recantation forced upon him by the church. Galileo’s celestial and terrestrial discoveries laid the foundation for the great advances and discoveries of Isaac Newton.
For most of history, the beginning of the universe has been understood through the many myths offered in various cultures. But in the modern age, scientific cosmology has emerged to offer new explanations for the beginning and evolution of the universe. By 1900, religious and scientific conceptions of creation were widely seen as incompatible. In the 15th century, Nicholas of Cusa anticipated modern relativistic physics by suggesting that the universe has no center, no circumference, and no beginning or end. In the 20th century, Edwin Hubble used statistical analysis to prove that the universe is infinite. Modern cosmology suggests that there are 200 billion billion stars in the universe, including a variety of structures such as the nova, supernova, nebula, quasar, white dwarf, neutron star, pulsar, and black hole. The behavior of stars is governed by the physics of nuclear combustion and gravitation; our theories about stars depend upon advances in particle physics to explain the nuclear reactions that appear to explain star behavior. Edwin Hubble also discovered that the universe is expanding, which tended to confirm the conception that the universe began with a "big bang". Various theories have suggested that the universe either is in a steady state, that it is inflating, that it may be oscillating, or perhaps even winding down.
