University of California, San Diego
Physics 7 - Introduction to Astronomy

Astronomy is among the oldest of the sciences. When Stoneage humans turned to an agrarian way of life and began to settle into communities, their interest must naturally have turned to the "heavens":

1. The seasons became important; during different times of the year, different stellar patterns appear in the sky. In the spring, Virgo and her accompanying constellations signalled the time to prepare the earth, to plant crops, and to be wary of floods. In the fall, Orion rises to indicate time to harvest and to prepare for winter.
2. The approximate equivalence of the human menstrual cycle and the 30 day orbital period of the moon which produces lunar phases led to the belief that the heavens, and the moon in particular, were related to fertility.
3. To early humans facing an uncertain and changeable future, the constancy of the heavens must have suggested perfection and certainly led to deification in many cultures.
4. We may expect that eclipses would have been especially frightening to early humans. After predicting the seasons, eclipse prediction may have been one of the earliest astronomical activities.

Stonhenge constructed between 3100-2000 BCE on England's Salisbury Plain may have been a Stoneage astronomical site (observatory is too strong a word), at least in part. Certainly the alignment of the "heelstone" with the rising Sun on Midsummer's Day (June 21, the Summer Solstice) represents a true astronomical alignment, and many other Megalithic sites have similar alignments. In Stonehenge Decoded, astronomer Gerald Hawkins argued that there exist a large number of astronomical alignments, though further study suggests that many of these are fortuitous.

Cosmologist Fred Hoyle has suggested that Stonehenge may have been used to keep track of the solar-lunar eclipse cycle. Far outside the still partially standing ring of Sarsen Stones is a ring of 56 holes, known as the Aubry holes. Hoyle has suggested that movement of a marking stone by 3 positions each time the Sun rose over the heelstone (or by one position three times yearly) would complete a circle in 18.67 years -- approximately the period for the "nodes", the intercepts of the lunar and solar paths in the sky, to complete a cycle. Certainly ritual use of Stonehenge would have been more important that its astronomical functions and much of this interpretation must remain speculation. We may be certain, however, that Stonehenge was indeed constructed by Stoneage humans without the assistance of alien astronauts as suggested in some pseudo-scientific books. Visit the Complete Stonehenge

Eastern observers, notably the Chinese, kept careful track of events in the skies, particularly the appearance of "guest stars" -- comets, novae and other transients. Chinese records of the guest star that we now call Comet Halley can be traced back to 240 BCE and possibly as early as 1059 BCE. One of the most important Chinese records is of a guest star that was bright enough to be seen during the daytime for nearly a month in the constellation that we call Taurus in July 1054. We believe this to be the supernova explosion that gave rise to the Crab Nebula and our knowledge of the date of the explosion itself is a very important key in understanding the deaths of massive stars. This event was also chronicled by the Anasazi in Chaco Canyon and by Native Americans elsewhere, but is curiously absent from European records in the Middle Ages.

As the above suggests, Archaeoastronomy is an active and exciting field of research.

Western scientific history begins with the ancient Greek civilization about 600 BCE.

The Ionian region of Asia Minor appears to have been a site of particular philosophical/scientific/mathematical activity for several centuries.

We will review the progress of science by highlighting a few key natural philosophers, scientists and mathematicians. As Isaac Newton said,"If I have seen further, it is by standing on the shoulders of Giants."

Pythagoras of Samos (~580-500 BCE)

Pythagoras developments in astronomy built upon those of Anaximander from whom, apparently, came the idea of perfect circular motion. The Pythagoreans believed that the planets were attached to crystalline spheres, one for each planet, which produced the Music of the Spheres. These spheres were centered on the Earth, which was itself in motion. Pythagoras is also credited with recognizing that the "morning star" and "evening star" are both the planet Venus.

Aristotle (384-322 BCE)

Aristarchus of Samos (~310-230 BCE)

Eratosthenes of Cyrene (276-197 BCE)

Claudius Ptolemy (~85-165AD)

Ptolemy's Geography remained the principal work in that field until the time of Columbus.

Copernicus Heliocentric Solar System vs. Ptolemy's Geocentric Model
Both models employed perfect circular motion with epicycles, equants ...

Nikolas Kopernig (Copernicus, 1473-1543)

Tyge (Tycho) Brahe (1546-1601)

Galileo Galilei (1564-1642)

Galileo was the first "modern scientist". He argued that mathematics, rather than being abstract perfection, is the true language of science. He performed many revolutionary experiments in mechanics and other fields of physics. Among his accomplishments in mechanics are:

• development of the concept of inertia, later refined by Newton.
• a variety of experiments on falling bodies which demonstrated that the acceleration of gravity is independent of mass. There is no evidence that Galileo actually dropped objects from the Tower of Pisa. Rather, his experiments were conducted with an inclined plane as shown in this animation.
• the first Theory of Relativity, valid for velocities much smaller than the speed of light.

• sunspots on the Sun and craters and mountains on the moon.
• The so called "Galilean satellites" which orbit Jupiter -- Io (with the volcanos), Europa, Callisto and Ganymede. Here's more on Jupiter and her satellites from the Siderius Nuncius and an animation showing what Galileo observed.
• rings of Saturn.
• the phases of Venus.

I highly recommend a tour of the Museum of the History of Science in Florence if you can tear yourself away from the Renaissance Art; here is a tour of the Museum which has an extensive collection of Galilean exhibits including the famous middle finger. Here is their biography of Florence' greatest scientist.

Also highly recommended is a new book about Galileo and his daughter Sister Maria Celeste. Galileo's Daughter by science journalist Dava Sobel, which sheds important new light on Galileo and his beliefs, his family relationships, and the position of women in Renaissance Society.

Johannes Kepler (1571-1630)

1. The orbits of the planets are ellipses with the Sun at one focus.
2. The planets sweep out equal areas during equal times of the orbit.
3. The square of the orbital period is proportional to the cube of the planet's distance from the Sun. (If you measure the period in Earth years and the distance in Astronomical Units (A.U.),(1 A.U.= the average distance of the Earth from the Sun), then Period² = Distance³.)

Here's a page with some nice animations of Kepler's Rules, and here is another way to play with them.

Obviously Kepler's Rules require that the Sun be the center of the Solar System, in contradiction with the Aristotilean ideal. The first rule eliminates the circular motion which had been fashionable for 2 millennia. The second replaces the idea that planets move at uniform speed around their orbits,with the empirical observation that the planets move more rapidly when they are close to the Sun and more slowly when they are farther away. The third rule is a harbinger of the Law of Gravitation which would be developed by Newton in the latter part of the 17^th century.

Isaac Newton (1642-1727)

Other selected pioneers and milestones in the advance of Astronomy:

• 18th Century, William Herschel discovered Uranus, a new planet beyond Jupiter. Barely visible with the unaided eye, Herschel made the observation with his telescope .
• Early in the 19th Century Adams (English) & LeVerrier (French) independently calculated that there must be another planet beyond Uranus that was producing small gravitational disturbances in Uranus' orbit. First observed in 1846 by Hohan Galle, it was named Neptune. (It was actually spotted earlier by Challis in Cambridge, but Challis did not note his discovery until Galle reported his observation.)
• 1930 Clyde Tombaugh discovered Pluto.
• 1910 Harlow Shapley estimated the size of the Milky Way.
• W. H.Pickering and Annie J. Cannon calculated the surface temperatures of the stars.
• Einstein (1905) developed the Theory of Special Relativity, based upon the idea that light travels at the same speed in all frames of reference. Modified Newton's Theory of Gravity by developing the General Theory of Relativity (1916).
• Cecilia Payne-Gaposchkin & Henry Norris Russell determined the composition of stars.
• 1924 Edwin Hubble established that the Andromeda nebula and other "spiral nebulae" are star systems like the Milky Way at great distances.
• 1929 Hubble & Milton Humason discovered that the Universe is expanding.
• 1938 Hans Bethe determined that the Sun's energy comes from thermonuclear fusion reactions.
• 1940 Karl Jansky observed that the nucleus of the Milky Way and other celestial objects are strong sources of Radio Waves.
• 1948 George Gamov developed the Hot Big Bang Theory of the origin of the Universe.
• 1950's chemical composition of the stars; stars build the heavy elements via nuclear fusion reactions.
• 1954 Radio Galaxies
• 1960-63 Quasars
• 1960s X-ray & Infrared astronomy
• 1968 Jocelyn Bell (Burnell) & Anthony Hewish discovered Pulsars

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