Chapter 3

The Science of Astronomy

Everyday Science

• Scientific Thinking is a fundamental part of human nature.

• Scientists apply the scientific method to their inquiries about the universe.

Ancient Observations

• Central Africa: c. 6500 B.C. People use the observations of the moon to predict the weather.

Graph depicts the annual rainfall pattern in central Nigeria, characterized by a wet season and a dry season.

Modern measures of time still reflect their ancient astronomical roots.

• 24 hour day – the time it takes the Sun to circle our sky.

• Month – comes from the lunar cycle.

• Calendar Year – Based on the cycle of the seasons.

• Days of the week – named after the seven “naked-eye” objects that appear to move among the constellations. (Sun, Moon and five planets)

• At night, the position and phase of the Moon give an indication of the time.

The Seven Days of the Week and the Astronomical Objects They Honor

Determining the time of day

• In the daytime, ancient peoples could tell the time of day by observing the Sun’s path through the sky.

The ancient Egyptians used huge obelisks as

simple clocks/ (Sundials).

Ancient Egyptian obelisk which now resides in St. Peter’s Square, Vatican in Rome.

Determining the Time of Year

• Many cultures built structures to help them mark the seasons.

• A good example of this is Stonehenge.

Stonehenge Reconstruction c. 1550 B.C.

The Sun Dagger– Chaco Canyon, New Mexico

The dagger pierces the center of the carved spiral each year at noon on the summer solstice

Interactive Astronomy Pages

Blue Moon:

The second full moon to occur in a particular calendar month.

See link below for more info about Blue Moons.

Lunar Cycles• Many ancient cultures used the lunar cycle as the

basis of lunar calendars.

• Metonic cycle: – The ancient Greek astronomer Meton (432 B.C.)

observed that the dates of the lunar cycle repeat every 19 years.

• Other cultures based their calendars on their ability to predict eclipses:

• Babylonian Calendar

• Mayan Calendar

Observation of Planets and Stars• Many cultures made careful observations of the

planets and the stars.

• Mayan observatories of Central America had windows placed to allow for observations of Venus.

In the desert regions of Peru, many large figures of animals may have represented constellations to the Incas who lived there.

The Big Horn Medicine Wheel in Wyoming may have been related to the month for the Native Americans.

From Observation to Science• Ancient Chinese

Astronomers kept very detailed records of astronomical observations beginning 5,000 years ago.

• They were the first to record an observation of a Supernova explosion, which we see today as the Crab Nebula.

• In the Americas, the Mayans had developed a modern system of numbers and mathematics, which included the invention of the concept of zero.

• It appears that virtually all cultures employed scientific thinking to varying degrees.

The Modern Lineage• 3000 B.C. The establishment of civilization

occurred in Egypt and Mesopotamia.

• 2700-2100 B.C. Egyptians build the Great Pyramids.

• 500 B.C. Greece rises as a military power.

• 330 B.C. Alexander the Great expands the Greek empire throughout the middle east.

• As a student of Aristotle, Alexander had a great interest in science and education.

Map of the Middle East in Ancient Times

• 300 B.C. The Library of Alexandria is established as the leading center of knowledge, housing more than half a million books.

• ~ 415 A.D. The destruction of the library of Alexandria, along with the loss of most of the knowledge stored there.

The Great Hall in the Library of Alexandria (Rendering)

• One of the most important scientific contributions from ancient Greece was the idea of creating models to represent natural phenomena.

• Models are still used today to help us in understanding natural phenomena.

• Claudius Ptolemy (100-170)A.D. developed an Earth centered (geocentric) model of the universe that included the motions of the Sun, moon, and planets.

Ptolemaic Model of the Universe • All heavenly motions proceed in perfect circles.

• To explain retrograde motion, Ptolemy maintained that each planet moved along a small circle that, in turn, moved around a larger circle.

• This allowed for the observed westward (retrograde) motion that some planets exhibited.

• The large circular orbit around the Earth was called the “Deferent”

• The smaller circular motion along a “deferent” was called an epicycle.

Ptolemy’s model of the Solar System/Universe

Epicycle

Deferent

The Copernican Revolution

• 1543 Copernicus publishes De Revolutionibus Orbium Caelestium, “Concerning the Revolutions of the heavenly Spheres”.

• Copernicus modifies the Ptolemaic model as noticeable discrepancies become more apparent with the improvements in observational astronomy.

Nicholas Copernicus (1473-1543)

The Copernican Model of the Solar System

• The Sun and not the Earth, is at the center of the solar system.

• The earth does move, and like the other planets, orbits the sun.

• The orbital paths follow perfect circles.

Tycho Brahe (1546-1601)

• Made detailed naked eye observations of the motions of the planets.

• Coined the term NOVA for the observation of a supernova (Nova means “New Star”)

• Showed that comets were distant objects rather than being phenomena of the Earth’s atmosphere, as argued by Aristotle.

• In the course of 30 years, he had amassed the best astronomical data of the day It was accurate to within 1 arc-minute.

Johannes Kepler (1571-1630)• A mathematician and a man of faith, who believed

that understanding the geometry of the heavens would bring him closer to God.

• Inherited Tyco Brahe’s astronomical data.

• With his mathematical skills, Kepler spent 20 years of his life trying to develop a consistent model to describe planetary motion.

• His efforts culminated in what is known today as Kepler’s Three Laws of Planetary Motion.

Kepler’s First law of planetary Motion

• The orbit of each planet about the Sun is an ellipse with the Sun at one focus.

Minor Axis

Major Axis

Kepler’s Second Law of Planetary Motion

• As a planet moves around its orbit, it sweeps out equal areas in equal times.

Kepler’s Third Law of Planetary Motion

• The square of a planet’s orbital period is proportional to the cube of its semi-major axis.

(Orbital period in years)2 = (Semi-major Axis in AU)3

P2 = a3

• The Copernican view of the solar system and Kepler’s three laws of planetary motion were very successful in explaining and predicting observations.

• However, due to the degree to which the Ptolemaic model was ingrained in the current thinking of the day, major objections were put forth with regard to the new Copernican/Keplerian model.

Main Objections to Copernican/Keplerian View

• The Earth could not be moving because if it did, objects such as birds, falling stones, and clouds would be left behind as the Earth moved along its way.

• The idea of noncircular orbits contradicted the ancient Greek belief that the heavens must be perfect and unchanging.

• Stellar parallax should be detectable if the Earth orbits the Sun. (True, but difficult to detect)

Galileo’s defuses all objections

Galileo (1564- 1642)

• Galileo performed careful experiments on the motions of moving bodies and showed that a moving object remains in motion unless a force acts to stop it or change its direction.

• Tycho’s observations of a comet and supernova showed that the heavens could change.

• The lack of noticeable stellar parallax was simply due to the fact that the stars were much farther away than anyone had previously thought.

Galileo Invents the Telescope

Galileo observes four moons clearly orbiting Jupiter.

This shows that objects do orbit planets other than the Earth.

One of Galileo’s Refracting Telescopes

Phases of Venus

• Galileo observes that Venus goes through phases much like the moon does. Only a Heliocentric model of the solar system can account for all observed phases.

Geocentric Model cannot account for all observed phases

Heliocentric model can account for all observed phases

Galileo’s Other Telescopic Discoveries

• Craters, mountains and cliffs on the Moon.

• Sunspots

End Of Section