September 25, 2016
Astronomy 122
Prof. J. Brau

Outline

• Our Place in Space
• Scientific Theory and the Scientific Method
• The "Obvious" View
• The Measurement of Distances
• Ancient Astronomy
• The Heliocentric Model of the Solar System
• Galileo, Kepler, and Newton
• Newtonian Mechanics
• Summary

Astronomy Today:

• Chapter 1: sections 1.1, 1.2, 1.3, 1.6
  
• Chapter 2: sections 2.1, 2.3, 2.7, 2.8

Our Place in Space

human dimension = 1.5 meters = 0.0015 kilometers size of Earth = 15,000 kilometers size of Sun = 1,500,000 kilometers size of galaxy = 1000 quadrillion kilometers = 1,000,000,000,000,000,000 kilometers = 100,000 light-years size of galaxy cluster = 1,000,000 light-years

Scientific Theory and the Scientific Method

Scientific theory results from observation leading to predictions, which can also be tested by observation must be testable must be simple should be elegant - tying together several phenomena

The "Obvious" View

When we see the points of light in the sky (stars) patterns are imagined and it is tempting to connect points Example, Orion the Hunter People long ago connected the brightest stars into configurations called constellations The ancients named the constellations after mythological beings, heroes and animals. Astrology grew out of mythical notions of the meaning of the constellations. Today, most people recognize astrology is an amusing diversion The stars that make up a constellation are not actually close to one another in space

The Measurement of Distances

Surveyors can use triangulation to measure distances on Earth. A similar method, parallax, is one method used to measure distances in space. Several other methods are used to build a cosmic distance scale

Ancient Astronomy

• Ancients studied changing nighttime sky for practical reasons

  navigation, tracking seasons for farming. Examples of ancient observatories: Stonehenge in England The Big Horn Medicine Wheel in Wyoming spokes that align with risings and settings of Sun. The Caracol temple in Mexico windows that align with astronomical events. Chaco Canyon in New Mexico sliver of light passes precisely through the center of the carved stone spiral at noon on the summer solstice, the time of the longest day of the year, June 21.

The Heliocentric Model of the Solar System

Greeks of antiquity and likely those before considered the Earth to be the center of the universe. The stars appeared fixed in the sky, scattered throughout space. The active elements of the universe were the planets of the solar system. This was the nature of the cosmological teachings of Aristotle (384-322 B.C.). Most complete explanation of the motion of the planets (including their retrograde motion) was that of Ptolemy (140 A.D.). Nicolaus Copernicus (1473-1543) revolutionized thinking about universe with heliocentric (Sun-centered) model in the 16th century In fact, Aristarchus of Samos (310-230 B.C.) had proposed that planets revolve around the Sun, but this view did not catch on then.

Nicolaus Copernicus (1473-1543)

Galileo, Kepler and Newton

Galileo Galilei (1564-1642) First observer to use a telescope to view the sky. Discovered terrain on the Moon. Discovered sunspots and observed rotation of the Sun. Discovered four moons of Jupiter. Observed orbit of Venus around Sun, evident from phases and varied apparent size. Finally, Galileo concluded that the Copernican theory was supported by his observations and published a book with his conclusions. In 1600, the astronomer Giordano Bruno had been burned at the stake in Rome for such heretical teachings. Galileo was banned by the Roman Catholic Church from continuing his work, but he did continue until 1633 when he was threatened with severe punishment and put under house arrest. Galileo was imprisoned for the last decade of his life for his belief in the Copernican theory. In 1992, nearly four hundred years later, he was forgiven by the Church		Galileo Galilei (1564-1642)
Johannes Kepler (1571-1630) Determined orbits of planets using measurements made from different times of the year, meaning the Earth was at a different point in its orbit around the Sun (triangulation) Kepler's laws of planetary motion First law: the orbital paths of planets are elliptical (not circular), with the Sun at one focus. Second law: An imaginary line connecting the Sun to any planets sweeps out equal areas of the ellipse in equal intervals of time. Third law:The square of a planet's orbital period is proportional to the cube of its semimajor axis.		Johannes Kepler (1571-1630)
Issac Newton (1642-1727) While Kepler's laws were powerful in describing the orbits of the planets, they were not understood from an underlying theory. Why does the Heliocentric model of the universe work? Why do the planets orbit the Sun? What prevents the planets from flying off into space? What prevents the planets from falling into the Sun? Why do the planets revolve around the Sun endlessly? Issac Newton developed the understanding of the way all objects move and interact. Newtonian mechanics - three basic laws of motion Newton's first law of motion: Every body continues in a state of rest or in a state of uniform motion in a straight line, unless it is compelled to change that state by a force acting on it. Newton's second law of motion:When a force F acts on a body of mass m, it produces in it an acceleration a equal to the force divided by the mass. Thus, a=F/m, or F=ma. Newton's third law of motion:To every action, there is an equal and opposite reaction. Newton's laws provide an excellent description of the motion of planets, stars, and galaxies When speeds approach the speed of light (186,000 miles/second or about 670 million miles per hour) these laws break down and Albert Einstein's theories of relativity take over.		Issac Newton (1642-1727)




• Gravity
  • Newton also developed a theory of gravity
    • Any object having mass exerts an attractive gravitational force on all other massive objects
    • The force of gravity obeys an inverse-square law
    • Newton's Law of Universal Gravitation: Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between their centers.
      
      
    • or     ,
      where G is the gravitational constant (6.67 x 10^-11 Newton/meter²/kilogram²)

Newtonian Mechanics

Solar Gravity The Sun's inward pull of gravity on a planet competes with the planet's tendency to continue moving in a straight line. This results in a stable orbit. Orbits The resulting orbits for two bodies are two ellipses with a common focus, and depend on the relative masses of two bodies. Escape speed With enough speed, a lighter body can escape the pull of the heavier one. Example of satellite

Summary

• Applications of the scientific theory and the scientific method have led to an advanced understanding of the cosmos.
  • Theory results from observations and leads to testable predictions.
• The "obvious" view of the sky can be incorrectly interpreted.
  • For example, the constellations are not as significant as they appear.
• Ancient astronomy was developed for practical reasons, such as navigation and farming.
• The Heliocentric model of the Solar System (advanced by Copernicus) overcame earlier, naive ideas with the Earth at the center.
• Galileo, Kepler, and Newton advanced our understanding.
  • Galileo - observational evidence for heliocentric model.
  • Kepler - 3 laws of planetary orbits.
  • Newton - 3 laws of motion and law of gravity.
• Newtonian Mechanics
  • Applying Newton's three laws of motion and the law of gravitation provides a solid theoretical foundation.