Introduction to Astronomy

• The study of celestial objects and phenomena in space.

  Astronomy
• During this time, the early Greeks describe observable celestial bodies and developed?

  Geometry and trigonometry
• Early astronomy can be traced back to Ancient Greece during the?  

  “Golden Age” of astronomy (600 B.C. – 150 A.D.)
• The early Greeks believed that the Earth was the center of the universe and that all the other heavenly bodies orbited around it.

  Geocentric model
• Aristarchus (312-230 BC) offered the possibility of a heliocentric model (from the Greek word helios meaning "Sun") of the universe, which suggested that the Earth and other planets revolved around the Sun, but the idea would not gain traction until the 15th century.
• Notable contributors to early astronomy:

  • Aristarchus (312-230 BC)
  • Parmenides (5th century BC)
  • Aristotle (384-322 BC)
  • Eratosthenes (276 – 194 BC)
  • Hipparchus (190 – 120 BC)
  • Claudius Ptolemy (100 – 170 AD) 
• Greek philosopher who believed in a spherical Earth, but this belief was purely based on philosophical assumptions.

  Parmenides
• Gave scientific credibility to the idea of a spherical Earth by observing the shape of the Earth's shadow that is cast upon the moon during eclipses.

  Aristotle
• Credited for successfully establishing the circumference of the Earth by observing the angles of the Sun's rays at noon in two Egyptian cities.

  Eratosthenes
• Eratosthenes calculated that the circumference of the Earth is around 39,400 km, close to the actual modern-day value of 40,075 km
• One of the most significant early Greek astronomers (190 - 120 BC).

  Hipparchus
• Hipparchuscontributions to the field of astronomy:


  • Developed trigonometry
  • Accurately estimated the distance between the Moon and Earth
  • Near accurate estimation of the length of a year
  • Created a star catalog of nearly 850 stars classified according to their brightness (adapted into the Hertzsprung-Russell diagram)
• Developed the Ptolemaic system, a geocentric model of the universe (100 - 170 AD).

  Claudius Ptolemy
• Despite using an incorrect model, Ptolemy could still predict the planets' positions.
• Even with the decline of the Roman Empire, the Ptolemaic system became the prevalent model of the universe for several years
• Accounted for the apparent motion of the planets as they revolve around a stationary Earth. Used a combination of large circles (deferents) and small circles (epicycles) to represent the planets' orbits.


  Ptolemaic system
• The center of astronomical study would shift to Baghdad, Iraq, after the fall of the Roman Empire during the 4th century.


  The Emergence of Modern Astronomy
• Polish astronomer who advocated for a heliocentric universe model, later called the Copernican system, after discovering Aristarchus' works. Even though his model was more correct than Ptolemy's, Copernicus could not account for planetary motion. His idea that the Earth was not the center of the universe was considered heretical then and, thus, met with much criticism.


  Nicolaus Copernicus
• German astronomer who served as an assistant to Tycho Brahe, a Danish astronomer. With an observatory at their disposal, Brahe and Kepler observed and measured the locations of different celestial bodies. Tycho did this to refute the Copernican system. Despite serving under Brahe, Kepler remained steadfast in his belief in the Copernican model. After Tycho's death, Kepler used the data they gathered to formulate the fundamental laws of planetary motion.


  Johannes Kepler
• Tycho Brahe was a Danish astronomer.
• Kepler formulated the fundamental laws of planetary motion after Tycho's death.
• All the planets move around the Sun in an elliptical orbit, not circular as previously believed.

  Law of Ellipses
• 1. If you trace an imaginary line from a point in the orbit to the Sun as a planet revolves, the line sweeps over equal areas in equal time intervals.

  2. Explains the variation in the speed at which planets orbit around the Sun.

  Law of Equal Areas
• Point where the planet's orbit is closest to the Sun.

  Perihelion
• Point where the planet's orbit is farthest from the Sun.

  Aphelion
• The square of a planet's orbital period is proportional to the cube of a planet's mean distance to the sun.

  Law of Harmonies
• Kepler could not establish why planets moved the way they did and what keeps planets in their orbits instead of floating away into space
• Galileo Galilei


  • Italian astronomer, contemporary of Kepler, supporter of the Copernican model
  • Built several telescopes, aided in making more detailed observations of heavenly bodies
  • Observed the surface of the Moon, which was previously believed to have been smooth as glass but has now proven to contain craters, mountains, and plains like the Earth
  • Discovered that Jupiter had four moons that revolved around it, further dispelling the notion that the Earth was the center of motion in the universe
  • Discovered the existence of sunspots, relatively darker and cooler areas on the surface of the Sun
  • Prolonged observation of the Sun damaged his eyesight and eventually completely blinded him
• Sir Isaac Newton


  • Prominent English scientist who significantly contributed to math, physics, and astronomy
  • Answered the question of what kept planets in orbit: the Law of Universal Gravitation
  • According to the law, everybody in the universe attracts every other body with a force proportional to the mass of the bodies and inversely proportional to the distance between the bodies
• The Three Basic Laws of Planetary Motion:

  • Law of Ellipses
  • Law of Equal Areas
  • Law of Harmonies
• The larger the mass of an object, the bigger the gravitational force it exerts. This would explain how the Moon affects the tides on Earth. The force of gravity and the tendency of a planet to move in a straight line contribute to the orbit of a planet around the Sun.


  Without the pull of gravity, the planets would move forward in a straight line out into space. Without the tendency of the planet to move in a straight line, the planets would fall into the Sun because of its immense gravitational pull.

  A) m1

  B) m2

  C) r2

  D) G

  E) F