History of Astronomy

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Chapter 2 : A Brief History of Astronomy

A Brief History of Astronomy arrow_upward

  • In the past, humans observed following (without telescope):
    • The Sun: It rises and sets.
    • The Moon: It has a monthly cycle of phases.
    • The Fixed Stars: The patterns stay fixed and the appearance of different constellations marks the different seasons.

    The Planets arrow_upward

  • In our solar system, the planets circle around our Sun.
  • The planets travel in the same direction (counter- clockwise looking down from the Sun's North Pole).
    • Mercury
    • Venus
    • Earth
    • Mars
    • Jupiter
    • Saturn
    • Uranus
    • Neptune

    Astronomy Models arrow_upward

  • A model can explain existing observations and be able to predict the outcome of future experiments.

  • Aristotle Model (385-322 B.C) arrow_upward

  • Aristotle (an influential Greek philosopher) favored a geocentric model for the Universe:
    • The Earth is at the center of the Universe.
    • The perfect shape of Earth is a sphere, and the natural motion is rotation.

    Aristotle’s Geocentric Model

  • The motion of the Sun around the Earth accounts for the rising and setting of the Sun.
  • The motion of the Moon around the Earth accounts for the rising and setting of the Moon.
  • The fixed stars were on the “Celestial Sphere” whose rotation causes the rising and setting of the stars.

  • Ptolemy’s model arrow_upward

  • Although Aristotle’s ideas were commonly accepted, there was a need for a more accurate way to predict planetary motions.
  • Ptolemy known as a mathematician, astronomer, geographer and astrologer presented a detailed model of the Universe.
    • It explained retrograde motion by using placement of circles.

    Ptolemy’s Epicycles based Model

  • By adding epicycles, complicated motion could be explained.

  • The Copernican Revolution arrow_upward

    The Sun-Centered Model

  • Nicolas Copernicus proposed a heliocentric model of the Universe.
  • The Sun was at the center, and the planets moved around it in perfect circles.
  • The Sun was the center of the Universe.
  • Each planet moved in a circle, and the speed of the planet’s motion decreased with increasing distance from the Sun.
  • Retrograde motion of the planets could be explained as a projection effect.
  • The philosophical shift was important i.e. the Earth is not at the center of the Universe.

  • Kepler’s Model (1571-1630) arrow_upward

  • Kepler was a mathematician.
  • He believed in the Copernican view:
    • The Sun at the center.
    • The motions of the planets on perfect circles.

    Kepler’s Laws of Planetary Motion arrow_upward

    Kepler’s First Law

    “The orbit of every planet is an ellipse with the Sun at one of the two foci.”

  • Planets move around the sun in elliptical paths with the sun at one focus of the ellipse.
  • An ellipse has two foci, F1 and F2
  • For any point P on the ellipse, F1 P + F2 P is a constant.
  • The orbits of the planets are nearly circular (F1 and F2 are close together), but not perfect circles.
  • A circle is an ellipse with both foci at the same point-the center.

  • Kepler’s Second Law

    "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time."

  • While orbiting, a planet sweeps out equal areas in equal times.
  • The green shaded sector has the same area as the red shaded sector.
  • Thus, a planet moves from C to D in the same amount of time as it moves from A to B.
  • This means a planet must move faster when it is closer to the sun.
  • For planets this affect is small, but for comets it is quite noticeable, since a comet’s orbit has much greater eccentricity.

  • Kepler’s Third Law

    "The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit."

    T2 R3

  • Assuming that a planet’s orbit is circular, then the mean distance from the sun is a constant-the radius (R).
  • F is the force of gravity on the planet.
  • F is also the centripetal force.
  • If the orbit is circular, the planet’s speed is constant, and v=2pR/T
  • Cancel m and simplify:
  • Rearrange:
  • Since G, M, and 4π2 are constants,

    ∴ T2 R3


    • T is the period.
    • G is the Gravitational constant.
    • M is the mass of the larger body.
    • R is the distance between the centers of mass of the two bodies.

    Galileo Galilei (1564-1642) arrow_upward

  • Galileo was the first to use a telescope to study astronomical objects.
  • His observations of the moons of Jupiter and the phases of Venus provided strong support for the heliocentric model.

  • Isaac Newton arrow_upward

  • Isaac Newton gave the law of gravity and three general laws of motion that apply to all objects:
    • Newton’s first law of motion states that an object remains at rest or continues in a state of uniform motion if no external force acts upon the object.
    • Newton’s second law of motion states that if a net force acts on an object, it will cause an acceleration of that object.

    Center of the Universe arrow_upward

  • Earlier astronomers incorrectly identified Earth and Sun as the center of the universe.
  • There is no center of the universe because there is no edge of the universe.
    • In a finite universe, space is curved so that if you could travel billions of light years in a straight line you would eventually finish back where you started.
    • It is also possible that our universe is infinite.
  • In both examples, groups of galaxies completely fill the universe and are moving apart at all points making the universe expand.

  • Thank You from Kimavi arrow_upward

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