Copernicus: Beginning of a Revolution

• The problems & complexities of the geocentric model caused Nicholas Copernicus (1473-1543) to examine an alternative sun-centered or heliocentric model of the solar system.
• His model proposed that:

  The Sun is at the center of the Universe.

  The distance between the Earth and the stars is much greater than the Earth-Sun distance. This would explain the lack of observed stellar parallax.

  The east to west daily motions of stars, planets, the Moon, and the Sun are caused by the rotation of the Earth on its axis.

  The Earth and all the planets revolve around the Sun on circular orbits. This produces the change in constellations observed from one time of year to the next.

  Retrograde motion is an illusion caused by the fact that we are observing other planets on the planet Earth which itself is moving around the Sun.

• Although Copernicus' idea of a heliocentric solar system is correct, his assumption of circular orbits made his model no more accurate than that of Ptolemy. But, it had the beauty of simplicity.

 

 

Implications of Heliocentric Model

• The Heliocentric Model offers additional explanations for some of the basic observations of the sky and the Earth environment made for thousands of years.
• Seasons:
  • Seasons are produced by the fact that the Earth's rotation axis is inclined by 23.5^o with respect to the line drawn perpendicular to the Earth's orbital plane. Seasons are NOT produced by the varying distance to the Sun as the Earth makes its elliptical orbit about the Sun.
  • The sunlight intensity (or amount of heating) depends on the angle that the Sun's light rays make with respect to the ground. When the Sun is highest in the sky at noon during the summer in Las Cruces, the Sun's rays are nearly perpendicular to the ground - this produces the greatest heating of the year. This is caused by the fact that the northern hemisphere is pointing most directly at the Sun. The opposite occurs during the winter.
  • What other terrestrial planet has seasons?
• Eclipses
  • Eclipses occur when one astronomical body moves into the shadow cast by a second astronomical body.
  • A Lunar Eclipse occurs at Full Moon when the Moon moves through the shadow of the Earth.
  • A Solar Eclipse occurs at New Moon when the Moon's shadow falls upon the Earth.
  • Question: Why isn't there a total solar eclipse at each New Moon?