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Physical Science Notes

Origin of Modern Astronomy

The History of Astronomy

Early civilization kept records of the location and motions of celestial objects.

Greeks believed in a geocentric view of the Universe. Earth was at th ecenter. All of the planets orbited the Earth. All the stars were attached to a transparent celestial sphere that rotated. They based their ideas on what they saw and felt. They didn't feel the Earth move.

The Greeks named the planets the wanderers because they changed positions relative to the stars. They included the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn.

Ptolemy (A.D. 141) developed the geocentric model and was able to account for retrograde movement of the planets by epicycles. His ideas were believed for 13 centuries.

Nicolaus Copernicus (1473 - 1543) deduced that Earth was a planet and placed the sun at the center of the universe. Copernicus also used perfectly circular orbits and epicycles to explain the motions of the planets. His ideas were published as he lay dying so he never had to personally defend them.

Tycho Brahe (1546 - 1601) contributed to astronomy by making careful measurements of th elocation of celestial bodies for twenty (20) years. He rejected the heliocentric (sun centered) model of the universe because he was unable to observe stellar parallax. He just didn't have a telescope!

Johannes Kepler (1571 - 1630) discovered three laws of planetary motion:

  1. The path of each planet around the sun is an ellipse with the sun at one focus.
  2. Each planet revolves so that an imaginary line connecting it to the sun sweeps over equal areas in equal time intervals.
  3. The orbital periods of the planet and their distances to the sun are proportional.

Galileo Galilei (1564 - 1642) Constructed a telescope based on an invention of a Dutch lens maker and turned it to the skies. He discovered:

  1. four moons of Jupiter; Earth was no longer the only center of motion in the universe.
  2. that the planets are circular disks
  3. that Venus has phases and must revolve around the sun
  4. the moon has mountains, craters and plains
  5. the sun has sunspots and it rotates

Galileo was sentenced to house arrest for the last ten years of his life for his support of the heliocentric model of the universe. He was condemned by the church and exonerated in 1992

Sir Isaac Newton (1643 - 1727) - developed laws of motion that for the first time provided a mechanism for planetary motion.

Newton developed the law of universal gravitation: Every body in the universe attracts every other body with a force that is diretly proportional to their masses and inversely proportional to the square of the distance between them.

Constellations

From con meaning with and stella meaning star

Named in honor of mythological characters or great heroes such as Orion.

88 constellations are recognized. The brightest stars are given proper names like Sirius, Arcturus and Betelgeuse. Other stars are designated by greek letters like alpha α and beta β

The constellations divide up the sky into areas for astronomers.

Motions of the Earth and Phases of the Moon

Motions of the Earth include: rotation, revolution and precession.

Synodic Month - 29.5 days.

Sidereal Month - 27.33 days.

Draw a diagram showing a lunar eclipse and a solar eclipse

Define: