From Ibstudy

Revision as of 17:07, 15 January 2008

Astronomical Observations

Stars and Constellations

• During the course of the night, the stars seem to rotate in a circle around the Polar star.
• During the course of a year, the polar star moves around the sky in a cycle, though it stays in roughly the same place.
• Which stars are visible depends on the latitude, time of year, time of night and, of course, the weather.

Planets

To the naked eye, planets look like stars, but they don't seem to move together with the other stars. Rather, they 'wander' among the other stars, showing up among the stars in a slightly different place each night. Thus the name 'planet' is derived from Greek for 'wanderer'.

• Planets generally move in a straight line across the other stars, in a cycle, however, sometimes they act wierd. This is called...

Retrograde Motion

Sometimes the planets stop moving, move backwards for a while, then continue on their way. This is called Retrograde Motion and was a key point in the development of models of the universe.

here is a useful applet that shows the retrograde motion of mars.

here is an excellent page explaining retrograde motion and its significance in the development of models of the universe.

Phases of the Moon

As the moon orbits the earth, only one side of it is lit by the sun, and only one side is visible from Earth. These aren't necessarily the same or the opposite side, so most of the time only part of the moon is visible in the sun's light.

Throughout the course of one lunar month (27.3 days), the visible half of the moon cycles from being fully lit, to completely unlit, and back again.

The

An animation of what the phases of the moon would look like if you could watch it for one month. The bobbing motion of the moon is called libration, you don't need to know about it, but it's caused by the moon's orbit. Be careful not to become hypnotized.

Development of Models of the Universe

• Aristotle (384 BC - 322BC)
  • Geocentric model (Earth at center)
  • Sun, Moon and planets circle it on concentric spheres which are turning all the time at slightly different speeds
  • All stars fixed on the outer sphere

• Ptolemy (85 AD - 165 AD)
  • Adapted Aristotle's model by adding epicycles to explain the retrograde motion of some of the planets
  • He called the big concentric spheres "deferents":

• • here is an animation of the Ptolmeic universe

• Aristarchus (2nd century BC) and Copernicus (1473 - 1543)
  • Aristarchus first proposed heliocentric model (sun centered), which is now more associated with Copernicus
  • Earth orbits the Sun
  • Orbits (spheres) are still circular
  • Stars are still fixed in the outer sphere
  • Model explains:
    • the retrograde motion of the planets (because Earth is moving around the Sun as well as other planets, and they are moving at different speeds; different angles of vision)

• • • Why Mercury and Venus always appear closer to the Sun
  • Copernicus added epicycles to the model to explain the slight variation between the planets' real motion and that of the model
  • Model was in conflict with the religious teachings of the Church
  • Ideas were not widely accepted until Galileo in the17th century
  • hereis an animation of the Ptolmeic (geocentric) and Copernican (Heliocentric) universes.

Check out this crazyass astronomical device that was once built for the King of England: