Modern Astronomy

1
Modern Astronomy

• Chapter 4
• 4-3 and 4-4

2
3 Astronomy Greats
Copernicus Brahe Kepler Galileo
Galileo Galilee Made his own telescope and
showed that the objections to Keplers laws were
wrong.
Tycho Brahe Best naked eye observer of all
time. Johannes Kepler Tried to match Tychos
data and had to abandon perfect circles. Came
upon 3 laws of planetary motion.
Nicholas Copernicus Sun in center, still had
perfect circles with circles on circles.
3
Keplers Laws

• First Law Planets orbit in ellipses with the
  Sun at one focus
• Second Law Planet Sun line sweeps out equal
  areas in equal time
• Third Law There is a relationship between a
  planets orbital period (P) and its average
  distance from the Sun (a), P2 a3

4
First Law Meaning

• An ellipse is like an elongated circle. Instead
  of one point in the center, an ellipse has two
  points called foci.
• For planetary motion then, each planet does not
  orbit the Sun in a perfect circle.

5
Second Law Meaning

• A planet does not orbit at a constant speed.
• As the planet orbits the Sun it will move faster
  at perihelion and slower at aphelion.

6
Third Law Meaning

• P is the period of a planets orbit
• Earths period is 1 year
• a is the average distance from the Sun, the
  semimajor axis as seen in the ellipse
• Earths average distance is 1 AU