History of Astronomy

The textbook web resources for Chapter 1 are here.

Celestial Sphere - an imaginary globe that surrounds the Earth and contains the stars, planets, sun, moon, etc.  This is not really the case, but we can think about the sky this way because the stars are so far away and seem to travel together. 

As we observe the sky, the sun, stars, moon, and planets seem to rise in the east and set in the west.  This is due to the Earth's rotation (spin on its axis).  Stars rise 3 min, 56 sec earlier each night, which adds up to 24 hours over the course of the year.

Different constellations are visible at different times of the year.  For example, Orion is a winter constellation.  This is due to the Earth's revolution (orbit around the sun).

The word planet comes from a Greek word that means "wanderer".  This is because the planets seem to move aross the Celestial Sphere separately from the stars.  They move along a path that follows the ecliptic (the path that the sun follows along the Celestial Sphere).  This path that the planets follow is called the zodiac, and it crossed the 12 zodiac constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces.

Planets move slower across the sky than stars do, because they are travelling around the sun with us - similar to a car that is travelling alongside you on the highway.  The car may be going faster or slower than your car, but it will seem to pass us slower than the surrounding landscape will.

Planets also display retrograde motion.  This means that they sometimes appear to reverse their direction as they travel across the Celestial Sphere.  This happens when Earth and the other planets pass each other.

The moon also moves across the Celestial Sphere.  It moves a distance about equal to its diameter in the sky every hour, and makes one revolution around the Earth in about 30 days.  As it goes around the Earth, it displays different phases - meaning that we see different amounts of its face lit up.  Phases include new moon (completely dark), waxing crescent, first quarter, waxing gibbous, full moon (completely lit), waning gibbous, third quarter, waning crescent, and back to new moon.  Waxing means that the lit portion of the moon is growing larger (as the moon becomes more full).  Waning means that the lit portion is growing smaller.  We tend to see the full moon all night long, while the crescent moon is out during the day (extending into early morning and late evening).

Because Earth is rotating as the moon goes around it, the moon seems to rise 50 min later each day.

Eclipses occur when the sun, Earth and moon all line up and the shadow of the Earth or the moon falls on the other.  These do not happen every month because the moon's orbit around the Earth is tilted about 5 degrees to the Earth's orbit around the sun.

Kepler's Laws of Planetary Motion
  1. Law of Ellipses
    • Planets move in elliptical orbits with the Sun at one focus of the ellipse.
  2. Law of Equal Areas
    • The orbital speed of a planet varies so that a line joining the Sun and the planet will sweep over equal areas in equal time intervals.
  3. Law of Periods
    • The amount of time a planet takes to orbit the Sun is related to the orbit’s size, such that the Period (P) squared is proportional to the semimajor axis (a) cubed.  The formula for this is:  P2 = a3


Homework from the Text:

  • Read Chapter 1.
  • Review Questions #1-11 on page 57.