Recall from last lecture:

Chapter 1: Observing the Sky: The Birth of Astronomy

Where did astronomy begin?

1.1 The Sky Above

Many of the ways we think about the sky come from antiquity. We'll begin our exploration of astronomy by connecting to these ancient ideas before moving to the modern era.

1.1.1 The Celestial Sphere

When you look up at the night sky, the Moon, stars, and planets appear to be fixed in a large clear sphere, with you (the Earth) at the center. At any time we can only see half of this sphere. During the course of a day, the sphere seems to rotate so that different stars and constellations are visible. During the course of a year, the sphere seems to tilt, so that the height of certain stars in the sky changes. From this point of view, it is understandable that until only a few hundred years ago, people believed that the Earth was the center of the Solar System, and more generally, the center of the universe.

It is still useful to imagine the stars as fixed in a large clear sphere that rotates around the Earth. This is a convenient way to understand how we locate objects in the sky. The large clear sphere is called the celestial sphere. The point on the sphere directly overhead is called the zenith. The zenith point changes with your location on the Earth, but from any location, it is always the point directly overhead. The boundary where the sky meets the Earth is called the horizon. We can't see objects below the horizon.

The stars appear fixed in the celestial sphere. They do move, but generally that motion is only apparent over years, and with a good telescope.

1.1.2 Celestial Poles and Celestial Equator

Like the Earth, the celestial sphere has a North and South pole, and an equator. We locate the celestial North and South poles by extending an imaginary line through the Earth's North and South poles until it intersects the celestial sphere. The points of intersection are the North and South celestial poles. Similarly, by extending a plane containing the Earth's equator to the celestial sphere, we can define the celestial equator.

As the celestial sphere (apparently) rotates, it rotates about an axis through the celestial poles. Therefore, the location of a celestial pole is fixed in the sky. A star located at the North celestial pole would seem to remain fixed during the night, while the other stars rotated around it (see Fig. 1.3). Such a star exists and it is called Polaris, or the North star. (A corresponding South star doesn't exist.)

1.1.4 Fixed and Wandering Stars

The Sun and the Moon do not remain fixed among the stars. During the year they seem to rise and set in different parts of the sky. There are other objects that also move relative to the stars in the course of weeks or months. The Greeks dubbed these objects "wanderers", or planets in Greek. Five planets (other than Earth) are visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn.

1.1.5 Constellations

Perhaps as an aid to orgainizing the stars, our ancestors divided the sky up into groups of stars called constellations. A number of cultures did this, but the 88 constellations astronomers use today are based largely on the constellations of the ancient Greeks. Their use now is as a convenience for locating and naming objects in the sky. For instance, Mars is presently in (or near) the constellation Capricorn. The back of the text contains star charts for each month, with the major constellations labelled. The CD included with the text ("The Sky") contains a program that will show you what the sky for any date and any location on Earth.

1.2 Ancient Astronomy

Many (possibly all) ancient civilizations made astronomical observations. One common purpose, as appeared in last week's discussion, was the creation of a calendar to keep track of significant events -- flooding of the Nile, summer and winter solstice, or religious celebrations. It is hard to think of a civilization essentially ignorant of astronomy (can anyone think of one?). Unfortunately, most of what they understood was not put in writing, but comes to us by inference from the construction of temples (Stonehenge), pyramids, drawings, and their exploits (Polynesians navigating across thousands of kilometers of open ocean).

Our own astronomy comes primarily from the Greeks and Romans by way of Arabic culture. Many star names (Algol, Betelgeuse, ...) and astronomy terms (zenith) come from Arabic.

2000 years before Columbus, the Greeks determined that the Earth is a sphere. Their arguments included:

  1. The Moon is a sphere (to explain the shapes seen in the phases), so the Earth should be as well.
  2. During a lunar eclipse, the shadow of the Earth on the Moon is curved.
  3. Travelers who went far south (up the Nile river for instance) see stars not visible from home (Greece), and the height of the pole star decreases.
The word geometry has Greek roots meaning Earth measurement.

The Greeks reasoned that if the Earth orbited the Sun then the stars should shift positions with the seasons due to parallax. This is correct, but due to the enormous distance to the stars, the shift is too small to detect with the unaided eye. The Greeks (at least Aristotle) couldn't imagine the enormous distances between stars and therefore concluded that the Sun orbited the Earth. Note that to us their conclusion was wrong, but their model agreed with all their observations.

Measurement of the Earth by Eratosthenes

About 200 B.C., Eratosthenes made the first (reasonable) determination of the Earth's diameter. See diagrams in the text for more information.


  1. Catalogued about 850 stars and their apparent (to him) magnitudes (brightness). We will discuss magnitudes in more detail in chapter 16.
  2. He noted that the position of the north celestial pole had changed over the previous 150 years. We now understand that this motion comes from the precession of the Earth's axis with a period of 26,000 years.


Ptolemy created a model of the solar system which predicted the positions of the planets for any date and time. This model became known as the Ptolemaic model.

  1. His model has a stationary Earth near the center of the solar system.
  2. The planets, the Sun, the Moon, and the stars orbit the Earth on circles.
  3. His model included epicycles to account for retrograde motion.
The Ptolemaic model was accepted by the Muslim world and Christian Europe, and endured for more than a thousand years.

1.3 Astrology and Astronomy

Astrology is the belief that our lives are governed by the motions of stars and planets. There is no evidence supporting this! To its credit, astrology increases knowledge of the zodiac, and has given us the infamous pick-up line "Hi, what's your sign".

1.4 The Birth of Modern Astronomy


In 1543 Copernicus published his sun centered (heliocentric) model of the solar system. Both the heliocentric and Ptolemaic models predict the positions of the planets equally well. So how can one test the Ptolemaic versus the Copernican models? Until the invention of the telescope, it was virtually impossible to choose in favor of one over the other.

Galileo and the Beginning of Modern Science

Galileo is generally considered the first modern scientist, the first to put the scientific method -- observe, hypothsize, measure, evaluate -- to effective use. And what an effect he had!

Galileo is perhaps most famous for his contributions in Physics. He observed that pendula swing with a period independent of the amplitude of the swing. Legend has it that he dropped objects from the leaning tower in Pisa, and concluded that bodies fall at the same rate, independent of their mass.

And he overturned the Aristotelian idea that all bodies eventually come to rest, because the natural state of matter is to be stationary. He accomplished this by sliding objects down a ramp, and observing that if he polished and greased the ramp, they slid further. He concluded that the polishing and greasing lessened the friction acting to slow down the object, and if he could completely remove the friction, the object would slide forever. Therefore, the natural state of an object is for it to move with constant speed in a straight line, until it is acted on by some external force (friction and gravity are common examples).

Galileo's Astronomical Observations

Galileo didn't invent the telescope, but he did bring its obvious military application to the attention of the Venetian government. Galileo was the first to use a telescope to view the night sky and document what he saw. First, he made ground based experiments to convince himself that what he saw through the telescope was real, not some figment generated by the telescope itself.

When he looked at the night sky, he saw many more stars than visible with the unaided eye. He saw that Milky Way is made up of many individual stars. He looked at the planets, and discovered four moons circling Jupiter.

He observed Venus, and saw it go through phases, like the Moon. As mentioned earlier, this effect cannot be accounted for in the Ptolemaic (Earth centered) model of the solar system, but is expected in the Copernican (Sun centered) model.

Discussion Questions

© Robert Harr 2005