Unit 2: Discovering Earth & Sky

Brass Armillary Sphere

Mapping the Earth and the Sky was among the first concrete physical applications of astronomy. Out of this comes many of our methods of measuring the surface of the earth and navigation today.

How the sky appears to us depends upon our location on the Earth, the time of day, and the season of the year. From our vantage point on a moving Earth, the sky appears to turn around us each day. Over the course of a month, the Moon waxes and wanes, changing from a thin crescent to a full disk and then back to a crescent. Over the course of the year the Sun follows a regular path along the Ecliptic that takes it first north, then south of the celestial equator. Finally there are the planets, the "wanderers" that travel near the Ecliptic, but with their own sometimes surprisingly complex motions.

Understanding these motions was not only the first step in learning the place of the Earth in the universe, it also had immense practical importance. Ancient and modern peoples used the regular motions of the Sun, Moon, and Stars to reckon the passage of time and the seasons of the year, giving us our systems of timekeeping and calendars.

In this unit we will examine the apparent daily & annual motions of the Sun, Moon, and stars, the causes of the Four Seasons, discuss the astronomical roots of our conventions for reckoning the passage of time, describe the phases of the Moon and Eclipses, and introduce the Motions of the planets. At each step, the motions we are considering become more complex, and the process of coming to an understanding of that complexity has led us into the modern world.


Measuring the Earth (Sept 24)

Mapping Earth & Sky (Sept 25)

Daily & Annual Motions (Sept 26)

The Four Seasons (Sept 27)

The Phases of the Moon (Sept 28)

Eclipses of the Sun & Moon (Oct 1)

Telling Time (Oct 2)

The Calendar (Oct 3)

The Wanderers: Planetary Motions (Oct 4)

Quiz 1: Friday Oct 5, in class

Lecture Podcasts Lecture Audio Podcasts

Supplement: Celestial Motions Revisited
In previous quarters which began on Tuesday instead of Wednesday, there was one extra lecture before Quiz 1. On one occasion, students asked lots of questions about all of the various motions, so I gave a lecture that revisited this material and put it together in context. Nothing in this supplement is new: everything has been covered in greater detail in the previous lectures. I include it here as some students said it was helpful in understanding celestial motions, and so I would like those of you interested in seeing the same material from a slightly different perspective to have benefit of it.

Another helpful thing to do is attend one of the Planetarium Programs presented early in the quarter. These are designed to illustrate many of the ideas we will discuss in this unit. Students from this class who attend one of these programs should go up to the TA giving the program afterwards and have them note your attendance. One-time extra credit on the final exam will be given to all students who attend a planetarium program.


See the Astro 161 Digital Movie Gallery for online copies of some of the animations shown during lectures. Beware! These are big files (many Megabytes). You should only try to download them if you are on a fast network link (cable or DSL). These could easily clog a slow dialup connection.

[ Return to the Astronomy 161 Main Page | Go back to Unit 1 | Go forward to Unit 3 ]
Updated: 2007 September 22
Copyright Richard W. Pogge, All Rights Reserved.