## LECTURE 1: INTRODUCTION TO COURSE

### THEMES OF COURSE

• Solar System Astronomy: What we know about the structure of the solar system, about the nature of planets, Moons, asteroids, and comets, and about the existence of planets around other stars.
• Science and the Scientific Revolution: How we arrived at the modern understanding of the solar system, and how we use observation and theory to learn about the universe.

A crucial discovery: The fundamental physical laws that apply on Earth and can be tested in terrestrial laboratories also apply to astronomical objects. Physics can teach us about astronomy, and astronomy can teach us about physics.

### OUTLINE

• Basic astronomical observations, from "Earth-centered" perspective.
• Development of modern understanding of solar system: ancient Greeks, Copernicus, Tycho, Kepler, Galileo.
• Present-day understanding of Earth, Moon, planets (leave Sun for A162). Heavily informed by (a) basic physics, (b) ground-based measurements with telescopes, and (c) space missions.
• Planets around other stars.

### SCIENTIFIC NOTATION

Astronomy deals with very large and very small numbers, so we will frequently use scientific notation.

100 = 1
101 = 10
102 = 10x10 = 100
103 = 10x10x10 = 1000
106 = 103x103 = 1 million
109 = 103x106 = 1 billion

10-1 = 1/10 = 0.1
10-2 = 1/102 = 0.01
10-3 = 1/103 = 0.001
etc.

### ANGLES

In a full circle, there are 360 degrees or 2\pi radians.
1 radian = 57.3 degrees (360/2\pi = 57.3)
1 degree ~ fingertip at arm's length
10 degree ~ fist at arm's length

Astronomers often give angles in arc-minutes or arc-seconds.
Related to a degree as minutes and seconds are to an hour.
1 arc-minute = 1/60 degree
1 arc-second = 1/60 arc-minute
1 degree = 60 arc-min = 3600 arc-sec

Angular size of Sun and full Moon = 0.5 degree
Smallest angle resolvable with naked eye = 1 arc-min
Typical angle resolvable with telescope = 1 arc-sec.

### ANGLES AND SIZES

The angle \theta at the apex of a triangle with two long sides of length d and one short side of length a is

\theta = a/d radians = 57.3 x (a/d) degrees.

If we know angle and distance, can solve for size:

a = d x (\theta / 57.3 degrees).

Example: The Sun is 1.5 x 108 km from Earth, and its angular size is 0.5 degrees. What is the Sun's physical diameter?

a = 1.5 x 108 km x (0.5 deg / 57.3 deg) = 1.4 x 106 km.