## 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.
10^{0} = 1

10^{1} = 10

10^{2} = 10x10 = 100

10^{3} = 10x10x10 = 1000

10^{6} = 10^{3}x10^{3} = 1 million

10^{9} = 10^{3}x10^{6} = 1 billion

10^{-1} = 1/10 = 0.1

10^{-2} = 1/10^{2} = 0.01

10^{-3} = 1/10^{3} = 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 10^{8} km from Earth, and its
angular size is 0.5 degrees. What is the Sun's physical diameter?

a = 1.5 x 10^{8} km x (0.5 deg / 57.3 deg) = 1.4 x 10^{6} km.

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Updated: 2005 March 27 [dhw]