The history of astronomy is a history of receding horizons. Edwin P. Hubble (1889-1953)

Scientific Notation

The system of recording very large or very small numbers by using powers of 10.

Ex. nanometer $= 0.000000001 m = 10^{-9}$, kilometer $= 1000 m = 10^3$

Units of measure

Lightyear (ly) - The distance that light travels in the period of one year. Roughly $3*10^8$ m/s.

Astronomical unit (AU) Average distance from Earth to the Sun. Roughly $1.5*10^8$ km.

Basic Definitions

Planet - A non-luminous body in orbit around a star, large enough to be spherical and to have cleared its orbital zone of other objects.

Star - A globe of gas held together by its own gravity adn supported by internal pressure of its hot gases, which generate energy by nuclear fusion.

Solar system - The Sun and its planets, asteroids, comets and so on.

Galaxy - A large system of stars, star clusters, gas, dust, and nebulae orbiting a common center of mass.

Spiral arms - Long spiral pattern of bright stars, star clusters, gas, and dust. Spiral arms extend from the center to the edge of the disk of spiral galaxies.

The Scientific Method

How do scientists learn about nature?

• The scientific method, the process by which scientists form hypotheses and test them against evidence gathered by experiment and observation.
• A hypothesis is a suggestion about how nature works, and the evidence is reality.