# Lesson 4

In Lesson 1 you can follow the definition of the Astronomical Unit based on a “test” particle, that takes to revolve around a major body, exactly one year. We saw, that based on this definition, the Earth’s distance from the Sun is slightly smaller, compare with the test particle’s distance around the Sun. The result is, that with a good approximation, the Astronomical Unit can be given as 150,000,000 kilometers, and we write this is ‘scientific notation’, considering the number to large to write many zeros. We write then 1.5 x 108 km. But this distance, as big as it may seem to be, is small for the dimensions of our Galaxy, the Milky Way, and much no viable to use in the universe. It would be like giving the height of a building in millimeters. Astronomers use units that are more suitable for the description of the Universe.

In the following video, I will describe the light-year (l-y), and the parsec (pc).

Because the expression “light-year” features the word “year”, let’s clarify that this unit is not a unit of time. We have to read the whole expression as “light-year”, not as “year” only. A light-year is a unit of distance. It is by definition, the distance light travels in one year, at a speed close to c=3×108 m/s (meters/second).

Watch the following video for the definition of a parsec.

http://www.showme.com/sh?h=fAJ93w0