Physics is concerned with the study of the Universe from the smallest () to the largest scale ().

In astronomy and cosmology, we need to measure times and distances.

**LENGTHS:**

The primary unit of length is the *meter* (m). However, lengths in the universe are so large, so we’ll want to use some larger units.

The average distance of the Earth from the Sun is about is 92 million miles. More precisely, this quantity is defined to be 1 **AU**= 1 **Astronomical Unit** = meters.

The distance to a star whose parallax is 1 second of arc is defined to be 1 **parsec** = 1 **pc **= 3.08568025×10^{16} m.

1 parsec (pc) is equivalent to:

The distance travelled by light in one year is called a **light year **
(**ly)**; it equals:

1 ly= ( days/year)*(seconds/day)=.

Thus

1″= ‘=

1 rad= ’’

d=

1 parsec is 3.261564 light years. A very important task in astronomy is the measurement of distances to things that are very far away.

**TIME**

The unit of time is the *second*. We also use **years**.

**SPEED**

We measure speed in *m/s* (meters per second). The speed of light is 3×10^{8} m/s.

Time and Space exist independently of one another!