Distances in Space

Interplanetary Distances Compared to how far away the stars are, separation of the planets is a relatively small distance. Examples: Distance from Earth to the Sun ~ km and the distance between Earth and Mars is km. Units of Distance used to describe the distance between planets is Kilometers

Kilometers are of limited use because distances in space are very large and we usually must express the number of kilometers in scientific notation. e.g. the average distance from Earth to the Sun is km, but we write it 1.5x10 8 km Bill Nye distances between planets (4:17 min): Bill Nye distances between planets soccer field (2:07 min):

Why use the sun as a reference point? 1 astronomical unit = 1 AU 1 AU = average distance between Earth and Sun Earth is km, or 1 AU away from the Sun. Mars is km, from the sun, or approximately 1.5 a.u. from the Sun. So Mars is 1.5 times as far from the sun as Earth is from the sun.

Interstellar Distances Interstellar distances are relatively very, VERY large distances. Examples: Distance between Earth and Alpha Centauri (the next closest star) is 3.8x10 13 km [or km] Units of Distance used to measure the distance from Earth to a star or galaxy are LIGHT YEARS

1 light year is the distance that light can travel in 1 year, at its speed of km/s 1 light year = 9.5x10 12 km

b Example: The nearest star (other than the Sun) to us is Alpha Centauri. Alpha Centauri is 4.3 light years away or 4.3x9.5x10 12 km = 3.8x10 13 km away from earth. This means that the light that we see from Alpha Centauri left that star 4.3 years ago. Is that like looking into the past?

The Andromeda Galaxy is 2.2 million light years away from Earth. This Means that the light we see from Andromeda Galaxy left there 2.2 million years ago. It is therefore very possible that some of the stars in Andromeda have exploded as a supernova or gone out long ago. The message of these star finishing events just has not gotten to us yet! 2.2 million light years