- Time is related to distance.
- Light takes time to get to us (at the speed of light).
- Everything we study in Astronomy is in the past.
- 1.0 AU = 1 Astronomical Unit = average distance from Earth to Sun
- ~93 million miles
- ~8 light minutes
- The distance light moves in 8 minutes
- Another unit: 1 parsec (pc) = 3.3 light years
- Our nearest star, Proxima Centauri, is 4.2 light years away.
- Most stars are 100-1,000 light years away.
- Milky Way galaxy
- Shaped like a frisbee
- 75,000 light years across
- Our nearest large galaxy is Andromeda
- 2.5 million light years away
- Solar System began forming 4.567 billion years ago
- The universe is 13.8 billion years old
- But the universe is ~45 billion light years wide (how is that possible? we’ll learn later)
Light
- Short wavelength and high energy = blue
- Long wavelength and low energy = red
- Anything with temperature gives off light