1. Measuring Distances in Space
The Light-Year (ly)
The correct SI abbreviation for light-year is all lower case as on slide.

2. distance light travels in one year
Light-year (ly)
distance light travels in one year

3. Reminders... Light travels in a straight line.
The speed of light is 300,000 km/s
The sun is 150 million km away
By 100 kph, it would take us 165 years to get to the sun
By 600 kph, 27.3 years
By 28,000 kph, 215 days
By 53,000 kph, 113 days
By light speed, it would take 8 minutes
NOTE: At the speed of light, the moon is 1.3 s away.

4. Next Closest Star Proxima Centauri 4.3 ly away
If it went supernova today…
You would see it at graduation
* Takes light 4.3 years to get here!
Proxima Cantauri is part of a binary star system (two stars that orbit each other). Its sister star is Alpha Centauri. Currently, Proxima Centauri is slightly closer that Alpha and will continue to be so for the next 9000 years.

5. The Milky Way Our galaxy… ~ 7,000 ly thick ~ 100,000 ly diameter
Light takes 100,000 years to travel from one side to the other
Earth about 26,000 ly from the core
Our solar system is located in the outer reaches of the Milky Way Galaxy, which is a spiral galaxy. The Milky Way Galaxy contains roughly 200 billion stars. Most of these stars are not visible from Earth. Almost everything that we can see in the sky belongs to the Milky Way Galaxy. Our solar system is about 26,000 light-years from the center of the Milky Way Galaxy, which is about 80,000 to 120,000 light-years across (and less than 7,000 light-years thick). We are located on one of its spiral arms, out towards the edge. It takes the sun (and our solar system) roughly million years to orbit once around the Milky Way. In this orbit, we (and the rest of the Solar System) are traveling at a velocity of about 155 miles/sec (250 km/sec).

6. Your Turn… Just how far does light travel in 1 year?
Your group will need paper & pencil (you may use a calculator)
With your team, calculate the distance to Proxima Centauri, 4.3 ly away…raise your hand when you think you are done and I will check it.
HINT: need to use Speed of light and time

7. distance light travels in 1 s 300,000 x 60 s in 1 min x 60 18,000,000
km/s
x 60 s in 1 min
x 60
s/min
18,000,000
km/min
x 60 min in 1 hr
min/hr
1,080,000,000
km/hr
x 24 hr in 1 day
x 24
hr/day
25,920,000,000
km/day
x days in 1 year x
days/yr
9,467,280,000,000
km/yr=1 ly
x 4.3 ly to Earth from PC
x 4.3 ly
distance to Proxima Centuari
40,709,304,000,000 km
Click though mathematical steps to calculate the distance light can travel in one year & the distance to Proxima Centauri (the next closest star); great group activity to do the actual math.