Measuring Distances in Space

Astronomical Units, Light Years, Parsecs

Measuring distances in space

• The universe is enormous!• Units of measurement on Earth are too small• Other units are used instead

Astronomical Units

• Used for measuring distances in the solar system

• Symbol: AU• The distance between the SUN and the EARTH

is 15 million km (15,000,000).• 15 million km = 1 AU

Distance to Other Planets

• Sun-Mercury = 0.39 AU• Sun-Venus = 0.72 AU• Sun to Mars = 1.52 AU• Sun to Jupiter = 5.20 AU• Sun to Saturn = 9.54 AU• Sun to Uranus = 19.18 AU• Sun to Neptune = 30.06 AJ• Sun to Pluto = 39.53 AU

Light Years• For objects that are VERY far away like stars,

astronomers use light years to measure distance.• Definition: The distance light travels in one year =

9,460,000,000,000 km (nearly 9.5 trillion km)• Light travels 300,000 km/sec• The closest star to earth (excluding the sun) is

4.22 light years away (Alpha Centauri)

Light Years

• If you could drive nonstop to the sun at 60 mph, it would take 180 years.

• Light makes the same trip in eight minutes. So the sun is about eight light-minutes away.

Distances in Light Years

• Sirius-brightest star in the sky: 8.6 light years• Rigel-brightest star in Orion: 777 light years• Center of the Milky Way galaxy: 27,700 ly• Furthest galaxy seen in the universe: 15 Bill ly• Sun to Pluto is 13 light hours (0.0015 light

years)

Parsecs & Parallax

• Parsec = 3.26 light years• Parsec = “Parallax of one arc second”

Parallax

• Definition: difference in the apparent position of an object viewed along two different lines of sight

• Used to determine the distance to celestial objects

Parallax

Units

What unit would you use to measure the distance between:• Jupiter – Saturn?• Earth – Rigel?• Edge of the Milky Way – Center of MW?• Sun-Edge of the solar system?