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
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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
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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?