9.1 A. Measuring Distances Beyond the Solar System
Recall: Solving for a variable (unknown) using cross multiplication
EXAMPLE 
Scale: 1 A.U. = 3 cm
Mercury is 0.39 A.U. from the Sun. How far from the Sun (in cm) would Mercury be drawn on your Solar
System Model?
𝑥
3𝑐𝑚
=
0.39 𝐴𝑈 1 𝐴𝑈
(1 𝐴𝑈) 𝑥 = (0.39 𝐴𝑈)(3 𝑐𝑚)
(1 𝐴𝑈)𝑥
1 𝐴𝑈
=
𝑥=
(0.39 𝐴𝑈)(3 𝑐𝑚)
(1 𝐴𝑈)
(0.39)(3 𝑐𝑚)
1
𝑥 = 1.17 𝑐𝑚
Measuring distances between two celestial objects
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Most stars are more than 100 trillion (1.0 x 1013) km from Earth.
The closest star to Earth is the Sun (1.5 x 108km).
Proxima Centauri is the next closest star (4.01x1013km).
The Light Year
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Unit of distance.
A Light Year is the distance that light travels in a vacuum (empty space) in one year.
Light travels at a constant speed of approximately 300 000 km/s in space.
Astronomers use light years to measure the distance to stars or other celestial objects
outside of the Solar System.
1 Light Year (ly) is approximately equal to 10 trillion (9.46 x 1012) km.
1 ly = 9.46 x 1012km
9.1 A. Measuring Distances Beyond the Solar System
Example 1: Calculate the distance from Earth to Proxima Centauri (4.01 x 1013km) in light
years.
Given:
Required:
Analysis:
Solution:
Statement:
Example 2: Vega is 25 ly from Earth. Calculate the distance from Earth to Vega in kilometres.
Given:
Required:
Analysis:
Solution:
Statement:
9.1 A. Measuring Distances Beyond the Solar System
Example 3: Betelgeuse is 5.7 x 1015 km from Earth. How many light seconds away is
Betelgeuse?
1 light second (ls) = 3.0 x 105 km
1 light minute (lm) = 1.80 x 107 km
WHAT DOES THIS ALL MEAN?
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The farther a star is from Earth, the longer it takes for the light from that star to reach Earth.
Vega is about 25 ly from Earth. The light from Vega travels at the speed of light; therefore it
takes about 25 years for the light from Vega to reach Earth.
When astronomers look at celestial objects through telescopes they’re actually looking back in
time.
HOMEWORK: Read p.365 -369 and answer questions # 1, 2, 3, 5, 6, 7, 8, 10, 11