# Light

## Speed of Light

• $3 \times 10^5 km/s$
• First sign that light's speed was finite came in 1675
• Ole Rømer observed eclipses of Jupiter's moons
• Noticed the eclipses happened at times different than predicted
• Earlier than predicted at opposition
• Later than predicted at conjunction
• Since the distance between opposition/conjunction is different, speed of light must be have an impact
• Takes ~8.3 minutes for light to get from the Sun to Earth
• 1 AU --> takes light 8 minutes to travel

Physics