2011.9.24

A Brief History of Light-Years

/NATIONAL GEOGRAPHIC  Daily News  Nov 22, 2010/

Sign-Spotting

• Definition of “Light-Year”
• Two Historical Events about Light-Year
• Understanding the Recent Astronomical Discovery and Complicated aspects of Light-Year 

Briefing

• What is “Light-Year” ?

- A technical term of Astronomy.

• How long light travels in one year in the outer space; about 6 trillion mile, 9.5 trillion km.

- Used when describing huge distance.

• A find that light-speed is limited and how fast it is. 

• Proved by Danish physicist Romer. 
• Measuring how long it takes for the Jupiter’s satellite to appear again after it get behind the planet.
• Result in that the amount of time varies according to the season, which means the time gap is caused by the length of light’s travel from Jupiter to earth.

• The first case in which light-year is used as astronomical term. 

• In the experiment of German astronomer Bessel.
• Calculating the distance between earth and the star far away solar system.
• Operated with a measuring method; parallax.

• The youngest black hole ever found is the oldest object in the universe ?

- The black hole is 30-years-old, but it is located 50 million light-years away.

Opinion

Light-year indication may cause confusion, because the unit does not describe time, but distance. However it is useful when handling astronomical distance, such as that between faraway star and our planet, and the figure of light-year tells us that the apparent shape is the one many years ago. That’s wired.