Expert: Josh - 10/14/2009

Question
In a.d. 1054, chinese astronomers observed the light from a supernova in the night sky.  This supernova was incredibly powerful origin of what is now Crab Nebula.
Using the velocity of light,devose and executea strategy for finding out when the supernova actually occured, relative to when the chinese astronomers saw the explosion.  Show work and please explain.

Answer
Hi Edward,

Here is a simplistic view of the problem, assuming that Crab Nebula is stationary relative to the astronomers on Earth when the event took place. That is, we view this as a basic high school physics problem and ignore any special relativity effects.

Suppose the light takes time "t" to travel a distance "D" to reach Earth. We need to find out the approximate distance of the supernova from the Earth using astronomical data.

Earth <-----------D------------< Supernova
1054 A.D.                       1054 A.D. -t

Since the speed of light C=D/t, as a rough estimate, the explosion of light must have taken place some interval "t" before 1054 A.D. Note: make sure you use the right units in your calculation. Distance is typically expressed in light years, so perhaps you may need to convert this to meter or kilometer. Speed is often measured in meter/sec. You have to reconcile the differences between these units to obtain the proper answer.