Expert: Josh - 7/18/2007

Question
In 1054, Chinese astronomers observed the light from a supernova in the
night sky.  This supernova was the incredibly powerful origin of what
is now the Crab Nebula.

Question: Using the velocity of light, devise and execute a strategy
for finding out when the supernova actually occurred, relative to when
the Chinese astronomers saw the explosion

Answer
Sean,

To answer this precisely, we need an accurate model for celestrial movement to determine the position of the supernova with respect to the Earth (our reference frame) when the event first took place.

Without this information, we would have to assume that the distance of the Crab Nebula from the Earth, at the present time, has not changed substantially from its position in 1054 when the astronomers first observed the light from the supernova. You can use some search engine to look up this distance "D" (usually expressed in light years).

Here is the physical principle. The time taken for light to travel from the origin (supernova) to the Earth (observation location) is given by T=D/C, where C denote the speed of light. Note: You may need to convert light years to standard SI unit like meter (m) if the speed of light is taken to be 299,792,458 (approx. 3x10^8) m/sec. The units of measurement have to be consistent.

In fact, given that the distance of the Crab Nebula is 6300 light years from the Earth, the explosion would have taken place around (1054 A.D.minus 6300) 5246 B.C. This is probably more straight forward.