Answer:
Two things we need to remember:
Speed of light = c = 299,792,458 m / s
This means that the light in one second covers a distance of 299,792,458 m
And for a circle of radius R, the is:
C = 2*pi*R
where:
pi = 3.14
Then if the radius of the earth is 6,378.1 km, then the of the earth is:
C = 2*(3.14)*6,378.1 km = 40,054.468 km
And we know that 1 km = 1000m
Then the circumference of the earth is:
C = (40,054.468*1000) m = 40,054,468 m
Now we need to see how many times the light can encircle the earth per second. (we assume that the light can move in a circular path, this is not really accurate to the reality)
We know that in one second the light can cover a distance of: 299,792,458 m
And the circumference of the earth is: 40,054,468 m
The quotient between these two distances is the number of times that the light can encircle the earth:
N = (299,792,458 m)/(40,054,468 m) = 7.5
So light could encircle the earth 7.5 times per second.
