Abell 2218 is a cluster of galaxies - a prodigious collection of mass,
all orbiting a common center of gravity.  Such clusters can tell us
a lot about the nature and distribution of matter in the universe.
(An image of the center of Abell 2218 can be found in the "Gravity and 
Light" section of the text.)

Using spectroscopy, you measure the hydrogen Balmer-alpha emission
line for several dozen galaxies in this cluster.  You know the rest
wavelength of this emission line is 656.3 nm.  In your spectroscopic
measurements, however, the average position of this emission line
is 789.0 nm.

(a) What is the apparent velocity of this cluster in km/sec?

(b) Using H = 70 km/s per Mpc as the value of the Hubble constant,
compute the approximate distance to this object.

(c) How long did the light you observed from this galaxy last
night take to reach Earth?