# Why do we need the formula #v=H_oD# to measure the recessional velocity of a galaxy when instead we can just do #v=(dD)/(dT)#?

##### 1 Answer

Feb 4, 2016

Hubble's formula is a correlation formula and is not in mathematical exactitude. The velocity of a galaxy is estimated by the Doppler effect. Galaxies appear to be receding from one another.

#### Explanation:

Velocity as rate of change of distance with respect to time is applicable to continuous variation. It stops working at discontinuities. Doppler effect: Light emitted by a source is shifted in wavelength, by the motion of light source..