V_r - radial velocity of the source with respect to the observer.
c - speed of light (3 x 10⁵ km/sec)
- the amount of the shift in nanometers
- unshifted wavelength in nanometers

For example, if a line in a spectrum should fall at 600 nanometers but instead lies at 600.1, what would the radial velocity be?

The solution to this equation only tells us the velocity of the source relative to the spectroscope. Whether the distance is increasing or decreasing is revealed by the direction of the shift to the red or blue end of the visible spectrum. It does not tell, however, if one or both objects are moving relative to some external reference point.

For Further Research

• Does the person twirling the whiffle ball hear the Doppler shift? Why or why not?
• Can the red/blue shift technique be used

for objects other than stars? Can you tell which way an emergency vehicle is travel-ing by the pitch of its siren?
• Transverse velocity is a motion that is perpendicular to radial velocity. Can this motion be detected by the Doppler effect?
• What has Doppler shift told astronomers about the size of the universe?

The electronic parts for this device are not specified exactly since there are many combinations that will work. Go to an elec-tronic parts store and select a buzzer, bat-tery holder, battery, and switch from what is available. Remember to purchase parts that will fit in a whiffle ball. The store clerk should be able to help you make a workable selection if you need assistance. If possible, test the buzzer before purchasing it to determine if it is loud enough. Test the buzzer and battery before soldering connections. The buzzer may be polarized. Reverse the connections if you do not hear a sound the first time.