# How to calculate relative doppler shift

1. Mar 10, 2005

I am to measure the rotational velocity of Io relative to Jupiter, and will do so using a spectrograph.

I will measure the frequency change of a specific mineral emmision band in the spectra, and then check how much it has shifted from its original wavelenght with a table. Then I use the formula:

$$v = (\Delta \lambda/\lambda) c$$

where $$\Delta \lambda$$ is the change of frequency and $$\lambda$$ is the rest wavelenght. C is speed of light.

to calculate the velocity for each object.

The problem is how do I calculate the velocities relative to each other? Io rotates around jupiter, using my method i believe i can only calculate its speed moving relative to earth?

Last edited: Mar 10, 2005
2. Mar 19, 2005

### Andrew Mason

Can you not also measure the same emission band for jupiter itself?

AM

3. Mar 22, 2005

but the problem is, that I do not know how to integrate the shift of Io relative to jupiter... in order to measure the radial velocity of Io around jupiter, i need to integrate them both into the same forumla in some way

4. Mar 22, 2005

### Andrew Mason

If you measure the Doppler shift from Jupiter itself you would simply subtract/add that from/to the Doppler shifts that you measure from Io.

AM