## Doplar effect calculation of date of Super Nova

In my astronomy lab we (part A) calculated how many arc seconds a super nova remnant had expanded from 1942 to 1976, and in further calculations (Part B) used the doplar effect to calculate a velocity, which was then used with the results of part A to calculate the distance from earth to the supernova remnant.
My problem is with how part B was done. I don't know if I'm misunderstanding something or what. there was a spectrum with the red shifted and blue shifted lines of oxygen, we found how many angstrums were between them. Then it said to calculate the relative speed of the red and blue shifted lines. (delta lambda C)/lamda0. My professor insisted that delta lamda was 1/2 the range of angstrums between the red and blue shifted lines(e.g. if blue shifted was 50A and redshifted was 100, use 75), and that lamda0 is what it should be when unshifted.
My problem with this method is that, aren't you calculating the speed of the either shifted end relative to the unshifted value, instead of the speed of the shifted lines relative to each other(like the experiment said)? I'm only really asking this here because i ended up with pretty darn close to 1/2 the known value of the distance.

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