The radial velocity of a star away from or towards the observer can be determined by measuring the red shift or blue shift of its spectrum. The American astronomer Vesto Slipher made a series of such measurements from 1902 to 1904 and discovered that the radial velocity of α Andromedae varied periodically. He concluded that it was in orbit in a spectroscopic binary star system with a period of about 100 days. A preliminary orbit was published by Hans Ludendorff in 1907, and a more precise orbit was later published by Robert Horace Baker.