We examine the motion of Earth's magnetopause for 16 dawnside traversals of this boundary by the sister spacecraft AMPTE/UKS and IRM in December, 1984, when their separation was 400--900 km. We compare magnetopause normal vectors, n, and speeds of motion, un, obtained separately from each spacecraft by use of three different methods, and also compare those un to corresponding speeds, un*, obtained from observed time lags between the two spacecraft. Agreement between un values and n vectors determined from the three methods ranges from poor to excellent. Comparing un* and un values, we find a clear tendency for |un*| to be larger than |un|: While slightly less than half of the results show reasonable agreement (0.5n/un*<2), there are about as many results in the range 0n/un*<0.5, and a few cases give the wrong sign of un. ¿ 2000 American Geophysical Union