The Dipole Observed in the COBE DMR 4 Year Data

The largest anisotropy in the cosmic microwave background (CMB) is the ~3 mK dipole assumed to be due to our velocity with respect to the CMB. Using the 4 year data set from all six channels of the COBE Differential Microwave Radiometers (DMR), we obtain a best-fit dipole amplitude 3.358 +/- 0.001 +/- 0.023 mK in the direction (l, b) = (264.31^deg^ +/- 0.04^deg^ +/- 0.16^deg^, +48.05^deg^ +/- 0.02^deg^ +/- 0.09^deg^), where the first uncertainties are statistical and the second include calibration and combined systematic uncertainties. This measurement is consistent with previous DMR and FIRAS results.