The Dipole Observed in the COBE DMR 4 Year Data

The largest anisotropy in the cosmic microwave background (CMB) is the {approx_equal}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 {ital COBE} Differential Microwave Radiometers (DMR), we obtain a best-fit dipole amplitude 3.358{plus_minus}0.001{plus_minus}0.023 mK in the direction ({ital l},{ital b})=(264.31{degrees}{plus_minus}0.04{degree}{plus_minus}0.16{degree} +48.05{degrees}{plus_minus}0.02{degree}{plus_minus}0.09{degree}), 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. {copyright} {ital 1996 The American Astronomical Society.}