Hubble's law is a statement in physical cosmology which states that the reshift in light coming from distant galaxies is proportional to their distance. The law was first formulated by Edwin Hubble and Milton Humason in 1929 after nearly a decade of observations. It is considered the first observational basis for the expanding space paradigm and today serves as one of the most often cited pieces of evidence in support of the Big Bang. The most recent calculation of the proportionality constant, using the satellite WMAP began in 2003 and a combination of other astronomical data, yielding a value of 70.1 ± 1.3 (km/s)/Mpc. In August, 2006, a less accurate figure was obtained independently using data from NASA's orbital Chandra X-ray Observatory: 77 (km/s)/Mpc or about 2.5×10−18 s−1 with an uncertainty of ± 15%.