of calculations to process. The data were collected by eight radio observatories at six geographical sites. Radio images are produced from data by aperture synthesis, usually from night long observations of stable sources. The radio emission from Sgr A* varies on the order of minutes, complicating the analysis. Their result gives an overall angular size for the source of . At a distance of , this yields a diameter of . For comparison, Earth is from the Sun, and Mercury is from the Sun at perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination. The telescope's measurement of these black holes tested Einstein's theory of relativity more rigorously than has previously been done, and the results match perfectly. In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft revealed that magnetic fields cause the surrounding ring of gas and dust, temperatures of which range from , to flow into an orbit around Sagittarius A*, keeping black hole emissions low. Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction by dust and gas between the source and Earth. History Karl Jansky, considered a father of radio astronomy, discovered in April 1933 that a radio signal was coming from a location in the direction of the constellation of Sagittarius, towards the center of the Milky Way. The radio source later became known as Sagittarius A. His observations did not extend quite as far south as we now know to be the Galactic Center. Observations by Jack Piddington and Harry Minnett using the CSIRO radio telescope at Potts Hill Reservoir, in Sydney discovered a discrete and bright "Sagittarius-Scorpius" radio source, which after further observation with the CSIRO radio telescope at Dover Heights was identified in a letter to Nature as the probable Galactic Center. Later observations showed that Sagittarius A actually consists of several overlapping sub-components; a bright and very compact component, Sgr A*, was discovered on February 13 and 15, 1974, by astronomers and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory. The name Sgr A* was coined by Brown in a 1982 paper because the radio source was "exciting", and excited states of atoms are denoted with asterisks. Since the 1980s, it has