second of arc

"Recently, at the Royal Greenwich Observatory in England, an analysis of occultations observed in 1958 and 1959 revealed a cyclic discrepancy [in the Moon's orbital latitude, up to] a large fraction of a second of arc, even after corrections for lunar limb irregularities have been applied.

According to Einstein's final version of the theory, completed in 1915, as their light rays curved around the sun during an eclipse, stars just grazing the sun should appear deflected from their normal positions by an angle of about 1.75 second of arc, about a thousandth of the width of a full moon.

The particular problem the Stanford team looked at, labeled Gravity Probe B, was Einstein's assertion that an object such as the Earth would cause a very slight alteration to the space-time around it - in Earth's case, actually causing space itself to rotate, by a mere 37 one-thousandths of one second of arc.