the Belt cover about 4.5° of angular size.) However, much finer units are needed to measure the angular sizes of galaxies, nebulae, or other objects of the night sky. Degrees, therefore, are subdivided as follows: 360 degrees (°) in a full circle 60 arc-minutes () in one degree 60 arc-seconds () in one arc-minute To put this in perspective, the full Moon as viewed from Earth is about °, or 30 (or 1800). The Moon's motion across the sky can be measured in angular size: approximately 15° every hour, or 15 per second. A one-mile-long line painted on the face of the Moon would appear from Earth to be about 1 in length. In astronomy, it is typically difficult to directly measure the distance to an object, yet the object may have a known physical size (perhaps it is similar to a closer object with known distance) and a measurable angular diameter. In that case, the angular diameter formula can be inverted to yield the angular diameter distance to distant objects as In non-Euclidean space, such as our expanding universe, the angular diameter distance is only one of several definitions of distance, so that there can be different "distances" to the same object. See Distance measures (cosmology). Non-circular objects Many deep-sky objects such as galaxies and nebulae appear non-circular and are thus typically given two measures of diameter: major axis and minor axis. For example, the Small Magellanic Cloud has a visual apparent diameter of × . Defect of illumination Defect of illumination is the maximum angular width of the unilluminated part of a celestial body seen by a given observer. For example, if an object is 40 of arc across and is 75% illuminated, the defect of illumination is 10. See also Angular diameter distance Angular resolution Solid angle Visual acuity Visual angle Perceived visual angle List of stars with resolved images Apparent magnitude References External links Small-Angle Formula Visual Aid to the Apparent Size of the Planets Elementary geometry Astrometry Angle