Angular separation in the context of Ray (geometry)

The angular diameter, angular width, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is the angular aperture (of a lens). The angular diameter can alternatively be thought of as the angular displacement through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side.

A person can resolve with their naked eyes diameters down to about 1 arcminute (approximately 0.017° or 0.0003 radians). This corresponds to 0.3 m at a 1 km distance, or to perceiving Venus as a disk under optimal conditions.

ν Serpentis, Latinized as Nu Serpentis, is a solitary star in the Serpens Cauda section of the equatorial constellation of Serpens. It is a white-hued star that is faintly visible to the naked eye with an apparent visual magnitude of 4.32. Based upon an annual parallax shift of 16.05 mas as seen from the Sun, it is about 203 light years from the Sun. The star is drifting further away with a radial velocity of +5 km/s.

This is an A-type main-sequence star with a stellar classification of A2V, and is generating energy through hydrogen fusion at its core. It is 350 million years old with a high rate of spin, showing a projected rotational velocity of 123 km/s. The star has 2.64 times the mass of the Sun and 3.0 times the Sun's radius. It is radiating 76 times the Sun's luminosity from its photosphere at an effective temperature of 9,120 K. Nu Serpentis has an optical companion, a magnitude +9.4 star at an angular separation of 46 arcseconds.

Lambda Geminorum, Latinized from λ Geminorum, is a candidate multiple star system in the constellation Gemini. It is visible to the naked eye at night with a combined apparent visual magnitude of 3.57. The distance to this system is 101 light years based on parallax, and it is drifting closer with a radial velocity of –7.4 km/s. It is a member of what is suspected to be a trailing tidal tail of the Hyades Stream.

Components A and B of this system form a wide binary. The secondary, component B, is a magnitude 10.7 stellar companion at an angular separation of 9.29″ from the primary along a position angle of 35.72°, as of 2009. The primary was identified as a spectroscopic binary by E. B. Frost in 1924. This companion was confirmed during a lunar occultation with a separation of 14.1±0.7 mas and magnitude 6.8.

HR 4796 is a binary star system in the southern constellation of Centaurus. Parallax measurements put it at a distance of 235 light-years (72 parsecs) from the Earth. The two components of this system have an angular separation of 7.7 arcseconds, which, at their estimated distance, is equivalent to a projected separation of about 560 Astronomical Units (AU), or 560 times the separation of the Earth from the Sun. The star and its ring resemble an eye, and it is sometimes known by the nickname "Sauron's Eye".