United States Naval Observatory

The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation and Timing (PNT) for the United States Navy and the United States Department of Defense. Located in Northwest Washington, D.C. at the Northwestern end of Embassy Row, it is one of the pre-1900 astronomical observatories located in an urban area; at the time of its construction, it was far from the light pollution thrown off by the (then-smaller) city center. The USNO operates the "Master Clock", which provides precise time to the GPS satellite constellation run by the United States Air Force. The USNO performs radio VLBI-based positions of quasars with numerous global collaborators, in order to produce Earth Orientation parameters.

Aside from its scientific mission, since 1974, the official residence of the Vice President of the United States has been Number One Observatory Circle, a house on the grounds of the Naval Observatory.

USNO (disambiguation)

USNO or Usno, may refer to:

RR Lyrae

RR Lyrae is a variable star in the Lyra constellation, located near the border with the neighboring constellation of Cygnus. As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astronomers. RR Lyrae variables serve as important standard candles that are used to measure astronomical distances. The period of pulsation of an RR Lyrae variable depends on its mass, luminosity and temperature, while the difference between the measured luminosity and the actual luminosity allows its distance to be determined via the inverse square law. Hence, understanding the period-luminosity relation for a set of local RR Lyrae-type variable stars allows the distance of more distant stars of this type to be determined.

The variable nature of RR Lyrae was discovered by the Scottish astronomer Williamina Fleming at Harvard Observatory in 1901.

The distance of RR Lyrae remained uncertain until 2002 when the Hubble Space Telescope's Fine Guidance Sensor was used to determine the distance of RR Lyrae within a 5% margin of error, yielding a value of 854 light-years (262 parsecs). When combined with measurements from the Hipparcos satellite and other sources, the result is a distance estimate of 860 ly (260 pc).