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The United States Naval Observatory (USNO)

3450 Massachusetts Ave, NW, Washington, DC 20392-5420
Urania Clear

USNO strengthens national security and critical infrastructure by serving as DoD’s authoritative source for the positions and motion of celestial bodies, motions of the Earth, and precise time. USNO provides tailored products, performs relevant research, develops leading edge technologies and instrumentation, and operates state of the art systems in support of the U.S. Navy, DoD, Federal Agencies, international partners, and the general public.

USNO products support activities in the following areas:


Integrated Image and Catalog Archive Service, star catalogs, fundamental reference frames, space astrometry.

Astronomical Applications

Astronomical data calculators (e.g., rise/set times, Moon phase, etc.), information on astronomical phenomena and related topics, astronomical and navigational almanacs, software products.

Earth Orientation

Daily, weekly, and long-term EO products (e.g. IERS Bulletin A), GPS-based products (rapids, ultra-rapids, UTGPS), VLBI-based products, background information, technical and scientific publications and software.

Precise Time

USNO Master Clock, Network Time Protocol (NTP) servers, web-based time synchronization, GPS timing products and services, Two-Way Satellite Time Transfer, and Loran-C timing products.

The James M. Gilliss Library

USNO's library holds one of the most complete collections of astronomical literature in the world.

Tours & Events

Information on tours of the Naval Observatory, upcoming scientific colloquia, and other events.

About Us

Information about the U.S. Naval Observatory.



USNO Master Clock Time
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