April 8, 2011
Seasons, the phases of the Moon and the changing aspect of the Sun in the sky have long provided us with opportune means to mark the changes in our environment. These astronomical phenomena supply repeating events that have been used to create timescales convenient in describing sequences of events. Devices of various degrees of sophistication have been developed throughout history to make the necessary observations, and along with those devices, we have created the appropriate formalism to take advantage of the appearance of the sky to mark the passage of time. However, it has become apparent that astronomical timekeeping is no longer capable of meeting modern requirements for precision. This may be due to the variability in the motions or to difficulties in accessing the astronomical phenomena, but the fact is that physical metrology is now the way to meet modern requirements for precise time and time interval. There remains a critical need to know the orientation of the Earth with respect to celestial reference systems, but precision timing now requires different repeating phenomena to create timescales. The evolution of devices, algorithms and the underlying philosophical concepts of time itself continues today, and is likely to continue into the future. A review of the past progress in timekeeping will be presented along with a description of the current state of the art and a glimpse into the future.
Dr. Dennis D. McCarthy received the B.S. degree in astronomy from Case Institute of Technology, Cleveland, OH, and M.A. and Ph.D. degrees in astronomy from the University of Virginia, Charlottesville. In 1982 he became Chief of the Earth Orientation Parameters Division of the U. S. Naval Observatory and in 1994, Head of the Earth Orientation Department. In 1996 he was appointed Director of the U.S. Naval Observatory Directorate of Time. At the Observatory he has received the Simon Newcomb Award for Research and the Superintendent's Award. Dr. McCarthy developed the Very Long Baseline Interferometry (VLBI) program of the U.S. Naval Observatory for the determination of Earth orientation. He has also developed the Observatory's use of the Global Positioning System observations for the determination of Earth orientation. He has served as Vice President and President of the International Astronomical Union (IAU) commission on Time, Vice President and President of the IAU Commission on the Rotation of the Earth, Secretary of the International Association of Geodesy Section on Geodynamics, member of the Organizing Committee of the IAU Commission on the Rotation of the Earth, and chairman of the IAU Working Group on Nutation. He was Chairman of the Directing Board of the National Earth Orientation Service (NEOS), the head of the International Earth Rotation Service (IERS) Product Center for Conventions, and Chairman of the IAU Working Group on the Definition of Coordinated Universal Time. He is currently President of IAU Division 1 on Fundamental Astronomy Dr. McCarthy is a co-author of Time : from Earth rotation to atomic physics, edited five books and authored numerous scientific articles, encyclopedia contributions. He was a Visiting Professor sponsored by the Japan Society for the Promotion of Science in 1979. He has served as a member of review panels, chairman of various international working groups, and member of the Directing Board of the International Earth Rotation Service. Dr. McCarthy is a member of the American Astronomical Society, International Astronomical Union, Sigma Xi and the American Geophysical Union. He is a fellow of the Royal Astronomical Society and the International Association of Geodesy.