JMSA conducts regular tidal observations at 28 tide stations throughout the nation to improve the forecasting accuracy of tide tables and to determine the datum level of nautical charts. In FY 1995, it completed the equipping of all its tide stations, except those on Okinotori Shima, with telemeters and established a real-time concentrated observation system.
(6) Observation of Tidal Streams and Ocean Currents
Straits and sea areas within bays, in addition to being heavily trafficked by ships, are also subject to the development of complex tidal streams. As a result, accurate tidal stream information is essential to ship navigation and the execution of construction work. Furthermore, complex currents can occur in coastal areas facing the open sea, because these areas are affected by ocean currents and the oceanographic conditions of the open sea. It is for this reason that JMSA must carry out observations and analyses of these tidal streams and coastal currents, and provide this information through tide tables and other means.
Therefore, in FY 1995, JMSA conducted observations of tidal streams and coastal currents in 24 sea areas, such as the Notsuke Channel. During FY 1996, it is conducting observations in 24 sea areas, such as the Tsugaru Strait area.
(7) Observation of Occultation and Grazing Occultation of Stars by the Moon (*Note 2)
Even at present, when positioning systems using radio waves are common, it is necessary to determine position through the use of celestial bodies. These position determinations are made using various nautical almanacs, such as the Nautical Almanac. These are published based on the Japanese Ephemeris, the most detailed almanac in Japan. In order to accurately measure the lunar orbit, which is required to maintain the accuracy of these kinds of nautical almanacs, hydrographic observatories at the JMSA regional headquarters, Shirahama (Shizuoka Prefecture), Shimosato (Wakayama Prefecture), and Bisei (Okayama Prefecture) conduct occultation observations regularly. In FY 1995, JMSA conducted observations of grazing occultation of stars by the moon at locations such as Niigata. During FY 1996, it is conducting observations of grazing occultation of stars by
*Note 2: Occultation and Grazing Occultation of Stars by the Moon
When viewed from the earth, an occultation is the phenomenon whereby a fixed star (the sun) is hidden from view by the moon, as a result of the moon orbiting. A grazing occultation is the phenomenon seen at the very limit of the region from where an occultation can be observed, It is a phenomenon whereby a fixed star flickers because of the irregularity of the moon's outline.
It is possible to determine the moon's orbit precisely and maintain and improve the accuracy of astronomical almanacs through the observation and analyses of occultation and grazing occultation.
