ABRUPT CHANGES OF THE EARTH’S ROTATION SPEED
M. SÔMA and K. TANIKAWANational Astronomical Observatory of JapanMitaka, Tokyo 181-8588, Japane-mail: Mitsuru.Soma@nao.ac.jp, tanikawa.ky@nao.ac.jp
ABSTRACT. In our recent work using ancient solar eclipse records we showed that the Earth’srotation rate changed abruptly in about AD 900 (Sôma and Tanikawa 2005). We show here thatmore abrupt changes in the Earth’s rate of rotation occurred in about AD 500.
For the past few years we have been deriving the changes of the Earth’s rate of rotation using
the solar eclipse records in ancient times (Tanikawa and Sôma 2002, 2004, Kawabata et al. 2004,Sôma et al. 2003, 2004), and in Journées 2004 we showed that the Earth’s rotation rate changedabruptly in about AD 900 so that the ∆T (TT − UT) values decreased between the years AD
873 and 912 by more than 600 sec (Sôma and Tanikawa 2005). We concentrate here the changesin about AD 500.
2. CHANGE OF THE EARTH ROTATION RATE AROUND AD 500
Sôma et al. (2004) deduced the following ranges of the possible ∆T values from the multiple
On 454 Aug 10 there was a solar eclipse in China, and it was recorded as total. As discussed
by Stephenson (1997, p. 242), this record was misplaced one calendar year. It can be assumedthat this eclipse was observed at Jiankang (Chien-k’ang), the capital at the time.
The 484 Jan 14 solar eclipse was recorded at Athens. The record says that the day was turned
into night and the darkness was deep enough for the stars to become visible, and therefore it isclear that the eclipse was total at Athens.
From the above two records, the range of the possible ∆T values can be obtained as follows:
Fig. 1 shows the variation of the ∆T values in around AD 500. The figure clearly shows
that the speed of the Earth’s rotation increased abruptly in around AD 450, and it graduallydecreased until about AD 600.
3. REFERENCESKawabata K., Tanikawa K., and Sôma M., 2004, “TT−UT in the Seventh Century Derived from
Astronomical Records in the Nihongi, the Suishu, and the Jiu- and Xin-Tangshu”, in Astro-nomical Instruments and Archives from the Asia-Pacific Region, W. Orchiston, F.R. Stephen-son, S. Debarbat, and I.-S. Nha (eds.), pp. 113–118, Yonsei University Press, Seoul.
Sôma M., Tanikawa K., and Kawabata K., 2003, “Earth’s Rotation in the 7th Century Derived
from Eclipse Records in Japan and in China”, in Proceedings of the “Journées 2002 Systèmesde Réfŕence Temps Espace” (Astronomy from Ground and from Space), N. Capitaine andM. Stavinschi (eds.), pp. 248–250, Bucharest.
Sôma M., Tanikawa K., and Kawabata K., 2004, “Earth’s Rate of Rotation between 700 BC and
1000 AD Derived from Ancient Solar Eclipses”, in Proceedings of the “Journées 2003 Systèmsde Référence Spatio-Temporels” (Astrometry, Geodynamics and Solar System Dynamics: frommilliarcseconds to microarcseconds), A. Finkelstein and N. Capitaine (eds.), pp. 122–127, St. Petersburg.
Sôma M. and Tanikawa K., 2005, “Variation of Delta T between AD 800 and 1200 Derived from
Ancient Solar Eclipse Records”, in Proceedings of the “Journées 2004 Systèmes de RéférenceSpatio-Temporels” (Fundamental Astronomy: New concepts and models for high accuracyobservations), N. Capitaine (ed.), pp. 265–266, Paris.
Stephenson, F.R., 1997, Historical Eclipses and Earth’s Rotation, Cambridge University Press. Tanikawa K. and Sôma M., 2002, “Reliability of the Totality of the Eclipse in AD 628 in the
Nihongi”, Astronomical Herald, 95, pp. 27–37 (in Japanese).
Tanikawa K. and Sôma M., 2004, “∆T and the Tidal Acceleration of the Lunar Motion from
Eclipses Observed at Plural Sites”, Publ. Astron. Soc. Japan, 56, pp. 879–885.
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From the above two records, the range of the possible ∆T values can be obtained as follows:
Fig. 1 shows the variation of the ∆T values in around AD 500. The figure clearly shows
that the speed of the Earth’s rotation increased abruptly in around AD 450, and it graduallydecreased until about AD 600.