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Our evolving Universe has four essential constituents: matter, radiation, space and time. All of them may be measured with great precision. Measurement of time. Any regularly repeated phenomenon can be used to record the passage of time. Nature has given to our ancestors three phenomena as units of time: One Day (period of alternation of sunshine and night) is given by the rotation of the Earth around its axis. A solar day lasts about 4 minutes longer than one rotation with respect to the stars (about one degree more than 360 degrees). The centre of the solar disc (true sun) moves on the ecliptic with varying speed - faster in winter, slower in summer. Therefore a fictitious point - mean sun - was introduced by astronomers, which moves with constant speed on the celestial equator. The mean sun and the true sun meet once a year at the vernal point. One revolution of the Moon around the Earth (= lunation) was used as a month and its quarter as a week. New moon, first quarter, full moon a

ESO European Organisation for Astronomical Research in the Southern Hemisphere" /> European Southern Observatory ESO - Reaching New Heights in Astronomy --> Public Science User Portal Subscribe Contact Site Map Go! 22 Jul 2026 ESO for the Public > Outreach > Education jump to navigation jump to content Venus Transit 2004 Introduction News Archive Final Event Background Brief InfoSheets Extended InfoSheets Observing Campaign Results Safety! Students and Teachers Media Amateurs Brandys Meeting VT-2004 Network In Your Region How to Participate Video Contest Gallery Writings Photo Archive Animatio

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