What is the period of the Earth's rotation around its axis?
As a rule, answer to this question is that that one turn around the axis of the Earth requires, approximately, 24 hours and that this period is cold the mean solar day. Either, more precisely, some four minutes shorter stellar day.
In fact, neither of these days stands for the Earth's full spin period. Earth turns around its axis considerably more slowly. The only frame of reference in which its intrinsic rotation is clearly defined, while its orbital motions are excluded, has to be the one related with the Moon. The full spin period of our planet is about 50 minutes longer lunar day – the time between one lunar zenith to the next. The term tidal day is also in use in oceanography.
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Koliko je vremena potrebno da se Zemlja jedanput obrne oko svoje ose?
Odgovor na to pitanje je, po pravilu, da njena puna rotacija traje, približno, 24 časa i da se taj period vremena zove srednji solarni dan. Uz to se obično dodaje da je tačno vreme te rotacije nekih četiri minuta kraći stelarni dan.
Ni jedan, ni drugi odgovor, međutim, nisu ispravni. Zemlja se obrće oko svoje ose znatno sporije. Jedini koordinatni sistem u kojem je njena sopstvena rotacija jasno definisana i u kojem su njena orbitalna kretanja isključena, mora biti vezan za Mesec. Period punog spina naše planete je, otprilike 50 minuta duži lunarni dan, koji se koristi u okeanografiji i meri od jednog do drugog Mesečevog zenita.