Soldøgn

Et middelsoldøgn er per definition eksakt 24 timer 0 minutter 0 sekunder. I løbet af et år er middelsoldøgnet det gennemsnitlige døgn mellem to påfølgende gange solen kulminerer (står nøjagtig i syd; fx fra sommersolhverv til sommersolhverv). Ifølge koordineret universaltid er et sekund defineret til at være 9.192.631.770 periodetider af en bestemt stråling fra cæsium-133-atomer (Cs).[1]

Et sandt soldøgn er den faktiske tiden mellem to påfølgende gange den virkelige sol kulminerer. Længden af dette sande soldøgn varierer lidt fra dag til dag og er næsten aldrig nøjagtig lig 24 timer.

Referencer

  1. ^ copradar.com: Coordinated Universal Time Citat: "...Mean solar time -- a measurement of the Earth's rotation with respect to the 'fictitious' mean sun...second -- One second (1 s or 1 sec.) is the duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom (Cs)...UT1 -- UT0 corrected for polar motion (widely used)...UTC -- civil atomic time scale on or about the Greenwich meridian kept within 0.9 seconds of UT1...", backup
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