Astronomy is the science of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as auroras and cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe. The word astronomy literally means 'law of the stars' and is derived from the Greek , astronomia, from the words (astron, 'stars') and (nomos, 'law').

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A day (symbol: d) is a unit of time equivalent to 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second. The term comes from the Old English dg, with similar terms common in all other Indo-European languages, such as dies in Latin and dive in Sanskrit. Contents 1 Definitions 1.1 International System of Units (SI) 1.2 Astronomy 1.3 Colloquial 2 Introduction 3 Civil day 4 Leap seconds 5 Astronomy 6 Boundaries of the day 7 Metaphorical days

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