The subdivision of a day?

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Nathan

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what time to what time is morning? what time to what time is afternoon? what time to what time is night?
 
As another answer indicates there is a definition of am and pm. However if you look back through history it's fairly easy to get a general consensus of what constitutes parts of the day. And although it has altered throughout the ages, people have generally settled on the following. The day is divided into four equal parts, morning (6am to 12pm) afternoon (12pm to 6pm) evening (6pm to 12am) night (12am to 6am)
 
ante meridian (a.m.) means "before the line" which means the Sun is heading toward the meridian which is the line that marks the highest point of a celestial object which for the Sun is noon. post meridian (p.m.) means "after the line" which is when the Sun passes noon and loses altitude. The division points are noon and midnight (which are not a.m. or p.m. since the Sun is ON the line and can neither be before or after.)
 
The answer is cultural, not scientific. To an astronomer, a day is defined as the time from noon to noon at any point on the earth's equator. "Night" is not a relevant astronomical term, except as needed locally to know when the stars will be visible.

Farm communities define morning from sunrise to midday (sun highest overhead) and afternoon from noon to sunset. Urban people are more likely to have arbitrary time definitions, regardless of the sun.

Some cultures have "evenings" which runs from an arbitrary time in the late afternoon to some time before midnight.

At the poles, at the solstices, there are either light or dark 24 hours, so no way to determine "morning" or "noon" except arbitrarily by reference to a clock.
 
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