Sonoma State University
Astronomy 100
J.S. Tenn

Natural time divisions

Time is nature’s way of keeping everything from happening at once

Day = Period of earth’s rotation with respect to Sun.

Varies slightly in length.

Can start at sunrise (Egyptians), sunset (Babylonians, early Greeks), noon (later Greeks), midnight (Romans, Chinese)

Month = Period of Moon’s phases

Varies in length.

Can start at new moon, full moon, etc.

Astronomers measure from new moon, when the moon is aligned with the sun and not visible.

Islamic, Jewish month begins 1 - 3 days later, when a crescent is first seen in the evening.

Year = Period of the seasons

Can start at or near spring equinox, summer solstice, fall equinox, winter solstice, etc.

Iranian calendar begins at spring equinox.

Gregorian calendar begins near winter solstice.

Jewish calendar begins near fall equinox, but formerly began near spring equinox.

Next: The Calendar Problem
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