What is a day?
And when does it begin?

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solar/synodic day versus sidereal day•

1→3: Earth repoints to thesun (rotation >360°)•A synodic or solar day is thetime between successive
astronomical noons. A meansolar day is 24 hours long.
The earth has to rotate morethan 360° for the sun to comeback to “noon”.

•

1→2: Earth rotates 360°

•One sidereal day is the time it takes for a star in the sky to come back to thesame place in the sky. For most purposes, the sky is “ xed”, so a sidereal
day is when the earth rotates 360°. A sidereal day is 23 hours 56 minutes
and 4.09 seconds long.

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using asundial:solar orastronomical noon
The shadow is shortest at “high noon”,i.e., when the sun is passing throughthe meridian and is at its highest point
during the day. This is also called“astronomical noon” or “solar noon”.

3meridian = noon = 正午 shōgo•

In local solar timekeeping, “12 noon” is the moment
when the sun is at its highest point in the sky.

•PM = p.m. = post meridian “after noon” = 午後 gogo

•AM = a.m. = ante meridian “before noon” = 午前 gozen•

11:59 a.m. < 12:00 noon < 12:01 p.m.
11:59 p.m. < 12:00 midnight < 12:01 a.m.

•

“12 a.m.” and “12 p.m.” are nonsense, because
the precise moment of 12 o’clock is neither before
noon nor after noon. When businesses incorrectly
write “12 AM” or “12 PM”, it can be very confusing.

4local time, mean time, GMT, and UTC•Local solar clock time is based on setting a clock to 12 at the moment of local
solar noon. This means that the local solar time in Tokyo is about 16 minutes earlier
than the local solar time in Kyoto.

•Mean clock time is based on dividing the world into time zones, where the clocks
in each time zone are all set to the same time. In the JST (Japan Standard Time)time zone, 12 noon in Tokyo and and 12 noon Kyoto are the same time.

•GMT (Greenwich Mean Time) is the mean time zone of London during the winter.
The IERS Reference Meridian (0° 00′ 00″ longitude) is historically related to themeridian calculated at the Greenwich Royal Observatory in England by George Airyin 1851 (currently at 0° 00′ 05.3101″ W longitude).

•UTC (Coordinated Universal Time) is the international standard for regulating clocktime. It is within about 1 second of mean solar time at the IERS Reference Meridian(0° 00′ 00″ longitude). GMT is the UTC 0 hour. In Japan, JST is UTC+9 hours.

5Mean Time Zones

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sunset in Cairo

When does the day begin?
When we awake?
When the sun comes up?
At noon?
When the sun sets?
At midnight?
sunrise in Tokyo
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Does the day begin at sunrise? or perhaps dawn?
•Hindus consider the Brahma
Muhūrta (04:24–05:12 AM on
the day of the spring equinox)
to be the best time to wake up.

•In US military tradition, BMNT(begin morning nauticaltwilight), i.e., nautical dawn, isthe time after which an attack
can begin.

•Some scholars have argued
that the Biblical day began atsunrise (cf. J. Amanda
McGuire, “Evening or morning:When does the Biblical daybegin?”, Andrews University
Seminary Studies, Vol. 46, No.
2, pp. 201-214).

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The Jewish day begins atnightfall and lasts until the nextnightfall.

•Jews often treat twilight asneither day or night. Jews
often begin their avoidance ofSabbath taboos just before
sunset on Friday and end theiravoidance after twilight on
Saturday, about a 25-hour
period.

•The Islamic day begins atsunset. The Maghrib prayer,
which is prayed immediatelyafter sunset, is the rst of theve obligatory daily prayers.

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Beginning the day at nightfall or sunset

The secular day starts at midnight.
•Astronomers start and end their days at noon.

•For all other scienti c and secular purposes, the daybegins and ends at midnight (i.e., 0:00 or 24:00).

•In accord with this international standard, the Japanese
civil day begins and ends at midnight.

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a Jewish/Hebrew birthday•

The Hebrew Calendar is important for Jews calculating birthdays,
death dates, and the dates of a bar or bat mitzva.

•For a child born after nightfall but before midnight on the Gregoriandate 31 December 2019, which was a Tuesday, the child’s Hebrew
birthday is actually Wednesday (which for Jews begins at sunset
on Tuesday), the 4th day of Tevet, Anno Mundi 5780 (which ended
at nightfall on 1 January 2020). The following year, Anno Mundi
5781, the 4th day of Tevet was on the Gregorian date 19December 2020. It will be a long time before the child’s Gregorian
Calendar birthday (31 December) and the Hebrew Calendar
birthday (the 4th day of Tevet) are on the same day, because the4th day of Tevet will not be on 1 January again until 2066.
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