- A complete rotation of the Earth
relative to the
observable universe
(or to good approximation the fixed stars) is
a
sidereal day = 86164.0905 s
= 1 day - 4 m + 4.0905 s (on average).
This is the Earth's rotation period:
i.e.,
true physical rotation period relative to
observable universe
(i.e., its absolute rotation period).
Note that the fixed stars define
a reference frame
with very low
absolute rotation
(i.e., very low rotation relative to
the observable universe)
and are used in most practical measurements of
absolute rotation
and this was done long before
our modern understanding of
absolute rotation.
This is where the adjective
"sidereal" (meaning relative to
fixed stars) comes from.
- On the other hand, the solar day is
solar noon to
solar noon and is
about 4
minutes
longer than the sidereal day because of
the revolution of the Earth
about the Sun as one can see in the
diagram.
- The fact that the sidereal day is
∼ 4 minutes shorter than the
solar day means, among other things,
that all
stars that are NOT
circumpolar stars rise
∼ 4 minutes earlier every day.
In a year, they cycle back to rising at the
same time.
Yours truly
mnemonicks this daily advance by the mnemonic:
The stars rise earlier every day.
- Sidereal time is defined
analogously to ordinary
timekeeping:
24
sidereal hours
in a sidereal day,
60
sidereal minutes
in a sidereal hour,
60
sidereal seconds
in a sidereal minute,
etc..
Sidereal time
is used in astronomy
since astronomy
has to keep track of the
motion of the
celestial sphere.
In the old days,
observatories
would have a standard time
clock
and a sidereal clock
superimposed somehow or mounted side-by-side.
But nowadays, the
computer tells
us everything
(e.g.,
Juergen Giesen: sidereal clock;
USNO: Compute Local Apparent Sidereal Time).
- By the by,
the mean solar day is increasing
with time due to tidal acceleration
caused by the Moon.
The rate of increase is about 1.70(5)
milliseconds
per
century based on historical records
since circa 700 BCE
(i.e., the last 2700 years)
(see Wikipedia: Day: Leap seconds;
Wikipedia:
Leap second: Slowing rotation of the Earth).
- There is also a
sidereal year = 365.256363004 days (J2000)
which is the rotation period
of the Earth in its
orbit around
the Sun
relative to the
observable universe.
The solar year = 365.2421897 days (J2000),
which determines the seasons,
is a bit shorter due to the
axial precession
of the Earth.
The Gregorian calendar,
of course, alternates
common years = 365 days
and
leap years =366 days
(which are made of integer numbers of the
standard metric day
= 24 h = 86400 s)
in such away that
the Gregorian calendar
synchronizes with the
count of solar years
on average to high
accuracy/precision:
i.e., 3
common years
then a leap year, except
centurial years
are NOT
leap years, unless
evenly divisible by
400: e.g.,
year 1900 was NOT
a leap year, but
year 2000 was a
leap year.
The
Julian year = 365.25 standard
metric days exactly by definition is slightly longer than the
average year
of the Gregorian calendar.