the-astrology-book

Sidereal Day
SIDEREAL DAY
A sidereal day is the period of time it takes for Earth to complete one rotation on its
axis with respect to a fixed point in space. Specifically, a sidereal day begins and ends
when the local meridian for any given location on Earth passes through 0° Aries (the
vernal point). Because of the motion of Earth around the Sun, sidereal days are slightly
shorter than ordinary solar days. A sidereal day is 23 hours 56 minutes and 4.09 seconds
in length; a sidereal hour is 1 ⁄ 24 the length of a sidereal day.
SIDEREAL MONTH
A sidereal month is the period of time it takes the Moon to complete an orbit of Earth
with respect to a fixed point in space, specifically, with respect to a fixed star (hence
the designation sidereal, from the Greek sidus, meaning “star”). Because of the motion
of Earth around the Sun, sidereal months are shorter than months measured from one
new moon to the next. A sidereal month is 27 days 7 hours 43 minutes and 11.5 seconds
in length.
SIDEREAL PERIOD
A sidereal period is the time it takes a celestial body such as a planet to complete an
orbit, as measured against the background of the fixed stars. Sidereal months (the time
it takes the Moon to complete an orbit) and sidereal years (the time it takes Earth to
complete an orbit) are examples of sidereal periods.
SIDEREAL TIME
Sidereal (from the Greek sidus, meaning “star”) time, like most ordinary measurements
of time, is based on the rotational and orbital motion of Earth. However, unlike
other ways of measuring the passing of time, sidereal time uses a fixed point in space
(usually one of the fixed stars; hence the name sidereal) as a point of reference for the
beginning and ending of a day, month, or year. By way of contrast, ordinary days and
years, as well as lunar months (from one new moon to the next), use the constantly
changing, relative positions of the Sun, the Moon, and Earth. As a result, there are
slight differences in length between sidereal days, months, and years and ordinary
days, months, and years. Sidereal time, which is also employed by astronomers, is used
in tables of planetary positions (ephemerides) as well as tables of houses. The first step
in casting a natal chart is to convert birth time to sidereal time.
SIDEREAL YEAR
A sidereal year is the time it takes Earth to complete an orbit of the Sun relative to
the fixed stars. The length of a sidereal year is 365 days 6 hours 9 minutes and 9.54
seconds, which is slightly longer than a solar year.
[612] THE ASTROLOGY BOOK