The new moon phase
In astronomical terminology, the phrase
new moon
is the
lunar phase that occurs when the
Moon, in its monthly orbital motion around
Earth, lies between Earth and the
Sun, and is therefore in
conjunction with the
Sun as seen from Earth. At this time, the dark (unilluminated)
portion of the Moon faces almost directly toward Earth, so that the
Moon is not visible to the naked eye.
The original meaning of the phrase
new moon was the first
visible
crescent of the Moon, after
conjunction
with the Sun. This takes place over the western
horizon in a brief period between
sunset and moonset, and therefore the precise time
and even the date of the appearance of the new moon by this
definition will be influenced by the geographical location of the
observer. The astronomical new moon, sometimes known as the
dark moon to avoid confusion,
occurs by definition at the moment of conjunction in
ecliptic longitude with the Sun, when the
Moon is invisible from the Earth. This moment is unique and does
not depend on location, and under certain circumstances it may be
coincident with a solar
eclipse.
The new moon in its original meaning of first crescent marks the
beginning of the
month in
lunar calendars such as the
Muslim calendar, and in
lunisolar calendars such as the
Hebrew calendar,
Hindu calendars, and
Buddhist calendar. But in the
Chinese calendar the beginning of the month
is marked by the dark moon.
Determining new moons: an approximate formula
The time interval between new moons—a
lunation—is
variable. The mean time between new moons, the
synodic month, is about 29.53... days. An
approximate formula to compute the mean moments of new moon
(
conjunction
between Sun and Moon) for successive months is:
- d = 5.597661 + 29.5305888610 \times N + (102.026 \times
10^{-12})\times N^2
where
N is an integer, starting with 0 for the
first new moon in the year 2000, and that is incremented by 1 for
each successive synodic month; and the result
d is
the number of days (and fractions) since 2000-01-01 00:00:00
reckoned in the time scale known as
Terrestrial Time (
TT) used
in
ephemerides.
To obtain this moment expressed in
Universal Time (
UT, world
clock time), add the result of following approximate correction to
the result
d obtained above:
- -0.000739 - (235 \times 10^{-12})\times N^2 days
Periodic perturbations change the time of true conjunction from
these mean values. For all new moons between 1601 and 2401, the
maximum difference is 0.592 days = 14h13m in either direction. The
duration of a lunation (
i.e. the time from new moon to the
next new moon) varies in this period between 29.272 and 29.833
days, i.e. −0.259d = 6h12m shorter, or +0.302d = 7h15m longer than
average . This range is smaller than the difference between mean
and true conjunction, because during one lunation the periodic
terms cannot all change to their maximum opposite value.
See the article on the
full moon
cycle for a fairly simple method to compute the moment of new
moon more accurately.
The long-term error of the formula is approximately: 1 cy² seconds
in TT, and 11 cy² seconds in UT (
cy is centuries since
2000; see section
Explanation of the formulae for
details.)
Explanation of the formula
The moment of mean conjunction can easily be computed from an
expression for the mean ecliptic longitude of the Moon minus the
mean ecliptic longitude of the Sun (Delauney parameter
D).
Jean Meeus gave
formulae to compute this in his popular
Astronomical Formulae
for Calculators based on the ephemerides of Brown and Newcomb
(ca. 1900); and in his 1st edition of
Astronomical
Algorithms formula 47.1 in Jean Meeus (1991):
Astronomical
Algorithms (1
^{st} ed.) ISBN 0-943396-35-2 based on
the ELP2000-85 (the 2
^{nd} edition uses ELP2000-82 with
improved expressions from Chapront
et al. in 1998). These
are now outdated: Chapront
et al. (2002) published
improved parameters. Also Meeus's formula uses a fractional
variable to allow computation of the four main phases, and uses a
second variable for the secular terms. For the convenience of the
reader, the formula given above is based on Chapront's latest
parameters and expressed with a single integer variable, and the
following additional terms have been added:
constant term:
- Sun: +20.496"
- Moon: −0.704"
- Correction in conjunction: −0.000451 days.
- For UT: at 1 January 2000, ΔT (=
TT − UT ) was
+63.83 s ; hence the correction for the clock time UT = TT − ΔT of
the conjunction is:
- −0.000739 days.
quadratic term:
- In ELP2000–85 (see Chapront et alii 1988),
D has a quadratic term of −5.8681"T²; expressed in
lunations N, this yields a correction of +87.403 N² days to the
time of conjunction. The term includes a tidal contribution of 0.5×(−23.8946
"/cy²). The most current estimate from Lunar Laser Ranging for the
acceleration is (see Chapront et alii 2002): (−25.858
±0.003)"/cy². Therefore the new quadratic term of
D is = -6.8498"T² . Indeed the polynomial provided
by Chapront et alii (2002) provides the same value (their
Table 4). This translates to a correction of +14.622 N² days to the
time of conjunction; the quadratic term now is:
- +102.026 N² days.
- For UT: analysis of historical observations show that ΔT has a
long-term increase of +31 s/cy² . Converted to days and lunations ,
the correction from ET to UT becomes:
- −235 N² days.
The theoretical tidal contribution to ΔT is about +42 s/cy² ; the
smaller observed value is thought to be mostly due to changes in
the shape of the Earth . Because the discrepancy is not fully
explained, uncertainty of our prediction of UT (rotation angle of
the Earth) may be as large as the difference between these values:
11 s/cy². The error in the position of the Moon itself is only
maybe 0.5"/cy² , or (because the apparent mean angular velocity of
the Moon is about 0.5"/s), 1 s/cy² in the time of conjunction with
the Sun.
Religious use
- The Islamic calendar has
retained an observational definition of the new moon, marking the
new month when the first Crescent Moon is actually seen, and making
it impossible to be certain in advance of when a specific month
will begin (in particular, the exact date on which Ramadan will begin is not known in
advance). In Saudi Arabia, if the weather is cloudy when the new moon is
expected, observers are sent up in airplanes. In Pakistan, there is a
"Central Ruet-e-Hilal
Committee" comprises of Ulemas (Religious Scholars), which
takes help from 150 Observatories of Pakistan Meteorological
Department all over the country and announces decision of the
sighting of new moon. In Iran a special
committee receives observations of every new moon to determine the
beginning of each month. This committee uses one hundred
groups of observers.
Recently an attempt to unify Muslims on a scientifically calculated
worldwide calendar has been adopted by both the
Fiqh Council of North America
and
European
Council for Fatwa and Research. The new calculation requires
that conjunction occur before sunset in Mecca, Saudi Arabia and
that moon set on the following day must take place after sunset.
These can be precisely calculated and therefore a unified calendar
is imminent if it becomes adopted worldwide.
- The new moon is quite significant in Hindu
(Indian) calendar. It is believed that new moon can create several
fluctuations(negative) in the mental plane. Goddess Kali is worshipped on new moon night to relax these
fluctuations.
- The new moon is the beginning of the month
in the Chinese calendar. Some Buddhist Chinese keep a vegetarian
diet on the new moon and full moon each month.
- The native messianic Pentecostal group, the New Israelites of Peru, keeps the new
moon as a Sabbath of rest. As an evangelical church, it follows the
Bible's teachings that God
sanctified the seventh-day
Sabbath, now largely known as Saturday,
and the new moons in addition to it. See Ezekiel 46:1, 3. No work
may be done from dusk until dusk, and the services run for 11
hours, although a large number spend 24 hours within the gates of
the temples, sleeping and singing praises throughout the
night.
- The Creation Seventh Day Adventist church
keeps the New Moon in a similar fashion to Sabbath observance in
that buying, selling, and secular labor are understood as
violations of the instructions attached to it in Scripture.
- The new moon is also an important event in Wicca.
See also
References
External links