New Moon - The Moon's unilluminated side is facing the Earth. The
Moon is not visible (except during a solar eclipse).
Waxing Crescent - The Moon appears to be partly but less than
one-half illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is increasing.
First Quarter - One-half of the Moon appears to be illuminated by
direct sunlight. The fraction of the Moon's disk that is illuminated is
increasing.
Waxing Gibbous - The Moon appears to be more than one-half but
not fully illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is increasing.
Full Moon - The Moon's illuminated side is facing the Earth. The
Moon appears to be completely illuminated by direct sunlight.
Waning Gibbous - The Moon appears to be more than one-half but
not fully illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is decreasing.
Last Quarter - One-half of the Moon appears to be illuminated by
direct sunlight. The fraction of the Moon's disk that is illuminated is
decreasing.
Waning Crescent - The Moon appears to be partly but less than
one-half illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is decreasing.
Following waning crescent is New Moon, beginning a repetition of the
complete phase cycle of 29.5 days average duration. The time in days
counted from the time of New Moon is called the Moon's "age". Each
complete cycle of phases is called a "lunation".
Because the cycle of the phases is shorter than most calendar
months, the phase of the Moon at the very beginning of the month
usually repeats at the very end of the month. When there are two Full
Moons in a month (which occurs, on average, every 2.7 years), the
second one is called a "Blue Moon".
Moon is not visible (except during a solar eclipse).
Waxing Crescent - The Moon appears to be partly but less than
one-half illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is increasing.
First Quarter - One-half of the Moon appears to be illuminated by
direct sunlight. The fraction of the Moon's disk that is illuminated is
increasing.
Waxing Gibbous - The Moon appears to be more than one-half but
not fully illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is increasing.
Full Moon - The Moon's illuminated side is facing the Earth. The
Moon appears to be completely illuminated by direct sunlight.
Waning Gibbous - The Moon appears to be more than one-half but
not fully illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is decreasing.
Last Quarter - One-half of the Moon appears to be illuminated by
direct sunlight. The fraction of the Moon's disk that is illuminated is
decreasing.
Waning Crescent - The Moon appears to be partly but less than
one-half illuminated by direct sunlight. The fraction of the Moon's disk
that is illuminated is decreasing.
Following waning crescent is New Moon, beginning a repetition of the
complete phase cycle of 29.5 days average duration. The time in days
counted from the time of New Moon is called the Moon's "age". Each
complete cycle of phases is called a "lunation".
Because the cycle of the phases is shorter than most calendar
months, the phase of the Moon at the very beginning of the month
usually repeats at the very end of the month. When there are two Full
Moons in a month (which occurs, on average, every 2.7 years), the
second one is called a "Blue Moon".
Lunar Phases
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What is Universal Time?
The times of various events, particularly astronomical and weather phenomena, are often given in
"Universal Time" (abbreviated UT) which is sometimes referred to, now colloquially, as "Greenwich Mean
Time" (abbreviated GMT). The two terms are often used loosely to refer to time kept on the Greenwich
meridian (longitude zero), five hours ahead of Eastern Standard Time. Times given in UT are almost
always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17
(2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z.
When a precision of one second or better is needed, however, it is necessary to be more specific about
the exact meaning of UT. For that purpose different designations of Universal Time have been adopted.
In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a
measure of the rotation angle of the Earth as observed astronomically. It is affected by small variations
in the rotation of the Earth, and can differ slightly from the civil time on the Greenwich meridian. Times
which may be labeled "Universal Time" or "UT" in data provided by the Astronomical Applications
Department of the U.S. Naval Observatory (for example, in the annual almanacs) conform to this
definition.
However, in the most common civil usage, UT refers to a time scale called "Coordinated Universal Time"
(abbreviated UTC), which is the basis for the worldwide system of civil time. This time scale is kept by
time laboratories around the world, including the U.S. Naval Observatory, and is determined using highly
precise atomic clocks. The International Bureau of Weights and Measures makes use of data from the
timing laboratories to provide the international standard UTC which is accurate to approximately a
nanosecond (billionth of a second) per day. The length of a UTC second is defined in terms of an atomic
transition of the element cesium under specific conditions, and is not directly related to any astronomical
phenomena.
UTC is the time distributed by standard radio stations that broadcast time, such as WWV and WWVH. It
can also be obtained readily from the Global Positioning System (GPS) satellites. The difference
between UTC and UT1 is made available electronically and broadcast so that navigators can obtain
UT1. UTC is the basis for civil standard time in the U.S. and its territories. Standard time within U.S. time
zones is an integral number of hours offset from UTC.
UTC is equivalent to the civil time for Iceland, Liberia, Morocco, Senegal, Ghana, Mali, Mauritania, and
several other countries. During the winter months, UTC is also the civil time scale for the United Kingdom
and Ireland.
One can think of UT1 as being a time determined by the rotation of the Earth, over which we have no
control, whereas UTC is a human invention. It is relatively easy to manufacture highly precise clocks that
keep UTC, while the only "clock" keeping UT1 precisely is the Earth itself. Nevertheless, it is desirable
that our civil time scale not be very different from the Earth's time, so, by international agreement, UTC
is not permitted to differ from UT1 by more than 0.9 second. When it appears that the difference
between the two kinds of time may approach this limit, a one-second change called a "leap second" is
introduced into UTC. This occurs on average about once every year to a year and a half.
Historical Note:
Greenwich Mean Time is a widely used historical term, but one that has been used in several ways.
Because of the ambiguity, its use is no longer recommended in technical contexts.
Prior to 1925, in astronomical and nautical almanacs, a day of Greenwich Mean Time began at noon.
This reckoning of Greenwich Mean Time is now called Greenwich Mean Astronomical Time, and is no
longer used. Persons using old editions of the almanacs for historical research should be aware of the
previous convention.
The times of various events, particularly astronomical and weather phenomena, are often given in
"Universal Time" (abbreviated UT) which is sometimes referred to, now colloquially, as "Greenwich Mean
Time" (abbreviated GMT). The two terms are often used loosely to refer to time kept on the Greenwich
meridian (longitude zero), five hours ahead of Eastern Standard Time. Times given in UT are almost
always given in terms of a 24-hour clock. Thus, 14:42 (often written simply 1442) is 2:42 p.m., and 21:17
(2117) is 9:17 p.m. Sometimes a Z is appended to a time to indicate UT, as in 0935Z.
When a precision of one second or better is needed, however, it is necessary to be more specific about
the exact meaning of UT. For that purpose different designations of Universal Time have been adopted.
In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a
measure of the rotation angle of the Earth as observed astronomically. It is affected by small variations
in the rotation of the Earth, and can differ slightly from the civil time on the Greenwich meridian. Times
which may be labeled "Universal Time" or "UT" in data provided by the Astronomical Applications
Department of the U.S. Naval Observatory (for example, in the annual almanacs) conform to this
definition.
However, in the most common civil usage, UT refers to a time scale called "Coordinated Universal Time"
(abbreviated UTC), which is the basis for the worldwide system of civil time. This time scale is kept by
time laboratories around the world, including the U.S. Naval Observatory, and is determined using highly
precise atomic clocks. The International Bureau of Weights and Measures makes use of data from the
timing laboratories to provide the international standard UTC which is accurate to approximately a
nanosecond (billionth of a second) per day. The length of a UTC second is defined in terms of an atomic
transition of the element cesium under specific conditions, and is not directly related to any astronomical
phenomena.
UTC is the time distributed by standard radio stations that broadcast time, such as WWV and WWVH. It
can also be obtained readily from the Global Positioning System (GPS) satellites. The difference
between UTC and UT1 is made available electronically and broadcast so that navigators can obtain
UT1. UTC is the basis for civil standard time in the U.S. and its territories. Standard time within U.S. time
zones is an integral number of hours offset from UTC.
UTC is equivalent to the civil time for Iceland, Liberia, Morocco, Senegal, Ghana, Mali, Mauritania, and
several other countries. During the winter months, UTC is also the civil time scale for the United Kingdom
and Ireland.
One can think of UT1 as being a time determined by the rotation of the Earth, over which we have no
control, whereas UTC is a human invention. It is relatively easy to manufacture highly precise clocks that
keep UTC, while the only "clock" keeping UT1 precisely is the Earth itself. Nevertheless, it is desirable
that our civil time scale not be very different from the Earth's time, so, by international agreement, UTC
is not permitted to differ from UT1 by more than 0.9 second. When it appears that the difference
between the two kinds of time may approach this limit, a one-second change called a "leap second" is
introduced into UTC. This occurs on average about once every year to a year and a half.
Historical Note:
Greenwich Mean Time is a widely used historical term, but one that has been used in several ways.
Because of the ambiguity, its use is no longer recommended in technical contexts.
Prior to 1925, in astronomical and nautical almanacs, a day of Greenwich Mean Time began at noon.
This reckoning of Greenwich Mean Time is now called Greenwich Mean Astronomical Time, and is no
longer used. Persons using old editions of the almanacs for historical research should be aware of the
previous convention.
