Sunset time

 

Sunset Time Complete Sun and Moon Data for One Day

sunset behind cloudsYou can obtain the times of sunrise, sunset, moonrise, moonset, transits of the Sun and Moon, and the beginning and end of civil twilight, along with information on the Moon’s phase by specifying the date and location in the form below and clicking on the “Get data” button at the end of the form.


Month:Day:Year:

State or Territory:

City or Town Name:

The place name you enter above must be a city or town in the U.S.
The place’s location will be retrieved from a file with over 22,000
places listed. Either upper- or lower-case letters or a combination can
be used. Spell out place name prefixes, as in “East Orange”, “Fort
Lauderdale”, “Mount Vernon”, etc. The only exception is “St.”, which is
entered as an abbreviation with a period, as in “St. Louis”. You need
only enter as many characters as will unambiguously identify the
place.

 


Rise, Set, and Twilight Definitions

Horizon: Wherever one is located on or near the
Earth’s surface, the Earth is perceived as essentially flat and,
therefore, as a plane. The sky resembles one-half of a sphere or
dome centered at the observer. If there are no visual
obstructions, the apparent intersection of the sky with the
Earth’s (plane) surface is the horizon, which appears as a circle
centered at the observer. For rise/set computations, the
observer’s eye is considered to be on the surface of the Earth,
so that the horizon is geometrically exactly 90 degrees from the
local vertical direction.

 

Rise, Set: During the course of a day the Earth
rotates once on its axis causing the phenomena of rising and
setting. All celestial bodies, stars and planets included, seem
to appear in the sky at the horizon to the East of any particular
place, then to cross the sky and again disappear at the horizon
to the West. The most noticeable of these events, and the most
significant in regard to ordinary affairs, are the rising and
setting of the Sun and Moon. Because the Sun and Moon appear as
circular disks and not as points of light, a definition of rise
or set must be very specific, for not all of either body is seen
to rise or set at once.

Sunrise and sunset conventionally refer to the
times when the upper edge of the disk of the Sun is on the
horizon, considered unobstructed relative to the location of
interest. Atmospheric conditions are assumed to be average, and
the location is in a level region on the Earth’s surface.
Moonrise and moonset times are computed for
exactly the same circumstances as for sunrise and sunset.
However, moonrise and moonset may occur at any time during a 24
hour period and, consequently, it is often possible for the Moon
to be seen during daylight, and to have moonless nights. It is
also possible that a moonrise or moonset does not occur relative
to a specific place on a given date.

 

Transit: The transit time of a celestial body refers
to the instant that its center crosses an imaginary line in the
sky – the observer’s meridian – running from north to south. For
observers in low to middle latitudes, transit is
approximately midway between rise and set, and represents
the time at which the body is highest in the sky on any given day.
At high latitudes, neither of these statements may be true – for
example, there may be several transits between rise and set.
The transit of the Sun is local solar (sundial) noon. The difference
between the transit times of the Sun and Moon is closely related to the
Moon’s phase. The New Moon transits at about the same time as the Sun;
the First Quarter Moon transits about 6 hours after the Sun;
the Full Moon transits about 12 hours after/before the Sun; and
the Last Quarter Moon transits about 6 hours before the Sun.

 

Twilight: Before sunrise and again after sunset
there are intervals of time, twilight, during which there is
natural light provided by the upper atmosphere, which does
receive direct sunlight and reflects part of it toward the
Earth’s surface. Some outdoor activities may be conducted
without artificial illumination during these intervals, and it is
useful to have some means to set limits beyond which a certain
activity should be assisted by artificial lighting. The major
determinants of the amount of natural light during twilight are
the state of the atmosphere generally and local weather
conditions in particular. Atmospheric conditions are best
determined at the actual time and place of events. Nevertheless,
it is possible to establish useful, though necessarily
approximate, limits applicable to large classes of activities by
considering only the position of the Sun below the local horizon.
Reasonable and convenient definitions have evolved.

Civil twilight is defined to begin in the
morning, and to end in the evening when the center of the Sun is
geometrically 6 degrees below the horizon. This is the limit at
which twilight illumination is sufficient, under good weather
conditions, for terrestrial objects to be clearly distinguished;
at the beginning of morning civil twilight, or end of evening
civil twilight, the horizon is clearly defined and the brightest
stars are visible under good atmospheric conditions in the
absence of moonlight or other illumination. In the morning
before the beginning of civil twilight and in the evening after
the end of civil twilight, artificial illumination is normally
required to carry on ordinary outdoor activities. Complete
darkness, however, ends sometime prior to the beginning of
morning civil twilight and begins sometime after the end of
evening civil twilight.

 

Nautical twilight is defined to begin in the
morning, and to end in the evening, when the center of the sun is
geometrically 12 degrees below the horizon. At the beginning or
end of nautical twilight, under good atmospheric conditions and
in the absence of other illumination, general outlines of ground
objects may be distinguishable, but detailed outdoor operations
are not possible, and the horizon is indistinct.

 

Astronomical twilight is defined to begin in the
morning, and to end in the evening when the center of the Sun is
geometrically 18 degrees below the horizon. Before the beginning
of astronomical twilight in the morning and after the end of
astronomical twilight in the evening the Sun does not contribute
to sky illumination; for a considerable interval after the
beginning of morning twilight and before the end of evening
twilight, sky illumination is so faint that it is practically
imperceptible.

 


Technical Definitions and Computational Details

Sunrise and sunset. For computational purposes,
sunrise or sunset is defined to occur when the geometric zenith
distance of center of the Sun is 90.8333 degrees. That is, the
center of the Sun is geometrically 50 arcminutes below a
horizontal plane. For an observer at sea level with a level,
unobstructed horizon, under average atmospheric conditions, the
upper limb of the Sun will then appear to be tangent to the
horizon. The 50-arcminute geometric depression of the Sun’s
center used for the computations is obtained by adding the
average apparent radius of the Sun (16 arcminutes) to the average
amount of atmospheric refraction at the horizon (34
arcminutes).

Moonrise and moonset. Moonrise and moonset
are defined similarly, but the situation is computationally more
complex because of the nearness of the Moon and the eccentricity
of its orbit. If the computations are carried out using
coordinates of the Moon with respect to the Earth’s center (the
usual method), then moonrise or moonset is defined to occur when
the geometric zenith distance of the center of the Moon is

90.5666 degrees + Moon’s apparent angular radius

– Moon’s horizontal parallax

Under normal atmospheric conditions at sea level, the upper
limb of the Moon will then appear to be tangent with a level,
unobstructed horizon. No account is taken of the Moon’s phase;
that is, the Moon is always regarded as a disk in the sky and the
upper limb might be dark. Here again, a constant of 34
arcminutes (0.5666 degree) is used to account for atmospheric
refraction. The Moon’s apparent radius varies from 15 to 17
arcminutes and its horizontal parallax varies from 54 to 61
arcminutes. Adding all the terms above together, the center of
the Moon at rise or set is geometrically 5 to 10 arcminutes above
the observer’s “geocentric horizon” – the horizontal plane that passes through
the Earth’s center, orthogonal to the observer’s local vertical.

Accuracy of rise/set computations. The times
of rise and set phenomena cannot be precisely computed, because,
in practice, the actual times depend on unpredictable atmospheric
conditions that affect the amount of refraction at the horizon.
Thus, even under ideal conditions (e.g., a clear sky at sea) the
times computed for rise or set may be in error by a minute or
more. Local topography (e.g., mountains on the horizon) and the
height of the observer can affect the times of rise or set even
more. It is not practical to attempt to include such effects in
routine rise/set computations.

The accuracy of rise and set computations decreases at high
latitudes. There, small variations in atmospheric refraction can
change the time of rise or set by many minutes, since the Sun and
Moon intersect the horizon at a very shallow angle. For the same
reason, at high latitudes, the effects of observer height and
local topography are magnified and can substantially change the
times of the phenomena actually observed, or even whether the
phenomena are observed to occur at all.

Twilight. There are three kinds of
twilight defined: civil twilight, nautical twilight, and
astronomical twilight. For computational purposes, civil
twilight begins before sunrise and ends after sunset when the
geometric zenith distance of the center of the Sun is 96 degrees
– 6 degrees below a horizontal plane. The corresponding solar
zenith distances for nautical and astronomical twilight are 102
and 108 degrees, respectively. That is, at the dark limit of
nautical twilight, the center of the Sun is geometrically 12
degrees below a horizontal plane; and at the dark limit of
astronomical twilight, the center of the Sun is geometrically 18
degrees below a horizontal plane.

This
information is derived from the
Explanatory Supplement to the Astronomical Almanac,
ed. P. K. Seidelmann (1992), pp 482ff.

All Information is Courtesy of the U.S. Naval Observatory