apparent sunrise/sunset - Due to
atmospheric refraction, sunrise occurs
shortly before the sun crosses above the horizon. Light from
the sun is bent, or refracted, as it enters earth's atmosphere. See
Apparent Sunrise Figure.
This effect causes the apparent sunrise to be earlier than the actual
sunrise. Similarly, apparent sunset occurs slightly later than actual sunset.
The sunrise and sunset times reported in our calculator have been corrected
for the approximate effects of atmospheric refraction. However, it should
be noted that due to changes in air pressure, relative humidity, and other
quantities, we cannot predict the exact effects of atmospheric refraction
on sunrise and sunset time. Also note that this possible error increases
with higher (closer to the poles) latitudes.
atmospheric refraction - as light from the sun (or another celestial
body) travels from the vacuum of space into Earth's atmosphere, the path of
the light is bent due to refraction. This causes
stars and planets near the horizon to appear higher in the sky than they
actually are, and explains how the sun can still be visible after it has
physically passed beyond the horizon at sunset. See also
apparent sunrise. Click here for a
graph of atmospheric refraction vs. elevation.
astronomical unit (AU) - the mean (average) distance from Earth
to the sun. Approximately 92,957,210 miles.
azimuth and elevation - an angular coordinate system for locating
positions in the sky. Azimuth is measured clockwise from true north
to the point on the horizon
directly below the object. Elevation is measured vertically from that
point on the horizon up to the object. If you know the azimuth of a
constellation is 135° from north, and the elevation is 30°,
you can look toward the southeast, about a third of the way up from the
horizon to locate that constellation. Because our planet rotates, azimuth
and elevation numbers for stars and planets are constantly changing with
time and with the observer's location on earth. See
calibration - the process of comparing a field instrument to a
measurement standard, in order to insure the instrument is reading the
correct values. All measuring devices (scales, volt meters, theodolytes)
must be calibrated to a standard before they can be used with any
celestial sphere - an imaginary spherical
"movie screen" upon which all stars, planets, and other bodies in space
can be thought of as being projected. Coordinates on the celestial sphere are
specified in degrees of declination and
right ascension, roughly analogous to latitude and
longitude coordinates on Earth's surface. See
Celestial Sphere Figure.
civil twilight - the time of morning or evening when the sun is
6° below the horizon. (Solar zenith angle is 96°, solar elevation
angle is -6°.) See also astronomical twilight
and nautical twilight.
Daylight Saving Time - Many countries have instituted a clock change over
the summer months to save energy. By moving sunset closer to the time when most
people go to bed, an hour's worth of electricity for lighting can be conserved.
In the U.S., except for Arizona, Hawaii and part of Indiana, daylight saving time
begins at 2:00 AM on the second Sunday of March,
and ends at 2:00 AM on the first Sunday of November. See
Saving Time, Saving Energy
by the California Energy Commission for further information.
Click here for a list of sites
with more daylight saving time information.
declination - along with right ascension, a
measure of celestial position. Declination is analogous to latitude on
Earth's surface, and measures an angular displacement north or south from the
projection of Earth's equator on the celestial
sphere to the location of a celestial body. See
Celestial Sphere Figure.
ecliptic plane - the imaginary plane defined by the projection of
Earth's orbit (the path the Earth follows around the sun) onto the
celestial sphere. See
electromagnetic spectrum - the scientific name for the full range of
wave radiation, from cosmic rays through UV, visible light, infrared, on to
radio waves and beyond. See Electromagnetic Spectrum Figure.
equinox - the time when the sun's path crosses
earth's equatorial plane, or when the sun's
declination is 0°. In the northern
hemisphere, the vernal equinox, which occurs on or around 21 March,
signals the start of spring. The autumnal equinox, around
22 September, is recognized as the beginning of fall (autumn).
Greenwich Mean Time - world standard time
implemented in Greenwich, England, in the 1840s. GMT, which is based on
earth's motion, was succeeded by
Universal Coordinated Time (UTC) in 1972.
Gregorian Calendar - in 1582, Pope Gregory XIII reformed the
Julian calendar to improve the accuracy.
The current scheme for leap years was introduced,
and 04 October 1582 (Julian calendar) was immediately followed by 15
October 1582 (Gregorian calendar) to correct for past errors due to the Julian
calendar's implementation of leap years. For
consistency, and to avoid having a 10-day discontinuity, astronomers
use Julian Day when dealing with historical
dates. The NOAA solar calculators simply extrapolate the Gregorian calendar
back through time, so dates before 15 October 1582 should be corrected by the user.
For more information, read the
Calendar page from Rice University.
international date line - roughly follows the
meridian at 180° longitude. This is the line that separates
today from tomorrow. When it is Saturday in New Zealand, it is still
Friday in Hawaii. This is because New Zealand is twelve hours ahead of
Greenwich Mean Time, and Hawaii is ten hours behind GMT.
(See time zone.)
Julian Calendar - calendar established by Julius Caesar in 46 BC,
setting the number of days in a year at 365, except for leap years which
have 366, and occurred every 4 years.
This calendar was reformed by Pope Gregory XIII into the
Gregorian calendar, which further refined
leap years and corrected for past errors by skipping 10 days in October of 1582.
Julian day - a time period used in astronomical
circles, defined as the number of days since 1 January, 4713 BCE (Before Common
Era), with the first day defined as Julian day zero. The Julian day begins
at noon UTC. Some scientists use the term julian day to mean the numerical day of
the current year, where January 1 is defined as day 001.
leap year - to account for the fact that it takes Earth 365.242196
days to orbit the sun, the
implemented a system by which every fourth
year would have an extra day (366 instead of 365). These years are called
leap years. Later, the
Gregorian Calendar improved this
correction by calling for every fourth year to
be a leap year, unless the year is divisible by 100. This corrects
the calendar to a year of 365.24 days, which is a good start but not perfect.
So there is another condition: if a year is divisible by 400, it is
a leap year. Therefore the year 1900 (divisible by 100, but not by 400)
was not a leap year, but the year 2000 was a leap year.
light year - a measure of distance, not time.
Defined as the distance light travels in one year. Roughly equal to 5.88 trillion
miles (5,880,000,000,000 miles). This unit of linear measurement is used for stating
interstellar distances. For example, Proxima Centauri is 4.2 light years from
our solar system.
magnetic north - Earth's magnetic north and
south poles are not located exactly at the rotational north and south poles. So
when your compass points north, chances are it is not pointing to
true north, but several (in the U.S., as many as 20) degrees off.
meridian - an imaginary line of
longitude, stretching over Earth's
surface from the north pole to the south pole.
month - a month is based on a lunar cycle, the time
it takes for Earth's moon to complete one complete orbit of the Earth. The moon's
cycle is approximately 29.53059 days.
nautical twilight - the time of morning or evening when the sun is
12° below the horizon. (Solar zenith angle is 102°, solar elevation
angle is -12°.) See also civil twilight and
prime meridian - the imaginary line of longitude passing from the north pole to the south pole through Greenwich, England. The
prime meridian is defined as 0° longitude, and is the point for which
UTC is defined.
radiation budget - a term used to refer to measurements of the solar
radiation (sunlight) coming in to the Earth's surface during the day, and the
long wave (infra red) radiation that is radiated
from Earth's surface to space at night. Because energy can neither be created
nor destroyed, all incoming energy should be accounted for as outgoing
energy, unless the Earth itself is experiencing an increase in temperature.
refraction - as light travels from one medium into a medium with
higher or lower density,
for example from air into water or from the vacuum of space into earth's
atmosphere, the path of the light is bent slightly.
This causes our legs to look shorter in a swimming pool, and allows us
to see the sun while it is still below the horizon. (See
right ascension - along with declination, a
means of defining the position of objects in space, referred to a known
point. Similar to the angular system used to define
latitude and longitude
on Earth's surface, right ascension is roughly analogous to longitude, and
defines an angular offset from the meridian of the
vernal equinox. See
Celestial Sphere Figure.
sidereal time - is defined as the time elapsed
since the most recent meridian passage of the vernal
equinox. This system is based on the rotation of the Earth with respect to
the stars, instead of the sun. (See solar time.)
A sidereal day is slightly shorter than 24 hours (3 minutes 55.91 seconds
shorter). See Jim McDonald's excellent
Time Page at the University of Connecticut for further explanation.
solar noon - (see also: solar time)
Defined for a given day for a specific longitude,
it is the time when the sun crosses the meridian
of the observer's location. At solar noon, a shadow cast by a vertical
pole will point either directly north or directly south, depending on the
observer's latitude and the time of year.
solar time - is defined as the time elapsed
since the most recent meridian passage of the sun. This system is based
on the rotation of the Earth with respect to
the sun. A mean solar day is defined as the time between
one solar noon and the next, averaged over the year.
solstice - is defined as the time of year
when the declination of the sun reaches a
minimum or maximum value. In the northern hemisphere, the summer solstice
occurs on or about 21 June, and the winter solstice occurs around 21 December.
The exact times of the solstices change from year to year.
speed of light - often considered a constant (3.0 x 10^8 m/s), the
speed that light travels depends on the medium through which it travels.
Light traveling through water goes slightly slower than through air or
through the vacuum of space.
time zone - longitudinally defined regions on the Earth that keep a
common time. A time zone generally spans 15° of longitude, and is
defined by its offset (in hours) from UTC. For example, Mountain Standard
Time (MST) in the US is 7 hours behind UTC (MST = UTC - 7). Click here for a
table of time zones for world cities.
true north - (see also:
magnetic north) the direction, along the surface of the Earth, toward the
point where Earth's imaginary axis of rotation intersects Earth's surface in the
ultraviolet - the region of the electromagnetic
spectrum that falls outside of the visible,
just beyond violet. Defined in the scientific community as electromagnetic
radiation between 200 and 390 nanometers in wavelength.
Universal Coordinated Time - replaced GMT as
the world reference for time. UTC is based on atomic clock time, with
leap seconds added when necessary to match earth-motion time.
Many data sets collected around the world
are recorded with a UTC time stamp to avoid the confusion associated with
time zones and daylight saving time. For more information, straight from
the United States' official timekeepers, see the
National Institute of Standards and Technology (NIST)
World Time Scales.
For the correct UTC time, see NIST's Official U.S. Time.
visible light - the region of the electromagnetic
spectrum that is seen by the human eye. Defined by scientists as falling between
390 and 780 nm in wavelength. Visible light contains
all of the colors seen in a rainbow: violet, indigo, blue, green, yellow, orange
and red. Also known, in scientific circles, as short wave radiation, because
the wavelengths involved are shorter than longwave
(infra red) radiation.
wavelength - simply, the length between consecutive peaks or troughs in
any wave. In electromagnetic radiation, wavelength defines the color of
light if it is visible, or the type of radiation
(infra-red, ultraviolet, etc.) if it is outside of the visible region of
the electromagnetic spectrum. Mathematically,
the wavelength times the frequency of the radiation (measured in cycles-per-second,
or Hertz) is equal to the speed of light.
year - the amount of time it takes planet Earth to complete one full
orbit of the sun. A year is defined as 365.242196 days. Because a year is
not evenly divisible by a day, our calendar has corrections in the form of
zenith angle - an angular measurement from straight up (zenith) to
a point in the sky. Zenith angle can be used along with azimuth
to indicate the position of a star or other celestial body. Zenith angle
is the complementary angle of the elevation (elevation = 90° - zenith).
(See azimuth and elevation.) See
of the solar zenith angle is used to calculate the vertical component
of direct sunlight shining on a horizontal surface.
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