Solar Calculator Glossary



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.
astronomical twilight - the time of morning or evening when the sun is 18° below the horizon. (Solar zenith angle is 108°, solar elevation angle is -18°.) See also civil twilight and nautical twilight.
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 Azimuth/Elevation/Zenith Figure.



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 certainty.
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 Ecliptic Figure.
elevation - see azimuth and elevation.
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.
equation of time - an astronomical term accounting for changes in the time of solar noon for a given location over the course of a year. Earth's elliptical orbit and Kepler's law of equal areas in equal times are the culprits behind this phenomenon. Click here to see a plot of the equation of time vs. day of the year. For more information on this phenomenon, see this offsite Analemma page.
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 Gregorian 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.



latitude - an angular measurement of north-south location on Earth's surface. Latitude ranges from 90° south (at the south pole), through 0° (all along the equator), to 90° north (at the north pole). Latitude is usually defined as a positive value in the northern hemisphere and a negative value in the southern hemisphere. Click here for an in-depth explanation of latitude and longitude from NASA. Click here for a list of sites to help determine your latitude and longitude.
leap year - to account for the fact that it takes Earth 365.242196 days to orbit the sun, the Julian Calendar 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.
longitude - an angular measurement of east-west location on Earth's surface. Longitude is defined from the prime meridian, which passes through Greenwich, England. The international date line is defined around +/- 180° longitude. (180° east longitude is the same as 180° west longitude, because there are 360° in a circle.) Many astronomers define east longitude as positive. For our solar calculators, we define west longitude to be positive. Click here for an in-depth explanation of latitude and longitude from NASA. Click here for a list of sites to help determine your latitude and longitude.
long wave radiation - another name for infra red radiation, or heat. Found between 4 and 100 microns wavelength in the electromagnetic spectrum.


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 astronomical twilight.



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 atmospheric refraction.)
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.


short wave radiation - see visible light.
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 Sidereal Time Page at the University of Connecticut for further explanation.
solar declination - the declination of the sun. The solar declination varies from -23.44° at the (northern hemisphere) winter solstice, through 0° at the vernal equinox, to +23.44° at the summer solstice. The variation in solar declination is the astronomical description of the sun going south (in the northern hemisphere) for the winter. Click on Solar Declination Graph to see how the solar declination varies over the year. See Solar Paths Figure to see the seasonal solar paths projected on the celestial sphere. For a ground-based view of the seasonal solar paths for different latitudes, see: 0° (the Equator), 23°N (the Tropic of Cancer), 40°N (Boulder, CO), 71°N (the Arctic Circle), and 90° (the North Pole).
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 northern hemisphere.


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.


vernal equinox - the point on the celestial sphere at which the plane of Earth's orbit crosses the plane of Earth's equator, moving from south to north. This point defines zero hours right ascension. See Ecliptic Figure.
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 leap years.


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 Azimuth/Elevation/Zenith Figure. The cosine of the solar zenith angle is used to calculate the vertical component of direct sunlight shining on a horizontal surface.