Photographic Golden Hour 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.
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.
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 first Sunday of April, and ends at 2:00 AM on the last Sunday of October.
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.
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 1542 (Julian calendar) was immediately followed by 15 October 1542 (Gregorian calendar) to correct for past errors. For consistency, and to avoid having a 10-day discontinuity, astronomers extrapolate the Gregorian calendar backward when dealing with historical dates. For more information, read the Gregorian Calendar page from Rice University.
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.
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.
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.
solar noon - 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.
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.
Universal Coordinated Time - replaced GMT (Greenwich Mean Time) 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.
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