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Astronomical twilight is the period when the center of the Sun is between 12 and 18 degrees below the horizon. It starts at astronomical dawn, early in the morning when the Sun is higher than 18 degrees below the horizon. From this point, it will be difficult to observe certain faint stars, galaxies, and other objects because the Sun starts to illuminate the sky. Astronomical twilight ends at astronomical dusk in the late evening, when those faint objects again can be visible because the Sun is lower than 18 degrees below the horizon. In locations north of 48°24' N or south of 48°24', it never gets darker than this near the middle of the summer solstice (June or December).
Technically, the start and end times are when the true geocentric position of the Sun is 108 degrees from the zenith position, or directly above the observer.Astronomical twilight is the period when the center of the Sun is between 12 and 18 degrees below the horizon.
Nautical twilight is the period when the center of the Sun is between 6 and 12 degrees below the horizon, when bright stars are still visible in clear weather and the horizon is becoming visible. It is too dark to do outdoor activities without additional lighting.
Nautical twilight starts at nautical dawn, at 12 degrees below the horizon, and nautical twilight ends at nautical dusk, when the Sun is lower than 12 degrees below the horizon. For locations north of 54°34' N or south of 54°34' S latitude, the Sun will never be lower than 12 degrees below the horizon for a period in the summer.
Technically, the start and end times are when the true geocentric position of the Sun is 102 degrees from the zenith position.
Civil twilight is the period when the Sun is below the horizon but its center is less than 6 degrees below. The "Civil Twilight Starts" time is the dawn or civil dawn, with the center of the Sun at exactly 6 degrees below the horizon. Equally, the "Civil Twilight Ends" time is dusk or civil dusk, when the Sun is 6 degrees below the horizon in the evening.
During civil twilight, the sky is still illuminated, and with clear weather it is brightest in the direction of the Sun. The Moon and the brightest stars and planets may be visible. It is usually bright enough for outdoor activities without additional lighting.
Near the equator, where the Sun sets and rises in an almost vertical direction, the civil twilight period can last only 21 minutes, a very fast nightfall compared to the much longer periods at southern and northern latitudes. In regions north of 60°24' N or south of 60°24' S, there will be at least one night when it does not get darker than this.
Technically, the start and end times are when the true geocentric position of the Sun is 96 degrees from the zenith position.
The times for sunrise and sunset are based on the ideal situation, where no hills or mountains obscure the view and the flat horizon is at the same altitude as the observer. Sunrise is the time when the upper part of the Sun is visible, and sunset is when the last part of the Sun is about to disappear below the horizon (in clear weather conditions).
If the horizon in the direction of sunrise or sunset is at a higher altitude than that of the observer, the sunrise will be later and sunset earlier than listed (and the reverse: on a high mountain with the horizon below the observer, the sunrise will be earlier and sunset later than listed).
The Earth's atmosphere refracts the incoming light in such a way that the Sun is visible longer than it would be without an atmosphere. The refraction depends on the atmospheric pressure and temperature. These calculations use the standard atmospheric pressure of 101.325 pascal and temperature of 15°C or 59°F. A higher atmospheric pressure or lower temperature than the standard means more refraction, and the sunrise will be earlier and sunset later. In most cases, however, this would affect the rising and setting times by less than a minute. Near the North and South Poles it could have greater impact because of low temperatures and the slow rate of the Sun's rising and setting.
For locations north of 66°34' N or south of 66°34' S latitude, the Sun will be above the horizon all day in the summer and below the horizon all day in the winter.
Technically, sunrise and sunset are calculated based on the true geocentric position of the Sun at 90°50' from the zenith position (directly above the observer).
The azimuth displayed is the horizontal direction of the Sun at sunrise or sunset at the times displayed in the Sunrise and Sunset columns. As on a compass, the azimuth is measured in degrees, with 360 in a full circle, counted in a clockwise direction from north. North has an azimuth value of 0 degrees, east is 90 degrees, south is 180 degrees, and west is 270 degrees. A small arrow is displayed after the azimuth value to indicate the map direction where the Sun will rise or set (for a map where north is upward).
Since the times used for the sunrise and sunset calculations are also used for the azimuth calculation, the actual height of the horizon and refraction as described for the Sunrise and Sunset columns can influence the real direction where the Sun rises or sets.
It is important to note that the directions refer to true north and not to magnetic north. True north refers to north according to the earth’s axis, not magnetic north. Magnetic north refers to the direction in which the north end of a compass needle will point in response to the earth’s magnetic field.
Length of day consists of two columns, "This day," showing the time from sunrise to sunset on that day, and "Difference," the difference compared with the previous date. The duration is shown in hours (h), minutes (m), and seconds (s). The difference is shown in minutes and seconds, with a plus sign (+) if this day is longer than the previous and a minus sign (−) if it shorter.
Because of rounding, there might be a difference of one second between the displayed numbers for "This day."
"Solar noon" consists of three columns that describe the local time and the altitude and distance when the Sun passes through the meridian (longitude) of the selected location.
"Time" shows the local time of this moment, when the Sun's position will be right above a location on the same longitude as the observer (directly south or north). Except in Polar Regions, where the Sun might be below the horizon, the Sun will appear in a direct southern or northern direction at this time. For locations near the equator, the Sun can be right over one's head, at the point nearest the zenith position (altitude 90 degrees).
"Altitude" shows the altitude of the Sun's center above the ideal horizon at the passing time. Typically this is the highest position it reaches in the sky that day (except near the South and North Poles, where the altitude increases or decreases all day and night). The altitude takes into account typical refraction in the Earth's atmosphere. If the Sun is below the horizon all day, the altitude will be labeled "below."
"Distance" is the distance from the Earth's center to the Sun's center in millions of kilometers, or 106 km, so a figure of 151 x 106 means 151,000,000 kilometers. (Subtract 660,000 km to get the approximate distance between the surfaces of the two bodies.) The distance varies because the Earth's orbit is elliptical rather than circular. The Sun is closest on perihelion, around January 3, and most distant on aphelion, around July 5. The mean distance is 149.6 * 106 km.
The distance to the Sun does not have as great an impact on the seasons as does the Earth's tilted angle. During the summer on the Northern Hemisphere, that hemisphere is tilted toward the sun. This means a longer day (more sunlight) and light rays striking the Earth's surface at higher angles, giving more intensive sunlight and more energy per square meter, compared with a location at the same distance from the equator on the Southern Hemisphere.
Times are rounded to the nearest minute and should generally match closely with those listed in the annual Astronomical Almanac by H.M. Nautical Almanac Office in the U.K. and the United States Naval Observatory.
A sample set of 150 records consisting of times for sunrise, sunset, and start and end times for civil, nautical, and astronomical twilight was compared with times listed in The Astronomical Almanac for 2007. Only two differed, both by only one minute, which means that just over 1.3 percent were different in that sample set.
Because times are calculated in local time, there is a chance that future times may be wrong, as changes might be made to dates of Daylight Saving Time or in the local time zone.