At far north or south latitudes, the sun moves nearly parallel to the horizon, making predictions of sunrise and sunset times less accurate. At 81 degrees N or S latitude, the sun moves about 10 degrees sideways for each 1 degree vertically.
At latitudes close to the arctic and antarctic circles, there are days when full darkness is never reached, because the sun never drops far enough below the horizon. In Alaska in the summer, the sky never darkens fully.
The times of sunrise and sunset are affected by atmospheric refraction - most pronounced near sea level where a thicker atmosphere and temperature differences cause sunlight to bend and reach the observer even after "geometric" sunset has occured.
Sunrise occurs earlier (and sunset occurs later) for an observer in an aircraft, where the observer looks down at an angle to see the horizon. For an observer on the ground, it is common to see airplanes lit by sunlight after sunset at ground level.
Altitude affects the times of sunrise and sunset, but only to the extent that the horizon is at a lower altitude than the observer. If the observer is on a high flat plain where the horizon is at the same altitude, there is no change in rise/set times.
HourWorld calculates the times of sunrise and sunset to occur when the center of the sun is 1/4 degree below the horizon, and the upper limb of the sun is on the horizon. This makes daytime just a little bit longer than nighttime.
The sun's angular size is just a little less than half a degree when viewed from the earth. Therefore, the center of the sun is about 1/4 degree below the horizon when upper limb is on the horizon at sunrise and sunset.
Twilight has the shortest duration at the Equator, where the sun's motion is perpendicular to the horizon. At 60 degrees N or S latitude, the sun moves at an angle of 60 degrees to the horizon, and twilight lasts twice as long as at the Equator.
Astronomical twilight is a period of near darkness where the sun is between 12 and 18 degrees below the horizon. There is enough light in the sky to interfere with some astronomical observations.
Nautical twilight is deep twilight when the sun is between 6 and 12 degrees below the horizon. It is too dark for most activities without artificial lighting. It becomes very dark toward the end of nautical twilight.
Civil twilight is the time when the sun is close to the horizon (within 6 degrees). During civil twilight, there is generally enough light to conduct outdoor activities without artificial lighting.
One lux is a unit of illuminance equal to one lumen of light flux per square meter. For rough comparison purposes, one lumen is about four times as bright as a clear moonlit night with the full moon directly overhead.
Daylight Saving Time is observed, for the most part, in latitudes distant from the equator where sunlight varies considerably during the year. Most tropical countries and locations stay on standard time all year, because sunrise and sunset vary little.
The start and stop of Daylight Saving Time, where it is observed, is set by the laws of the countries involved. In the United States, DST starts at 2:00 AM on the first Sunday in April, and ends at 2:00 AM on the last Sunday in October.
Daylight Saving Time does not actually save any daylight. No political decision actually adjusts the rotation of the earth. Daylight Saving Time (or Summer Time) just shifts the hours of daylight to better match the waking hours of local residents.
During parts of the year in some countries, usually in the summer, laws have been enacted to advance the clocks one hour ahead of the standard time normally used. This is called Daylight Saving Time, Daylight Time, Summer Time, or Advanced Time.
Solar heating of the earth's surface depends on the hours of sunlight in a day, and the angle of the sun's rays. Heat transfer is greatest in the summer when the sun is up longer, and the rays strike the surface at the steepest angle.
The sun's illumination on the earth's surface is approximately 130,000 lux (lumens per square meter) when the sun is directly overhead. This is approximately 500,000 times as much illumination as a full moon directly overhead.
