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	day - HighBeam Encyclopedia
		The ordinary day, or solar day, is measured relative to the sun, being the time between successive passages of the sun over a stationary observer's celestial meridian.
		The length of a solar day varies during the course of a year, so for purposes of time measurement an average, or mean, solar day is used (see solar time), equal to exactly 24 hr.
		The sidereal day, used by astronomers, is measured relative to the fixed stars rather than the sun (see sidereal time); it is about 4 min shorter than the mean solar day.
	www.encyclopedia.com /doc/1E1-day-ent.html (489 words)

	Solar time (Site not responding. Last check: 2007-10-12)
		Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon.
		Apparent solar time is based on the apparent solar day, which is the interval between two successive returns of the sun to the local meridian.
		Mean solar time is based on a fictional mean sun which travels at a constant rate throughout the year.
	www.ebroadcast.com.au /lookup/encyclopedia/me/Mean_solar_time.html (212 words)

	Day (Site not responding. Last check: 2007-10-12)
		The solar day, measured by the interval between meridian passages of the sun, varies in length because of the variation in speed of the earth in its orbit.
		In consequence, the length of the solar day is averaged over the period of a year, and the mean solar day thus obtained is used for all civil and many astronomical purposes.
		The day is still often regarded as starting with sunset in ecclesiastical (particularly Jewish ecclesiastical) usage; until recently, the astronomical day started at noon, and the Julian day still starts at noon.
	skybooksusa.com /time-travel/timeinfo/day.htm (261 words)

	day
		The apparent solar day is the interval between two consecutive upper culminations (or upper transits) of the Sun, i.e., the period between one passage of the Sun at maximum altitude across the observer's meridian and the next.
		The apparent solar day varies with the time of year because the Sun moves in the ecliptic instead of along the celestial equator, and also because the Sun moves along the ecliptic at a variable rate (due to the varying distance of Earth from Sun during the year).
		The mean solar day is the average of the apparent solar day over a whole year or –; what amounts to the same thing – the length of day reckoned according to the mean sun.
	www.daviddarling.info /encyclopedia/D/day.html (295 words)

	AllRefer.com - solar time (Astronomy, General) - Encyclopedia
		The solar day is the time it takes for the sun to return to the same meridian in the sky.
		Such time, called mean solar time, may be thought of as being measured relative to an imaginary sun (the mean sun) that lies in the earth's equatorial plane and about which the earth orbits with constant speed.
		The difference between the local solar time and the mean solar time at a given location is known as the equation of time.
	reference.allrefer.com /encyclopedia/S/solartim.html (354 words)

	day - definition by dict.die.net
		See under Clock, Day, etc. Sideral time, time as reckoned by sideral days, or, taking the sidereal day as the unit, the time elapsed since a transit of the vernal equinox, reckoned in parts of a sidereal day.
		Thus, if this is the sun, the day (the interval between two successive transits of the sun's center over the same meridian) is called a solar day; if it is a star, a sidereal day; if it is the moon, a lunar day.
		Day The Jews reckoned the day from sunset to sunset (Lev.
	dict.die.net /day (534 words)

	Time and Frequency from A to Z
		Since the Earth's rotational period (one day) rate is much more variable than its period of revolution about the Sun (one year), the mean solar day is more useful for timekeeping, since it averages the length of the day over the course of a year.
		The difference between apparent and mean solar time is known as the "equation of time" and is a measure of the apparent Sun preceding or following the mean Sun by an interval than can be as much as 16 minutes.
		Therefore, mean solar time (based on the length of an average day) is more useful for uniform timekeeping than apparent solar time.
	tf.nist.gov /general/enc-s.htm (576 words)

	Day - definition from Biology-Online.org
		see anniversary, astronomical day, a period equal to the mean solar day, but beginning at noon instead of at midnight, its twenty-four hours being numbered from 1 to 24; also, the sidereal day, as that most used by astronomers.
		Civil day, the mean solar day, used in the ordinary reckoning of time, and among most modern nations beginning at mean midnight; its hours are usually numbered in two series, each from 1 to 12.
		The mean or average of all the apparent solar days of the year.
	www.biology-online.org /dictionary/Day (558 words)

	Time
		The mean sun is a point which moves uniformly around the earth along the plane of the ecliptic, but is usually not is the same position as the real sun.
		A solar year is sometimes called a tropical year and is equal to 365 days, 5 hours, 48 minutes, and 45.51 seconds, or 365.24219 days, in solar time.
		Mean sidereal time is measured with the mean vernal equinox, apparent sidereal time is measured with the true vernal equinox.
	library.thinkquest.org /29033/begin/time.htm (1915 words)

	ENQUIRERWORLD.COM: Time Standards
		Solar time is based on the solar day, which is the period of time between one solar noon and the next.
		However, because the Earth's orbit around the sun is elliptical, and the day's variation depends on the observer's lattitude, solar time varies as much as 15 minutes from mean solar time.
		Julian day number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar.
	www.enquirerworld.com /standards.html (1287 words)

	day
		The simplest definition of a day's duration, and the one used in most sciences, including astronomy, is that it is 86,400 seconds as the second is defined in SI.
		The sidereal day is subdivided in the same way as the solar day, into 24 sidereal hours; each sidereal hour into 60 sidereal minutes, and each sidereal minute into 60 sidereal seconds.
		Because the vernal equinox itself moves (due to the precession of the Earth's axis), the sidereal day is not quite the same as the period of earth's rotation with respect to a fixed direction in space.
	www.sizes.com /time/day.htm (1160 words)

	The Sirius Research Group
		While variations of the mean time interval would equally affect the sidereal day and the solar day, the length of the solar day depends upon Earth’s orbital velocity and/or the path it has to travel around the Sun.
		This would imply that at the exact position of the equinox, for instance, the length of the 'mean solar day' is equal to the period of the 'mean sidereal day' (equinoctial day), which in turn is equal to Earth’s absolute rotation period.
		The assertion that ‘lunisolar precession’ affects the mean sidereal day and not the mean solar day is therefore false, because with each solar day our Sun would have to move around a wobbling Earth for the equinox to occur earlier.
	siriusresearchgroup.com /articles/MeasurementOfTime/absolute.shtml (442 words)

	Time Equations and Notes
		mean solar day - average solar day across a year - 24 hours on average.
		Universal time (UT) is counted from 0 hours at midnight, with unit of duration the mean solar day, defined to be as uniform as possible despite variations in the rotation of the Earth.
		The Julian Date is, then, the Julian day number followed by the fraction of the day elapsed since the preceding noon.
	www.csgnetwork.com /timeequations.html (717 words)

	Cycles and Seasons: Motions in the sky
		The length of the day is a natural unit of time on which all the living beings have adapted.
		The local mean solar time of each standard meridian is the time to be kept by the timepieces in the entire zone within 7°30' east and west of that meridian.
		The length of the solar day is variable, so the average value of it is known as a mean solar day.
	www.ii.metu.edu.tr /~astr201/demo/lecture_notes/section4/index.html (587 words)

	The Dialist's Companion - Data Elements
		While the difference in the length of a true solar day from one day to the next is small, the variations add up over weeks and months to make the sun time recorded by a dial vary from a regular, uniformly running clock by as much as 16 minutes.
		A mean solar day is the average time between crossings of the meridian by the sun; a mean sidereal day is the average time between crossings of the meridian by a fixed point on the ecliptic.
		EST the solar time is 4:00:05 (accounting for the equation of time); however, the effect of refraction will be on average to increase the sun's apparent altitude by 9 arcminutes and to decrease its apparent hour angle so that the time recorded (assuming sufficient precision) will be 3:59:39.
	sundials.org /publications/dcomp/dcomp2.htm (2083 words)

	apparent time and mean time (Site not responding. Last check: 2007-10-12)
		The use of a 24-hour day, with 60 minutes (minutiae prima) in an hour and 60 seconds (minutiae secundae) to the minute does not require that all hours be the same length.
		Thus some days (day in the sense of day plus night) are longer than others, and hence some hours in a year will be longer than others even if the day is divided into 24 equal hours.
		An alternative is to average the lengths of all the days in a year to obtain a mean solar day, and divide that day into 24 equal mean solar hours.
	www.sizes.com /time/apparent_mean.htm (472 words)

	Journal of Theoretics
		Mean solar time is based on the motion of a hypothetical sun traveling at an even rate throughout the year, and it is obtained in practice from observations of stars.
		The difference between the mean solar day of 86400 seconds (s) and the mean sidereal day is exactly 235.9094618 s.
		For more than a century the mean solar day has essentially remained a constant, implying a non-precessing or fixed axis of rotation with respect to the position of the sun and the equinoctial points.
	www.journaloftheoretics.com /Articles/3-3/Uwe-pub.htm (1319 words)

	Basics of Space Flight Section I. The Environment of Space
		The Earth rotates on its axis relative to the sun every 24.0 hours mean solar time, with an inclination of 23.45 degrees from the plane of its orbit around the sun.
		Mean solar time represents an average of the variations caused by Earth's non-circular orbit.
		Its rotation relative to "fixed" stars (sidereal time) is 3 minutes 56.55 seconds shorter than the mean solar day, the equivalent of one solar day per year.
	www2.jpl.nasa.gov /basics/bsf2-1.html (1286 words)

	Reckoning Days and Hours (Site not responding. Last check: 2007-10-12)
		The mean solar day is the average length of a day as determined by noting one passage of the sun across the meridian of an observer and calculating the time that it takes for the sun to cross the same point in the sky a second time.
		The mean siderial day is determined by a procedure similar to that of fixing the solar day; however, this procedure uses a star's passage across a reference point on the celestial sphere (that point now being the vernal equinox) instead of the sun's passage.
		That means that the solar day appears to be about 4 minutes longer than the sidereal day because Earth in its solar orbit has to move a little farther to get back to the point at which the sun crosses the same meridian.
	www.circ.uab.edu /nypldr/1time/reckon.htm (544 words)

	The Martian Calendar
		This day is not suitable as a basis for chronology because time reckoning in civil life depends on the course of the Sun and not of the stars.
		The length of the mean Gregorian year is 365 + 1/4 - 1/100 + 1/400 days = 365.2425 days, while the length of the mean solar year is 356.2422 days.
		Mean time averages the seasonal variation in the length of the true solar day, as the author attempts to explain in the next paragraph.
	pweb.jps.net /~gangale3/other/vertregt.htm (4449 words)

	Time Cycles-25
		by using leap days corresponding to the 126 cycles of the 3rd mean motion of the Sun in a week of precessional years, the year of 360 + 5 mean solar days may be used interchangeably with the year of 360 + 6 Earth revolutions.
		We have shown that cycles of sixty days and sixty years as well as the sexagesimal number system are derived from the interaction between the first and second mean motions of the Sun intrinsic to Hindu cosmological time cycles.
		The ancient Egyptians are known to have counted the cycle of the third mean motion of the Sun separately, using the heliacal rising of Sirius to mark the beginning of their sidereal year.
	www.aaronsrod.com /time-cycles/original/time-cycles-25.html (474 words)

	ENQUIRERWORLD.COM: Time Standards - Leap Second
		Take care not to confuse the difference between the length of the mean solar day and the SI day, with the leap second adjustment (which is approximately 0.7 seconds per year).
		The correct reason for leap seconds is not the difference, but rather, the sum of the difference between the length of the SI day and the mean solar day (currently about 0.002 seconds), over a given period of time.
		All references to UTC will have to be reviewed to ascertain whether the original intent was to be mean solar time or to be atomic time or simply to be the conventional civil time scale which happens to be in current use.
	www.enquirerworld.com /standards/leap.html (1820 words)

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