The solar time refers to the time calculated by taking the solar day as the standard.It can be divided intoTrue solar timeandFlat solar time。
The sun hour is calculated according to the sun hour angle. The time of the sun hour is 12h (or 180 °) different from the sun hour angle, that is, the sun hour sun hour angle+12h[1]。
Calculated according to the standard of true solar dayTrue solar time，sundialThe time represented is the true solar time.Calculated according to the standard of weekdaysFlat solar timeThe time represented by a clock is the flat solar time.In fact, our daily timing is the flat sun time. The flat sun time assumes that the earth is in a standard circle around the sun, and it is even every day of the year.Beijing time is flat sun time, 24 hours a day.If the earth's orbit around the sun is elliptical, then the earth'srotationIt is not even. Every day is not 24 hours, sometimes less and sometimes more.Considering this factor, we can get the true solar time.
True solar time requires 12 o'clock at noon every day, with the sun at the top of the head.Traditionally, the true sun time is used to determine the time, so the flat sun time should be adjusted to the true sun time.
The time calculated by taking the solar day as the standard.The real sun time calculated according to the standard of the true sun day is the time indicated by the sundial.The mean solar time calculated according to the standard of ordinary solar day, such as the time indicated by clocks and watches, is[2]。
True solar time
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The solar time is the time measured based on the period of the earth's rotation to the sun.The solar time is the general term of the true solar time and the solar time.The true sun is based on the sunDiurnal visual movementAs a basis, the time measured at noon is the time when the sun is in the due south direction.The mean solar time is the time measured on the basis of the average speed of the sun's diurnal apparent motion, that is, one solar day is divided into 24 mean solar hours.In fact, the apparent speed of the sun is not uniform. The sun moves faster when it is near the earth, and slower when it is far from the earth.The true solar time is several minutes to ten minutes ahead or behind the normal solar time, up to 16 minutes and 24 seconds.The difference between the two is called time difference. The true solar time can be obtained by adding or subtracting the time difference from the mean solar time[3]。
1. For example, on February 25 of a year, somewhereFlat solar timeIt is 7:5, and the time difference on February 25 is - 12 minutes and 54 seconds according to the difference table, that is, subtract 12:54 seconds from 7:5True solar timeThat is 6:52m6s.
2. Since the Gregorian calendar year andRegression yearAndleap yearAndPeacetimeThe actual time difference may be the data of the previous day or the next day, and the error is negligible.
True solar time is also called apparent solar time.An astronomical timing system.SolarDiurnal visual movementTime measurement system established for the basis.The true sun refers to the center of the sun's apparent circle.The true solar hour is measured by the hour angle of the true sun. The starting point is true noon, which is defined as the hour angle of the true sun plus 12h.Because the earth's orbit around the sun is an ellipse, and is perturbed by the moon and planets, it reflects that the sun's movement on the ecliptic is uneven, and because the sun does not run on the celestial equator, but on the ecliptic, the equator and the ecliptic have an intersection angle ε, even if the sun runs evenly on the ecliptic, the increase of its right ascension is uneven,So the true solar time is uneven.In order to measure time, people have established a uniform flat solar time system that is not much different from the true solar time[4]。
Flat solar time
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The time of flat sun is called peacetime for short.An astronomical timing system.It refers to an astronomical timing system based on the Earth's rotation period.It is measured by the hour angle of the flat sun, and the flat sun hour of the middle sky under the flat sun is zero.The flat sun is an imaginary reference point introduced by American astronomer Newcomb, S. in the 19th century. It moves at a uniform speed on the celestial equator, and its speed is equal to the average speed of the sun on the ecliptic.The time angle of the sun is the apparent solar time, and the difference between the apparent solar time and the normal solar time is called the time difference.Because the flat sun is an imaginary point on the celestial sphere and cannot be observed, it is actually converted into the flat sun time when the star is directly measured.Since the flat solar time is measured by the meridian circle of each place, the flat solar time of different geographical longitude places at a certain instant is different, which is expressed by the local flat solar time.The difference between the local mean solar time (m) of the two places is numerically equal to the difference between the geographical longitude (λ) of the two places, that is
。
In order to avoid the inconvenience caused by the constant change of the flat sun time with longitude, the earth is divided into 24 time zones, each of which has a longitude of 15 °, and the standard longitude in the center of the time zone is 0 °, 15 °, 30 °, etc.The flat solar time of the central meridian of the time zone is the unified time of the whole time zone, which is called zone time or standard time.Taking the time of a certain meridian circle (such as the meridian circle passing through the capital) as the civil time standard of a region or country is also called standard time.District time is a widely used standard time.China adopts the zone time of the East Eight Time Zone, which is 120 ° east longitude, as the national standard time, also known as Beijing Time.Due to economic reasons and other reasons, the time used by the country (or region) in advance of one hour, half an hour or several hours as stipulated by law is called legal time. For example, China used to use daylight saving time one hour in advance in summer[4]。
The flat solar time is based on the rotation of the earth, and the basic unit of time is the flat solar day (the cycle of the daily movement of the flat sun).It is measured by the hour angle of the flat sun (imaginary sun) to the local meridian circle, and the instant of the flat sun in the underground midheaven is taken as the zero hour of the flat sun.The time of flat sun is called "normal" for short."Peacetime" is widely used in science, technology and daily life[5]。
