Equation of Time: True Solar Time vs Mean Solar Time

The term 均时差 denotes a seasonal correction used when converting between clock time and apparent solar time, or true solar time. In Chinese calendrical and astrological practice, true solar time is based on the actual position of the Sun in the local sky: when the Sun crosses the local meridian, or reaches upper culmination, that moment is taken as local noon, or twelve o’clock in true solar time. A conventional clock, however, does not follow the visible Sun exactly. The difference between clock time and true solar time contains two distinct components: a longitude correction and the 均时差. The longitude correction arises from the difference between the longitude of the place in question and the standard meridian of the time zone. The 均时差 arises from seasonal irregularities caused by the Earth’s elliptical orbit around the Sun and the tilt of the Earth’s rotational axis. Only the second component is properly called 均时差.

钟表的一天与太阳的一天不一样长

A clock is designed to divide the day into equal hours, minutes, and seconds. The Sun, as observed from the Earth, does not move across the sky at a perfectly uniform rate. The interval from one solar noon to the next, sometimes called an apparent solar day, varies over the course of the year. This happens for two main reasons. First, the Earth’s orbit around the Sun is not a perfect circle, so the Earth’s orbital speed changes during the year. Second, the Earth’s axis is tilted relative to the plane of its orbit, so the Sun’s apparent motion along the ecliptic is not spread evenly across the celestial equator. As a result, a sundial and a mechanical clock can drift apart by a small but measurable amount, even for an observer located exactly on the standard meridian of the time zone. The quantity that describes this drift is the 均时差. It is the difference between the time shown by an ideal, uniformly running clock set to mean solar time and the time shown by the Sun itself.

全年在什么区间里摆动

The 均时差 is not a fixed number. It changes continuously through the year, following the combined effect of the Earth’s orbital motion and axial tilt. Over the course of a full year, the value of the 均时差 swings between roughly minus fourteen minutes and plus sixteen minutes. The exact bounds can be stated as approximately −14 minutes to +16 minutes, depending on the source and the level of rounding. This range is comparatively small, but it is not negligible when precision matters. The sign convention matters for practical use: at certain times of year the apparent Sun is ahead of the mean Sun, and at other times it is behind. The 均时差 depends only on the date, not on the observer’s location. Two observers in different cities, even in different provinces, will use the same value of 均时差 on the same date. For this reason, tables of the 均时差 are universal across the country for a given calendar date.

与经度时差是两回事

It is important not to confuse the 均时差 with the longitude correction. The longitude correction depends on the birthplace or observation site, because it measures how far the site’s local meridian is from the standard meridian of the time zone. That correction is constant throughout the year for a given place. If a location is east of the standard meridian, the Sun reaches local noon earlier than the clock indicates; if west, later. The size of the correction depends on the longitudinal difference, with roughly four minutes of time for every degree of longitude. The 均时差, by contrast, does not depend on location at all. It depends only on the date and is the same across the entire country. The two quantities also have different causes: one is geographic and administrative, tied to the choice of time zone; the other is astronomical and seasonal, tied to the Earth’s orbit and axial tilt. Because they are different in kind, they must be handled separately. The correct procedure is to compute the longitude correction for the specific birthplace, compute the 均时差 for the specific date, and then add the two results.

两项相加之后能差多少

Once the two corrections are added, the total difference between clock time and true solar time can become substantial. The 均时差 alone reaches about sixteen minutes at its positive extreme and about fourteen minutes at its negative extreme. The longitude correction can be larger or smaller depending on the distance from the standard meridian. In China, where a single standard time zone covers a very wide span of longitude, the longitude correction can be considerable for places far from the standard meridian. When the seasonal term and the geographic term are combined, the total difference between clock time and true solar time can amount to several tens of minutes. The exact figure varies with place and date, but a cumulative difference of tens of minutes is a realistic possibility. This is why, in practices that require true solar time, simply reading the clock without adjustment can place an event in the wrong hour.

什么时候可以不必理会它

In Chinese timekeeping, one 时辰 lasts two hours. The effect of the combined correction therefore does not always change the assigned 时辰. If the recorded birth time lies comfortably inside a two-hour 时辰, then adding or subtracting a few minutes, or even tens of minutes, may not push the moment across the boundary into the preceding or following 时辰. The correction can be ignored in that case. It becomes relevant only when the recorded clock time is close to the boundary between two 时辰. More precisely, if the distance from the recorded time to the nearest 时辰 boundary is smaller than the sum of the longitude correction and the 均时差, then the conversion may change which 时辰 is used, and therefore may change the resulting 四柱. The four pillars are calculated from the clock time as entered. The system does not automatically apply a true-solar-time adjustment behind the scenes. Whether to convert the entered time to true solar time is a decision that must be made by the user before input. The output will reflect whichever convention was chosen at the input stage.