Equation of time
The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of "reconcile a difference". The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion. Apparent solar time can be obtained by measurement of the current position of the Sun, as indicated by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would have a mean of zero.
The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth. The equation of time values for each day of the year, compiled by astronomical observatories, were widely listed in almanacs and ephemerides.
The concept
During a year the equation of time varies as shown on the graph; its change from one year to the next is slight. Apparent time, and the sundial, can be ahead by as much as 16 min 33 s, or behind by as much as 14 min 6 s. The equation of time has zeros near 15 April, 13 June, 1 September, and 25 December. Ignoring very slow changes in the Earth's orbit and rotation, these events are repeated at the same times every tropical year. However, due to the non-integral number of days in a year, these dates can vary by a day or so from year to year.The graph of the equation of time is closely approximated by the sum of two sine curves, one with a period of a year and one with a period of half a year. The curves reflect two astronomical effects, each causing a different non-uniformity in the apparent daily motion of the Sun relative to the stars:
- the obliquity of the ecliptic, which is inclined by about 23.44 degrees relative to the plane of the Earth's equator; and
- the eccentricity of the Earth's orbit around the Sun, which is about 0.0167.
Sign of the equation of time
The United States Naval Observatory states "the Equation of Time is the difference apparent solar time minus mean solar time", i.e. if the sun is ahead of the clock the sign is positive, and if the clock is ahead of the sun the sign is negative. The equation of time is shown in the upper graph above for a period of slightly more than a year. The lower graph has the same absolute values but the sign is reversed as it shows how far the clock is ahead of the sun. Publications may use either format - in the English-speaking world, the former usage is the more common, but is not always followed. Anyone who makes use of a published table or graph should first check its sign usage. Often, there is a note or caption which explains it. Otherwise, the usage can be determined by knowing that, during the first three months of each year, the clock is ahead of the sundial. The mnemonic "NYSS", for "new year, sundial slow", can be useful. Some published tables avoid the ambiguity by not using signs, but by showing phrases such as "sundial fast" or "sundial slow" instead.In this article, and others in English Wikipedia, a positive value of the equation of time implies that a sundial is ahead of a clock.
History
The phrase "equation of time" is derived from the medieval Latin aequātiō diērum, meaning "equation of days" or "difference of days".The word :wikt:aequatio|aequātiō was used in medieval astronomy to tabulate the difference between an observed value and the expected value.
Gerald J. Toomer uses the medieval term "equation" from the Latin aequātiō, for Ptolemy's difference between the mean solar time and the apparent solar time. Johannes Kepler's definition of the equation is "the difference between the number of degrees and minutes of the mean anomaly and the degrees and minutes of the corrected anomaly."
The difference between apparent solar time and mean time was recognized by astronomers since antiquity, but prior to the invention of accurate mechanical clocks in the mid-17th century, sundials were the only reliable timepieces, and apparent solar time was the generally accepted standard. Mean time did not supplant apparent time in national almanacs and ephemerides until the early 19th century.
Early astronomy
The irregular daily movement of the Sun was known to the Babylonians.Book III of Ptolemy's Almagest is primarily concerned with the Sun's anomaly, and he tabulated the equation of time in his Handy Tables. Ptolemy discusses the correction needed to convert the meridian crossing of the Sun to mean solar time and takes into consideration the nonuniform motion of the Sun along the ecliptic and the meridian correction for the Sun's ecliptic longitude. He states the maximum correction is time-degrees or of an hour. However he did not consider the effect to be relevant for most calculations since it was negligible for the slow-moving luminaries and only applied it for the fastest-moving luminary, the Moon.
Based on Ptolemy's discussion in the Almagest, values for the equation of time were standard for the tables in the works of medieval Islamic astronomy.
Early modern period
A description of apparent and mean time was given by Nevil Maskelyne in the Nautical Almanac for 1767: "Apparent Time is that deduced immediately from the Sun, whether from the Observation of his passing the Meridian, or from his observed Rising or Setting. This Time is different from that shewn by Clocks and Watches well regulated at Land, which is called equated or mean Time." He went on to say that, at sea, the apparent time found from observation of the Sun must be corrected by the equation of time, if the observer requires the mean time.The right time was originally considered to be that which was shown by a sundial. When good mechanical clocks were introduced, they agreed with sundials only near four dates each year, so the equation of time was used to "correct" their readings to obtain sundial time. Some clocks, called equation clocks, included an internal mechanism to perform this "correction". Later, as clocks became the dominant good timepieces, uncorrected clock time, i.e., "mean time", became the accepted standard. The readings of sundials, when they were used, were then, and often still are, corrected with the equation of time, used in the reverse direction from previously, to obtain clock time. Many sundials, therefore, have tables or graphs of the equation of time engraved on them to allow the user to make this correction.
The equation of time was used historically to. Between the invention of accurate clocks in 1656 and the advent of commercial time distribution services around 1900, there were three common land-based ways to set clocks. Firstly, in the unusual event of having an astronomer present, the sun's transit across the meridian was noted, the clock was then set to noon and offset by the number of minutes given by the equation of time for that date. Secondly, and much more commonly, a sundial was read, a table of the equation of time was consulted and the watch or clock set accordingly. These calculated the mean time, albeit local to a point of longitude. The third method did not use the equation of time; instead, it used observations to give sidereal time, exploiting the relationship between sidereal time and mean solar time.
The first tables to give the equation of time in an essentially correct way were published in 1665 by Christiaan Huygens. Huygens, following the tradition of Ptolemy and medieval astronomers in general, set his values for the equation of time so as to make all values positive throughout the year.
Another set of tables was published in 1672–73 by John Flamsteed, who later became the first Astronomer Royal of the new Royal Greenwich Observatory. These appear to have been the first essentially correct tables that gave today's meaning of Mean Time. Flamsteed adopted the convention of tabulating and naming the correction in the sense that it was to be applied to the apparent time to give mean time.
The equation of time, correctly based on the two major components of the Sun's irregularity of apparent motion, was not generally adopted until after Flamsteed's tables of 1672–73, published with the posthumous edition of the works of Jeremiah Horrocks.
Robert Hooke, who mathematically analyzed the universal joint, was the first to note that the geometry and mathematical description of the equation of time and the universal joint were identical, and proposed the use of a universal joint in the construction of a "mechanical sundial".
18th and early 19th centuries
The corrections in Flamsteed's tables of 1672–1673 and 1680 gave mean time computed essentially correctly and without need for further offset. But the numerical values in tables of the equation of time have somewhat changed since then, owing to three factors:- general improvements in accuracy that came from refinements in astronomical measurement techniques,
- slow intrinsic changes in the equation of time, occurring as a result of small long-term changes in the Earth's obliquity and eccentricity, and
- the inclusion of small sources of additional variation in the apparent motion of the Sun, unknown in the 17th century, but discovered from the 18th century onwards, including the effects of the Moon, Venus and Jupiter.
As the apparent daily movement of the Sun is one revolution per day, that is 360° every 24 hours, and the Sun itself appears as a disc of about 0.5° in the sky, simple sundials can be read to a maximum accuracy of about one minute. Since the equation of time has a range of about 33 minutes, the difference between sundial time and clock time cannot be ignored. In addition to the equation of time, one also has to apply corrections due to one's distance from the local time zone meridian and summer time, if any.
The tiny increase of the mean solar day due to the slowing down of the Earth's rotation, by about 2 ms per day per century, which currently accumulates up to about 1 second every year, is not taken into account in traditional definitions of the equation of time, as it is imperceptible at the accuracy level of sundials.
Major components of the equation
Eccentricity of the Earth's orbit
The Earth revolves around the Sun. As seen from Earth, the Sun appears to revolve once around the Earth through the background stars in one year. If the Earth orbited the Sun with a constant speed, in a circular orbit in a plane perpendicular to the Earth's axis, then the Sun would culminate every day at exactly the same time, and be a perfect time keeper. But the orbit of the Earth is an ellipse not centered on the Sun, and its speed varies between 30.287 and 29.291 km/s, according to Kepler's laws of planetary motion, and its angular speed also varies, and thus the Sun appears to move faster at perihelion and slower at aphelion a half year later.At these extreme points this effect varies the apparent solar day by 7.9 s/day from its mean. Consequently, the smaller daily differences on other days in speed are cumulative until these points, reflecting how the planet accelerates and decelerates compared to the mean. As a result, the eccentricity of the Earth's orbit contributes a periodic variation which is a sine wave with an amplitude of 7.66 min and a period of one year to the equation of time. The zero points are reached at perihelion and aphelion ; the extreme values are in early April and early October.
Obliquity of the ecliptic
Even if the Earth's orbit were circular, the perceived motion of the Sun along our celestial equator would still not be uniform. This is a consequence of the tilt of the Earth's rotational axis with respect to the plane of its orbit, or equivalently, the tilt of the ecliptic with respect to the celestial equator. The projection of this motion onto our celestial equator, along which "clock time" is measured, is a maximum at the solstices, when the yearly movement of the Sun is parallel to the equator and yields mainly a change in right ascension. It is a minimum at the equinoxes, when the Sun's apparent motion is more sloped and yields more change in declination, leaving less for the component in right ascension, which is the only component that affects the duration of the solar day. A practical illustration of obliquity is that the daily shift of the shadow cast by the Sun in a sundial even on the equator is smaller close to the solstices and greater close to the equinoxes. If this effect operated alone, then days would be up to 24 hours and 20.3 seconds long near the solstices, and as much as 20.3 seconds shorter than 24 hours near the equinoxes.In the figure on the right, we can see the monthly variation of the apparent slope of the plane of the ecliptic at solar midday as seen from Earth. This variation is due to the apparent precession of the rotating Earth through the year, as seen from the Sun at solar midday.
In terms of the equation of time, the inclination of the ecliptic results in the contribution of a sine wave variation with an amplitude of 9.87 minutes and a period of a half year to the equation of time. The zero points of this sine wave are reached at the equinoxes and solstices, while the extrema are at the beginning of February and August and the beginning of May and November.
Secular effects
The two above mentioned factors have different wavelengths, amplitudes and phases, so their combined contribution is an irregular wave. At epoch 2000 these are the values :| Point | Value | Date |
| minimum | −14 min 15 s | 11 February |
| zero | 0 min 0 s | 15 April |
| maximum | +3 min 41 s | 14 May |
| zero | 0 min 0 s | 13 June |
| minimum | −6 min 30 s | 26 July |
| zero | 0 min 0 s | 1 September |
| maximum | +16 min 25 s | 3 November |
| zero | 0 min 0 s | 25 December |
On shorter timescales the shifts in the dates of equinox and perihelion will be more important. The former is caused by precession, and shifts the equinox backwards compared to the stars. But it can be ignored in the current discussion as our Gregorian calendar is constructed in such a way as to keep the vernal equinox date at 20 March. The shift of the perihelion is forwards, about 1.7 days every century. In 1246 the perihelion occurred on 22 December, the day of the solstice, so the two contributing waves had common zero points and the equation of time curve was symmetrical: in Astronomical Algorithms Meeus gives February and November extrema of 15 m 39 s and May and July ones of 4 m 58 s. Before then the February minimum was larger than the November maximum, and the May maximum larger than the July minimum. In fact, in years before −1900 the May maximum was larger than the November maximum. In the year −2000 the May maximum was +12 minutes and a couple seconds while the November maximum was just less than 10 minutes. The secular change is evident when one compares a current graph of the equation of time with one from 2000 years ago, e.g., one constructed from the data of Ptolemy.
Graphical representation
Practical use
If the gnomon is not an edge but a point, the shadow will trace out a curve during the course of a day. If the shadow is cast on a plane surface, this curve will be a conic section, since the circle of the Sun's motion together with the gnomon point define a cone. At the spring and fall equinoxes, the cone degenerates into a plane and the hyperbola into a line. With a different hyperbola for each day, hour marks can be put on each hyperbola which include any necessary corrections. Unfortunately, each hyperbola corresponds to two different days, one in each half of the year, and these two days will require different corrections. A convenient compromise is to draw the line for the "mean time" and add a curve showing the exact position of the shadow points at noon during the course of the year. This curve will take the form of a figure eight and is known as an analemma. By comparing the analemma to the mean noon line, the amount of correction to be applied generally on that day can be determined.The equation of time is used not only in connection with sundials and similar devices, but also for many applications of solar energy. Machines such as solar trackers and heliostats have to move in ways that are influenced by the equation of time.
Civil time is the local mean time for a meridian that often passes near the center of the time zone, and may possibly be further altered by daylight saving time. When the apparent solar time that corresponds to a given civil time is to be found, the difference in longitude between the site of interest and the time zone meridian, daylight saving time, and the equation of time must all be considered.
Calculating the equation of time
The equation of time is obtained from a published table, or a graph. For dates in the past such tables are produced from historical measurements, or by calculation; for future dates, of course, tables can only be calculated. In devices such as computer-controlled heliostats the computer is often programmed to calculate the equation of time. The calculation can be numerical or analytical. The former are based on numerical integration of the differential equations of motion, including all significant gravitational and relativistic effects. The results are accurate to better than 1 second and are the basis for modern almanac data. The latter are based on a solution that includes only the gravitational interaction between the Sun and Earth, simpler than but not as accurate as the former. Its accuracy can be improved by including small corrections.The following discussion describes a reasonably accurate algorithm for the equation of time that is well known to astronomers. It also shows how to obtain a simple approximate formula, that can be easily evaluated with a calculator and provides the simple explanation of the phenomenon that was used previously in this article.
Mathematical description
The precise definition of the equation of time isThe quantities occurring in this equation are
- EOT, the time difference between apparent solar time and mean solar time;
- GHA, the Greenwich Hour Angle of the apparent Sun;
- GMHA = Universal Time − Offset, the Greenwich Mean Hour Angle of the mean Sun.
According to the definitions of the angles on the celestial sphere
where:
- GAST is the Greenwich apparent sidereal time. This is a known function of UT.
- is the right ascension of the apparent Sun.
Like the formula for GHA above, one can write, where the last term is the right ascension of the mean Sun. The equation is often written in these terms as
where. In this formulation a measurement or calculation of EOT at a certain value of time depends on a measurement or calculation of at that time. Both and vary from 0 to 24 hours during the course of a year. The former has a discontinuity at a time that depends on the value of UT, while the later has its at a slightly later time. As a consequence, when calculated this way EOT has two, artificial, discontinuities. They can both be removed by subtracting 24 hours from the value of EOT in the small time interval after the discontinuity in and before the one in. The resulting EOT is a continuous function of time.
Another definition, denoted to distinguish it from EOT, is
Here, is the Greenwich mean sidereal time. Therefore, GMST is an approximation to GAST ; eqeq is called the equation of the equinoxes and is due to the wobbling, or nutation of the Earth's axis of rotation about its precessional motion. Since the amplitude of the nutational motion is only about 1.2 s the difference between EOT and can be ignored unless one is interested in subsecond accuracy.
A third definition, denoted to distinguish it from EOT and, and now called the Equation of Ephemeris Time is
here is the ecliptic longitude of the mean Sun.
The difference is 1.3 s from 1960 to 2040. Therefore, over this restricted range of years is an approximation to EOT whose error is in the range 0.1 to 2.5 s depending on the longitude correction in the equation of the equinoxes; for many purposes, for example correcting a sundial, this accuracy is more than good enough.
Right ascension calculation
The right ascension, and hence the equation of time, can be calculated from Newton's two-body theory of celestial motion, in which the bodies describe elliptical orbits about their common mass center. Using this theory, the equation of time becomeswhere the new angles that appear are
- , is the mean anomaly, the angle from the periapsis of the elliptical orbit to the mean Sun; its range is from 0 to 2 as increases from to ;
- = days is the length of time in an anomalistic year: the time interval between two successive passages of the periapsis;
- , is the ecliptic longitude of the periapsis;
- is dynamical time, the independent variable in the theory. Here it is taken to be identical with the continuous time based on UT, but in more precise calculations the small difference between them must be accounted for as well as the distinction between UT1 and UTC.
- is the value of at the periapsis.
- , the Sun's eccentric anomaly ;
- , the Sun's true anomaly;
- , the Sun's true longitude on the ecliptic.
Now given a value of, one can calculate by means of the following well-known procedure:
First, given, calculate from Kepler's equation:
Although this equation cannot be solved exactly in closed form, values of can be obtained from infinite series, graphical, or numerical methods. Alternatively, note that for,, and by iteration:
This approximation can be improved, for small, by iterating again,
and continued iteration produces successively higher order terms of the power series expansion in. For small values of two or three terms of the series give a good approximation for ; the smaller, the better the approximation.
Next, knowing, calculate the true anomaly from an elliptical orbit relation
The correct branch of the multiple valued function to use is the one that makes a continuous function of starting from. Thus for use, and for use. At the specific value for which the argument of is infinite, use. Here is the principal branch, ; the function that is returned by calculators and computer applications. Alternatively, this function can be expressed in terms of its Taylor series in, the first three terms of which are:
For small this approximation is a good one. Combining the approximation for with this one for produces
The relation is called the equation of the center; the expression written here is a second-order approximation in. For the small value of that characterises the Earth's orbit this gives a very good approximation for.
Next, knowing, calculate from its definition:
The value of varies non-linearly with because the orbit is elliptical and not circular. From the approximation for :
Finally, knowing calculate from a relation for the right triangle on the celestial sphere shown above
Note that the quadrant of is the same as that of, therefore reduce to the range 0 to 2 and write
where is 0 if is in quadrant 1, it is 1 if is in quadrants 2 or 3 and it is 2 if is in quadrant 4. For the values at which tan is infinite,.
Although approximate values for can be obtained from truncated Taylor series like those for, it is more efficacious to use the equation
where. Note that for, and iterating twice:
Equation of time
The equation of time is obtained by substituting the result of the right ascension calculation into an equation of time formula. Here is used; in part because small corrections, that would justify using, are not included, and in part because the goal is to obtain a simple analytical expression. Using two term approximations for and, allows to be written as an explicit expression of two terms, which is designated because it is a first order approximation in and in.This equation was first derived by Milne, who wrote it in terms of. The numerical values written here result from using the orbital parameter values, =, = ° = radians, and = ° = radians that correspond to the epoch 1 January 2000 at 12 noon UT1. When evaluating the numerical expression for as given above, a calculator must be in radian mode to obtain correct values because the value of in the argument of the second term is written there in radians. Higher order approximations can also be written, but they necessarily have more terms. For example, the second order approximation in both and consists of five terms
This approximation has the potential for high accuracy, however, in order to achieve it over a wide range of years, the parameters,, and must be allowed to vary with time. This creates additional calculational complications. Other approximations have been proposed, for example, which uses the first order equation of the center but no other approximation to determine, and which uses the second order equation of the center.
The time variable,, can be written either in terms of, the number of days past perihelion, or, the number of days past a specific date and time :
Here is the value of at the chosen date and time. For the values given here, in radians, is that measured for the actual Sun at the epoch, 1 January 2000 at 12 noon UT1, and is the number of days past that epoch. At periapsis, so solving gives =. This puts the periapsis on 4 January 2000 at 00:11:41 while the actual periapsis is, according to results from the Multiyear Interactive Computer Almanac, on 3 January 2000 at 05:17:30. This large discrepancy happens because the difference between the orbital radius at the two locations is only 1 part in a million; in other words, radius is a very weak function of time near periapsis. As a practical matter this means that one cannot get a highly accurate result for the equation of time by using and adding the actual periapsis date for a given year. However, high accuracy can be achieved by using the formulation in terms of.
When, M is greater than 2 and one must subtract a multiple of 2 from it to bring it into the range 0 to 2. Likewise for years prior to 2000 one must add multiples of 2. For example, for the year 2010, varies from on 1 January at noon to on 31 December at noon, the corresponding values are and and are reduced to the range 0 to 2 by subtracting 10 and 11 times 2 respectively. One can always write, where is the number of days from the epoch to noon on 1 January of the desired year, and .
The result of the computations is usually given as either a set of tabular values, or a graph of the equation of time as a function of. A comparison of plots of,, and results from MICA all for the year 2000 is shown in the figure on the right. The plot of is seen to be close to the results produced by MICA, the absolute error,, is less than 1 minute throughout the year; its largest value is 43.2 seconds and occurs on day 276. The plot of is indistinguishable from the results of MICA, the largest absolute error between the two is 2.46 s on day 324.
Remark on the continuity of the equation of time
For the choice of the appropriate branch of the relation with respect to function continuity a modified version of the arctangent function is helpful. It brings in previous knowledge about the expected value by a parameter. The modified arctangent function is defined as:It produces a value that is as close to as possible. The function rounds to the nearest integer.
Applying this yields:
The parameter arranges here to set to the zero nearest value which is the desired one.
Secular effects
The difference between the MICA and results was checked every 5 years over the range from 1960 to 2040. In every instance the maximum absolute error was less than 3 s; the largest difference, 2.91 s, occurred on 22 May 1965. However, in order to achieve this level of accuracy over this range of years it is necessary to account for the secular change in the orbital parameters with time. The equations that describe this variation are:According to these relations, in 100 years, increases by about 0.5%, decreases by about 0.25%, and decreases by about 0.05%.
As a result, the number of calculations required for any of the higher-order approximations of the equation of time requires a computer to complete them, if one wants to achieve their inherent accuracy over a wide range of time. In this event it is no more difficult to evaluate using a computer than any of its approximations.
In all this note that as written above is easy to evaluate, even with a calculator, is accurate enough for correcting sundials, and has the nice physical explanation as the sum of two terms, one due to obliquity and the other to eccentricity that was used previously in the article. This is not true either for considered as a function of or for any of its higher-order approximations.
Alternative calculation
Another calculation of the equation of time can be done as follows. Angles are in degrees; the conventional order of operations applies.is the Earth's mean angular orbital velocity in degrees per day.
is the date, in days starting at zero on 1 January. 10 is the approximate number of days from the December solstice to 1 January. is the angle the earth would move on its orbit at its average speed from the December solstice to date.
is the angle the Earth moves from the solstice to date, including a first-order correction for the Earth's orbital eccentricity, 0.0167. The number 2 is the number of days from 1 January to the date of the Earth's perihelion. This expression for can be simplified by combining constants to:
is the difference between the angles moved at mean speed, and at the corrected speed projected onto the equatorial plane, and divided by 180 to get the difference in "half turns". The value 23.44° is the obliquity of the Earth's axis. The subtraction gives the conventional sign to the equation of time. For any given value of, has multiple values, differing from each other by integer numbers of half turns. The value generated by a calculator or computer may not be the appropriate one for this calculation. This may cause to be wrong by an integer number of half turns. The excess half turns are removed in the next step of the calculation to give the equation of time:
The expression Nearest integer function| means the nearest integer to. On a computer, it can be programmed, for example, as. It is 0, 1, or 2 at different times of the year. Subtracting it leaves a small positive or negative fractional number of half turns, which is multiplied by 720, the number of minutes that the Earth takes to rotate one half turn relative to the Sun, to get the equation of time.
Compared with published values, this calculation has a root mean square error of only 3.7 s. The greatest error is 6.0 s. This is much more accurate than the approximation described above, but not as accurate as the elaborate calculation.
Addendum about solar declination
The value of in the above calculation is an accurate value for the Sun's ecliptic longitude, so the solar declination becomes readily available:which is accurate to within a fraction of a degree.