sunlightIts radius is about 696300 km.We areAverage distance between the sun and the earthWhen watching the sun, the radius angle of the sun is about 960 angular seconds, which is converted into a linear scale of 1 angular second (arcsec)=725.3 kilometers (Km).The radius of the sun is not invariable. The sun itself has a process of contraction and expansion, so we generally refer to the radius of the sun as an average.
The radius of the sun is about 696300 kilometers.We areAverage distance between the sun and the earthWhen watching the sun, the radius angle of the sun is about 960 angular seconds, which is converted into a linear scale of 1 angular second (arcsec)=725.3 kilometers (Km).The radius of the sun is not invariable. The sun itself has a process of contraction and expansion, so we generally refer to the radius of the sun as an average.
The sun is formed by the gravitational collapse of the original nebula. After the sun is formed, it enters the main sequence star stage.The figure briefly shows the evolution process of the sun, which has a history of about 4.6 billion years from its formation to the present.The sun enters the stage from main sequence star to red giant star, and its radius and luminosity increase with time.Unlike rocky planets, the sun has a specific boundary.The solar radius refers to the distance from the center of the solar sphere to the outer boundary of the photosphere.For the solar model, the solar radius refers to the wavelengthλ=5000 ∨ Depth of chronologyτ_5000=1. The corresponding level.More than 2000 years ago, the method of measuring the diameter of the sun was recorded in the ancient Zhoubi Suanjing.This is the first time in the world to measure the diameter of the sun.At the end of the 19th century, Auwers measured the solar radius as 959.63 ″, which was later published as the standard solar radius by IAU.Since the systematic measurement of the solar radius in the 19th century, people have studied how large the solar radius is and whether it changes with different instruments and methods.With the continuous improvement of instrument precision and the continuous enrichment of measurement data, people gradually found some laws of the change of the solar radius.The change of solar radius involves major solar physical problems such as solar internal structure, radiation mechanism and evolution. At the same time, it also has important theoretical and practical significance for the study of many scientific problems such as solar terrestrial relationship, atmospheric physics, space weather, etc.If the radius of the sun changes, what is the law of its change?What is the cause of the change?What is the physical mechanism driving its change?These need to be further studied.
Measurement history and method
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China is the first country in the world to measure the diameter of the sun.The method of measuring the diameter of the sun in the Zhoubi Suanjing is: "Take an inch of the hollow diameter of the bamboo. It is eight feet long. Look at the shadow. The sky is covering the sun. The sun should be a hole in the sky. From this point of view. The rate is 80 inches. The path is an inch. Therefore, the sentence is the first. Take the leg as the leg. From the leg to the sun, the distance is 60000 li. The leg is invisible. From above to the sun, the distance is 80000 li. The rate is 80 li. The path is one li.A hundred thousand li leads a thousand two hundred and fifty li.So it is called.A sundial path.One thousand two hundred and fifty li ".Take an eight foot long diabolo pole with an inner diameter of one inch and aim it at the sun.When the sun passes the meridian, measure the length of the pole shadow. At this time, the sun circle just fills the inner tube of the bamboo pole.When the shadow length is exactly six feet, the ratio of the inner diameter of the bamboo tube to the length of the bamboo pole is one inch eight feet, so it is considered that the diameter of the sun is one eightieth of the distance between the sun and the earth.This value is close to the diameter of the sun, which is one hundredth of the average distance between the sun and the earth.In Lingxian, Zhang Heng described the angular diameter of the sun and the moon as "hanging like the moon, which is as big as the sun and the moon. Its diameter is one seventh of the week of the day".It is converted to the current 360 degree system, namely 29 '. 21' '. Compared with the average angular diameters of the sun and the moon obtained by modern astronomical measurements, 31' 59 '' and 31 '. 5' ', the error is only 2'.Under the scientific and technological level and observation conditions at that time, this value was quite accurate[1]。
French astronomer Picard measured the solar radius more accurately during the Monde minimum. In order to commend his contribution to the study of the solar radius, a satellite launched by France in 2010 to measure the solar diameter was named after him.The system began to measure the solar radius in the 19th century. In 1891, Auwers measured the solar radius as 959.63 '', which was later published as the standard solar radius.The measurement of solar radius is mainly based on geometric principles. The common measurement methods are:
Meridian circle measurement;
Eclipses and transits of Mercury;
Telescope drift scanning technology;
Contour method;
Satellite angular distance measurement.
Meridian circle measurement
By recording the time when the sun passes through the meridian circle and measuring the angle from the zenith to the upper and lower edges of the sun, this method isRoyal Observatory Greenwich An early effort to measure the radius of the sun.This work lasted from 1836 to 1953.Before 1851, the time when the sun passed through the meridian circle was recorded by listening to the sound of a pendulum.After 1854, this method of "eyes and ears" observation was replaced by the chronograph method, which can automatically record the time when the sun passes through the meridian circle;The introduction of this method also leads to the discontinuity of data.In 1891 and 1906, the objective lens of the telescope was polished twice, which may affect the image of observing the solar radius.In 1915, the "personal micrometer" was installed on the telescope, which would affect the measurement of the horizontal solar radius.The human error caused by the measurement of different observers will also affect the measured value. During the period from 1861 to 1883, there were 9 observers;The error range of the average value of the vertical diameter they observed reached 4.8 ", and the error range of the horizontal diameter reached 2.2".From 1915 to 1949, there were 7 observers in total, 5 of whom had no obvious change trend in their measurement results, and 2 had relatively large and unstable personal errors in their results;Therefore, the average value measured by seven people is gradually decreasing, giving a false trend of decreasing solar radius.In addition, the method is limited by weather and observation conditions, such as "cloud cover", "instability", "unclear definition", "bad image", "extremely poorly defined" and other factors.Therefore, the meridian circle measurement is not suitable for studying the possible changes of the solar radius.
Solar eclipses and planetary transits
The principle of measuring the solar radius is similar to that of measuring the solar radius by the phenomenon of solar eclipse and planetary transit, both of which are based on the principle of the revolution of the moon, the earth and the planets.A solar eclipse is when the moon moves between the sun and the earth. When the three are in the same straight line, the moon blocks the light from the sun to the earth, and the shadow behind the moon falls on the earth. At this time, a solar eclipse occurs.Assuming that the sun and the moon are ideal spheres, since their distances are known, the size of the sun's radius can be calculated by accurately measuring the four contact times.The relative distance between the sun and the moon edge moves at the rate of 0.5 ″ per second. If the measurement accuracy at the contact time can reach the order of one tenth of a second, the measurement accuracy of the solar radius will be higher than 0.1 ″.Using solar eclipse to measure the solar radius, the most widely used is the Bayliju event.During a solar eclipse, when the moon obscures the solar sphere, sunlight can still be emitted through the concave part due to the uneven surface of the moon, forming bright spots similar to pearls. It was named after the British astronomer Bailey who first observed it in 1836.The change of solar radius can be estimated by measuring the time of appearance and disappearance of bailey bead, and the accuracy can reach 0.01 ″.The line length of 0.1 ″ on the lunar surface is about 190m, but on the lunar surface, the maximum deviation between the real edge and the average edge can reach ± 2 ″, far more than 190m.Therefore, to measure the solar radius, it is necessary to know the terrain of the moon edge accurately, and the accuracy is higher than that of the general, which is difficult to achieve.However, the situation can be greatly improved by measuring near the food belt boundary, because:
Due to geometric reasons, the impact of the optical libration of the moon on the contact time is much lower in the polar region of the moon than in other regions of the moon;
In recent decades, the long-term observation of lunar occultation has greatly improved the correction of the lunar polar terrain.
Therefore, during the total eclipse or annular eclipse of the day, the observation near the southern and northern boundaries of the eclipse zone can provide valuable data for the measurement of changes in the solar radius.Dunham analyzed three solar eclipses in 17151776 and 1979, and found that the solar radius had almost no change from 1976 to 1979;However, from 1715 to 1979, the radius decreased by 0.34 '' ± 0.2 ''.Parkinson et al. systematically studied the solar eclipses of 17151842185118781900 in 1925 and 1966 in 1980.During the annular eclipse on September 23, 1987, two observation teams of the Shanghai Observatory made measurements near the southern and northern boundaries of the annular eclipse zone;They compared the measured value of the solar radius correction number with the previous four solar eclipses, and the result showed that from 1715 to 1987, the solar radius had a shrinking trend.Adassuriya et al. studied the annular eclipse on January 15, 2010, and the results showed that the radius correction number was 0.26 ″ ± 0.18 ″, which was not related to the solar cycle.Kilcik systematically summarized the correction numbers of the solar radius of the main solar eclipse observations from 1715 to 2006, as shown in the table.
Mercury transitThe principle of occurrence is similar to that of solar eclipse.When Mercury or Venus moves between the sun and the earth, and the three happen to be in a straight line, Mercury or Venus blocks part of the solar surface. On the earth, a small black spot on the sun can be observed moving slowly. This phenomenon is called Mercury or Venus transit.The surveyors can calculate the solar radius by recording the time when Mercury or Venus passes through the solar circle, using the parameters such as the distance between the sun and the earth, and the distance between Mercury and the earth.Mercury transits occur about 13 times every 100 years, and the diameter of the sun has been measured by the Mercury transits method for more than 300 years.Morrison and Ward summarized 30 transits of Mercury in the past 250 years in 1975.In the past, the diameter of the sun was measured mainly through the transit of Mercury.Mercury transit is a method of measuring the radius of the sun with high accuracy. According to the orbit of the Earth and Mercury, the Mercury transit phenomenon occurs in May or November, and the longest duration (crossing the solar circle) in May is 8 hours, and in November is 6 hours.If the recording time of Mercury just touching the edge of the sun can be accurate to 1s, then the resolution of the obtained solar radius can reach 0.1 ″.However, because it is difficult for observers to distinguish the first contact of Mercury with the edge of the sun, the standard deviation of each observation can reach 0.5 ″~1 ″.
Mercury transitThe conclusion of the research on measuring the solar radius has not been unified.Shapiro believes that the sun's radius has shrunk.Shapiro et al. believe that the change of the solar radius does not exceed 0.1 '' in 100 years, and the change of the radius has an 80 year cycle.Sveshnikov found that the change of solar radius has 80 year and 11 year change periods.Emilio et al. measured the solar radius with the transit of Mercury on May 7, 2003 and November 8, 2006. This was the first time in the history of measuring the solar radius with high accuracy from space, and their measurement value was 960.12 "± 0.09".The record of measuring the solar radius through the transit of Mercury and Venus has a long history, but due to the limitation of observation time, it is impossible to carry out frequent measurements, and the data obtained are sporadic.
Telescope drift scanning technology
Telescope drift scanning technology is also called time delay integral readout technology.The principle of CCD charge gradual transfer is used to control the speed of charge parallel transfer along the column direction (realized by vertical clock) and the speed of serial data reading along the line direction (realized by horizontal clock) through the sequential circuit, so that the speed of parallel transfer matches the size of the target drift speed;The purpose of charge tracking is realized while the charge accumulates. Since there is no relative movement between the target image on the CCD photosensitive surface and the CCD electronic image, the image formed on the CCD photosensitive surface drifts with the target, so that a good round star image of moving objects can be obtained.Wittman et al. used this technology to measure the solar radius.Wittman et al. measured the solar radius 1122 times from July to October 1990 (472 times in Izana and 650 times in Localo), and the average solar radius R=960.56 '' ± 0.03.Their research shows that the amplitude of the variation of the solar radius will not exceed 0.3 ''.
Contour method
Laclare of the French Observatory has measured the solar radius with an astrolabe since 1976.Xu Jiayan and others summarized their measurement principle: observe the time when the upper and lower edges of the sun pass through the same contour circle.In order to improve the observation accuracy, the upper and lower edges of each edge of the sun before and after passing the contour circle should be recorded for many times, and these records should be reduced to the time when the edge is tangent to the contour circle, and then their average values should be taken.Although the apparent circle of the sun is large, the atmospheric refraction correction is carried out on the same zenith distance because the time measured is the time when the upper and lower edges of the sun pass the same contour circle, so the influence of the atmospheric refraction coefficient error is small and can be ignored.On this basis, Xu Jiayan and others proposed to usePhotoelectric altimeterTwo schemes for measuring the solar radius (single image scheme and dual image scheme) have been simulated, and the results show that both schemes can be better than 0.1. The solar radius data measured by French Calern Observatory is the longest in the history of measuring the solar radius, with a relatively continuous set of data years（1976-2006), this group of data is widely used for the periodic analysis of the solar radius (Golbasi et al. mainly summarized the results of the drift scanning method and the contour method to measure the solar radius).
Satellite angular distance measurement
At present, there are mainly two satellites measuring the solar radius, namely SOHO/MDI and Picard satellites.The advantage of satellite observation is that it avoids the influence of atmospheric disturbance and seasonal factors, so the observation value is more accurate and has better continuity.MDI uses the solar theoretical model to obtain the solar earthquake radius by observing the solar mode oscillation.The measured data are characterized by high stability and low noise. The solar earthquake radius measured by MDI corresponds to the height range of 5000-10000km below the photosphere, which is different from the solar radius measured from the solar sphere center to the surface of the photosphere. The two cannot be directly compared.Schou analyzed the f-mode frequency observed by MDI, and pointed out that the solar daily earthquake radius observed by MDI was R=(695.68 ± 0.03) Mm, which was nearly 300 km smaller than the standard value of the solar radius R=(695.99 ± 0.07) Mm (million meters).Emilio et al. compared the number of Fourier smoothed sunspots, the solar radius observed by the ground telescope of CERGA and the solar radius observed by SOHO/MDI, and found that the amplitude of increasing the solar radius observed by MDI was far less than the solar radius observed by the ground telescope of CERGA.Sofia et al. pointed out that the change in the radius of the photosphere is far greater than that of the sun below the photosphere.Kuhn et al. studied the MDI solar radius data and found that the annual variation of the solar radius would not exceed 15 mas/a.Similarly, Bush et al. showed that the variation of the daily earthquake radius does not exceed 1.2 mas/a.Emilio et al. measured the solar radius of 959.28 "+/- 0.15" with MDI;This value is slightly smaller than the value measured by the ground telescope, but it is in good agreement with the value measured by Schou.Picard satellite was launched on June 15, 2010. Its main goal is to measure the solar irradiance, solar radius and solar edge contour more accurately and synchronously, and to study the internal conditions of the sun through the method of solar earthquakes.The diameter of the sun in Picard satellite and the surface imager measure the diameter of the sun once every minute, with an accuracy of several thousandths of an angular second.We also expect that the data measured by Picard satellite can accurately give the characteristics of the change of solar radius.
