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Heliometry is the science of measuring the properties of the Sun.


Main source: Astronomy
This figure shows the extraterrestrial solar spectral irradiance of the Sun. Credit: Sch.
CO2, temperature, and sunspot activity are diagrammed since 1850. Credit: Leland McInnes.

"The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.[1]"[2] The effective temperature of the surface of the Sun's photosphere is 5,778 K.[3] The "[t]emperature at [the] bottom of [the Sun's] photosphere [is] 6600 K", while the "[t]emperature at [the] top of [the] photosphere [is] 4400 K".[3] The photosphere is "~400 km" in thickness.[3]

The peak emittance wavelength of 501.5 nm (~0.5 eV) makes the photosphere a primarily green radiation source. The figure at the right shows the extraterrestrial solar spectral irradiance as compared with a blackbody spectrum. The sharper than black-body cutoff at the shorter wavelength end indicates an even lower likelihood that X-rays are emitted from the photosphere.


Main source: Radiation
Solar irradiance spectrum is diagrammed above atmosphere and at the Earth's surface. Credit: Robert A. Rohde.
One composite is graphed of the last 30 years of solar variability. Credit: Robert A. Rohde.

"Direct irradiance measurements have only been available during the last three cycles and are based on a composite of many different observing satellites.[4] [5] However, the correlation between irradiance measurements and other proxies of solar activity make it reasonable to estimate past solar activity. Most important among these proxies is the record of sunspot observations that has been recorded since ~1610. Since sunspots and associated faculae are directly responsible for small changes in the brightness of the sun, they are closely correlated to changes in solar output. Direct measurements of radio emissions from the Sun at 10.7 cm also provide a proxy of solar activity that can be measured from the ground since the Earth's atmosphere is transparent at this wavelength. Lastly, solar flares are a type of solar activity that can impact human life on Earth by affecting electrical systems, especially satellites. Flares usually occur in the presence of sunspots, and hence the two are correlated, but flares themselves make only tiny perturbations of the solar luminosity."[6]

"Solar irradiance and insolation are measures of the amount of sunlight that reaches the Earth. The equipment used might measure optical brightness, total radiation, or radiation in various frequencies."[6]

"High precision radiometric observations of the Sun carried out by several satellites since the late-1970s have shown that the Sun undergoes small cyclic variations in brightness. These brightness changes are closely related to the ~ 11 year sunspot cycle. Over the last three solar sunspot cycles (Cycles 21, 22, & 23), the observed bolometric brightness of the Sun varied by 0.12 percent (see, e.g., Lean 1997). However, surprisingly (at least at first) the Sun is brightest during the times of maximum sunspot number and faintest during the sunspot minima. This has been explained (and modeled) by the larger changes in the areal coverage and intensity of magnetic white light facular regions that peak near sunspot maximum. Thus, the observed light variations of the Sun arise from the blocking effect of sunspots and increased facular contribution to brightness in which the facular contributions slightly offset the light blocking effects of sunspots."[7]

"Even though the observed light (bolometric) variations are small over its activity cycle, variations over the sunspot cycle are much larger at shorter wavelengths (Lean 1997; Rottman, this volume). [The] typical variations of solar coronal X-ray emissions from the minimum to the maximum of the ~ 11 year activity cycle are nearly 500 percent. The cyclic changes arising from variations in the chromospheric and transition region emission range from 10 to 200 percent at NUV, FUV and EUV wavelengths. Also, the frequencies and intensities of flaring events and coronal mass ejections (CME) are strongly correlated with the Sun's activity cycle. For example, the rate of CME occurrences is larger during the sunspot maxima than during the [...] sunspot minima (Webb & Howard 1994). [The] changes in the XUV flux of the Sun over a typical activity cycle are significantly larger and these high energy solar emissions are absorbed and heat the Earth's stratosphere and thermosphere. Although the deposited energy is small, non-linear feedback mechanisms could amplify the effect on climate by, for example, altering the tropospheric heat exchange between the equator and polar regions."[7]

Theoretical heliometry[edit]


Notation: let the symbol '"`UNIQ--postMath-00000001-QINU`"' represent the net solar charge.

"[A] variety of geophysical and astrophysical phenomena can be explained by a net charge on the Sun of -1.5 x 1028 e.s.u."[8] This figure was later reduced by a factor of five.[9]



"The luminosity of stars is measured in two forms: apparent (visible light only) and bolometric (total radiant energy). (A bolometer is an instrument that measures radiant energy over a wide band by absorption and measurement of heating.) When not qualified, "luminosity" means bolometric luminosity, which is measured either in the SI units, watts; or in terms of solar luminosities, '"`UNIQ--postMath-00000003-QINU`"', that is, how many times as much energy the object radiates as the Sun".[10]

Notation: let the symbol '"`UNIQ--postMath-00000004-QINU`"' represent the solar bolometric luminosity.

"The solar luminosity, ['"`UNIQ--postMath-00000005-QINU`"'], is a unit of radiant flux (power emitted in the form of photons) conventionally used .. to measure the luminosity of stars. One solar luminosity is equal to the current accepted luminosity of the Sun, which is 3.839×1026
, or 3.839×1033
.[11] The value is slightly higher, 3.939×1026
(equivalent to 4.382×109
or 1.9×10−16
) if the solar neutrino radiation is included as well as electromagnetic radiation.[12] The Sun is a weakly variable star and its luminosity therefore fluctuates. The major fluctuation is the eleven-year solar cycle (sunspot cycle), which causes a periodic variation of about ±0.1%. Any other variation over the last 200–300 years is thought to be much smaller than this.[12]"[13]

"The solar luminosity is related to the solar irradiance measured at the Earth or by satellites in Earth orbit. The mean irradiance at the top of the Earth's atmosphere is sometimes known as the solar constant, ['"`UNIQ--postMath-00000006-QINU`"']. Irradiance is defined as power per unit area, so the solar luminosity (total power emitted by the Sun) is the irradiance received at the Earth (solar constant) multiplied by the area of the sphere whose radius is the mean distance between the Earth and the Sun:


where A is the unit distance (the value of the astronomical unit in metres) and k is a constant (whose value is very close to one) that reflects the fact that the mean distance from the Earth to the Sun is not exactly one astronomical unit."[13]

Solar radius[edit]

Notation: let the symbol '"`UNIQ--postMath-00000008-QINU`"' indicate the solar radius.

The "[s]olar radius is a unit of distance used to express the size of stars in astronomy equal to the current radius of the Sun:


The solar radius is approximately 695,500 kilometres (432,450 miles) or about 110 times the radius of the Earth ('"`UNIQ--postMath-0000000A-QINU`"'), or 10 times the average radius of Jupiter. It varies slightly from pole to equator due to its rotation, which induces an oblateness of order 10 parts per million."[14]


Main source: Physics
The graph demonstrates the motion of the barycenter of the solar system relative to the location of the center of the Sun. Credit: Carl Smith derivative of work by Rubik-wuerfel.

"The center of mass plays an important role in astronomy and astrophysics, where it is commonly referred to as the barycenter. The barycenter is the point between two objects where they balance each other; it is the center of mass where two or more celestial bodies orbit each other. When a moon orbits a planet, or a planet orbits a star, both bodies are actually orbiting around a point that lies away from the center of the primary (larger) body."[15]

"The Sun's motion about the center of mass of the Solar System is complicated by perturbations from the planets. Every few hundred years this motion switches between prograde and retrograde.[16]"[17]

Def. "the mass of the Sun" is called the astronomical unit of mass.[18]

Notation: let the symbol '"`UNIQ--postMath-0000000B-QINU`"' indicate the solar mass.

"The solar mass ('"`UNIQ--postMath-0000000C-QINU`"') is a standard unit of mass in astronomy, used to indicate the masses of other stars, as well as clusters, nebulae and galaxies. It is equal to the mass of the Sun, about two nonillion kilograms:"[19]


"This is about 332,946 times the mass of the Earth or 1,048 times the mass of Jupiter."[19]

"Because the Earth follows an elliptical orbit around the Sun, the solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass.[22] Based upon the length of the year, the distance from the Earth to the Sun (an astronomical unit or AU), and the gravitational constant (G), the mass of the Sun is given by:"[19]


"The value of the gravitational constant was derived from 1798 measurements by Henry Cavendish using a torsion balance. The value obtained differed only by about 1% from the modern value.[23] The diurnal parallax of the Sun was accurately measured during the transits of Venus in 1761 and 1769,[24] yielding a value of 9″ (compared to the present 1976 value of 8.794148″). When the value of the diurnal parallax is known, the distance to the Sun can be determined from the geometry of the Earth.[25]"[19]


Main source: Technology
SR20 is a solar radiation sensor that can be applied in scientific grade solar radiation observations. Credit: Hukseflux.
In front is a Normal Incidence Pyrheliometer (NIP) mounted on a Solar tracker. Credit: Prillen.

Def. a "device used to measure the heating power of electromagnetic radiation, especially that of solar radiation"[26] is called an actinometer.

Def. an "actinometer used to measure solar radiation incident on a surface"[27] is called a pyranometer.

At right is an SR20 solar radiation sensor. It complies with the "secondary standard" specifications within the latest ISO and WMO standards.

Def. the "total solar radiation from sun and sky on a horizontal surface"[28] is called the global radiation.

The Radiation Observatory, University of Bergen, Bergen, Norway, latitude 60° 24' N and longitude 5° 19' E, at 45 m elevation above sea level, uses one or more pyranometers to measure the Global Radiation.[28]

A sensitivity check is made of each pyranometer against a standard using the sun/shade method on a cloudless day.[28]

A sensitivity may be similar to 4.818 V/Wm-2, which should be a small factor such as 1.0165 times the original sensitivity when first manufactured.[28]

The diffuse (sky) radiation is measured by a pyranometer. "When measuring the sky radiation, the direct solar radiation is constantly shadowed off by means of a 6 cm diameter circular disc mounted on a 30 cm long rotating arm."[28]

Def. a "device that measures the intensity of solar radiation received on the surface of the earth"[29] is called a pyrheliometer.

The normal incidence beam radiation is measured by a normal incidence pyrheliometer with a known and calibrated sensitivity, e.g. 8.15 V/Wm-2.[28]

The pyrheliometer is mounted on an automatic solar tracker.[28]

Def. a "device that measures radiant energy"[30] is called a radiometer.

Ultraviolet radiation is measured by means of a total ultraviolet radiometer with a specific wavelength response such as 290 - 385 nm.[28]

"For the measurement of long-wave radiation, a ventilated [...] pyrgeometer [...] with coated silicon hemisphere [is] used. This makes it possible to compute the [Downward Atmospheric Radiation], since the temperature of the instrument is also recorded."[28]

"The [Duration of Sunshine] is measured by a Campbell-Stoke sunshine recorder with blue paper strips. The strips are read according to the rules of [the World Meteorological Organization] WMO [3]. Maximum possible duration gives the number of hours the sun is above the natural horizon, as found from the records on days with clear skies at sunrise or sunset. The [Duration of Sunshine] is also given as the number of minutes during which the [NIP records] irradiance above 120 Wm-2 (with one instantaneous recording counted as 20 seconds)."[28]

"The necessary routine calibrations of the pyranometers and the NIP pyrheliometer are carried out by means of the absolute self-calibrating cavity pyrheliometer [which in turn is] compared to the World Radiation Reference Scale (WRR)".[28]


Main source: Hypotheses
  1. The Sun may not be gaseous all the way through.

See also[edit]


  1. "The Colour of Stars". Australian Telescope Outreach and Education. Retrieved 2006-08-13. 
  2. "Star, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. June 15, 2012. Retrieved 2012-07-06. 
  3. 3.0 3.1 3.2 David R. Williams (September 2004). "Sun Fact Sheet". Greenbelt, MD: NASA Goddard Space Flight Center. Retrieved 2011-12-20. 
  4. Active Cavity Radiometer Irradiance Monitor (ACRIM) solar irradiance monitoring 1978 to present (Satellite observations of total solar irradiance); access date 2012-02-03
  6. 6.0 6.1 "Solar variation, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. November 17, 2012. Retrieved 2012-11-23. 
  7. 7.0 7.1 E. F. Guinan and I. Ribas (2002). Benjamin Montesinos, Alvaro Gimenez and Edward F. Guinan, ed. Our Changing Sun: The Role of Solar Nuclear Evolution and Magnetic Activity on Earth's Atmosphere and Climate, In: The Evolving Sun and its Influence on Planetary Environments. 269. Astronomical Society of the Pacific. pp. 85–106. Bibcode:2002ASPC..269...85G. Retrieved 2017-07-31.  Text "Template:Isbn " ignored (help)
  8. Ludwig Oster & Kenelm W. Philip (January 1961). "Existence of Net Electric Charges on Stars". Nature 189 (4758): 43. doi:10.1038/189043a0. 
  9. V. A. Bailey (January 1961). "Existence of Net Electric Charges on Stars". Nature 189 (4758): 43-4. doi:10.1038/189043b0. 
  10. "Luminosity, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. November 17, 2012. Retrieved 2012-11-23. 
  11. Lua error in Module:Citation/CS1 at line 3556: bad argument #1 to 'pairs' (table expected, got nil).
  12. 12.0 12.1 Noerdlinger, Peter D. (2008). "Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System". Celest. Mech. Dynam. Astron. 0801: 3807. 
  13. 13.0 13.1 "Solar luminosity, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. October 23, 2012. Retrieved 2012-11-23. 
  14. "Solar radius, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. May 9, 2012. Retrieved 2012-07-05. 
  15. "Center of mass, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. November 12, 2012. Retrieved 2012-11-20. 
  16. Javaraiah (2005). "Sun's retrograde motion and violation of even-odd cycle rule in sunspot activity". Monthly Notices of the Royal Astronomical Society 362 (4): 1311–8. doi:10.1111/j.1365-2966.2005.09403.x. 
  17. "Sun, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. November 18, 2012. Retrieved 2012-11-20. 
  18. P. K. Seidelmann (1976). "Measuring the Universe The IAU and astronomical units". International Astronomical Union. Retrieved 2011-11-27. 
  19. 19.0 19.1 19.2 19.3 "Solar mass, In: Wikipedia". San Francisco, California: Wikimedia Foundation, Inc. September 17, 2012. Retrieved 2012-11-23. 
  20. 2013 Astronomical Constants
  22. Harwit, Martin (1998), Astrophysical concepts (3 ed.), Springer, ISBN 0-387-94943-7, 
  23. Holton, Gerald James; Brush, Stephen G. (2001). Physics, the human adventure: from Copernicus to Einstein and beyond (3rd ed.). Rutgers University Press. p. 137. ISBN 0-8135-2908-5. 
  24. Pecker, Jean Claude; Kaufman, Susan (2001). Understanding the heavens: thirty centuries of astronomical ideas from ancient thinking to modern cosmology. Springer. pp. 291–291. ISBN 3-540-63198-4. 
  25. Barbieri, Cesare (2007). Fundamentals of astronomy. CRC Press. pp. 132–140. ISBN 0-7503-0886-9. 
  26. "actinometer, In: Wiktionary". San Francisco, California: Wikimedia Foundation, Inc. October 8, 2013. Retrieved 2013-10-28. 
  27. "pyranometer, In: Wiktionary". San Francisco, California: Wikimedia Foundation, Inc. March 4, 2013. Retrieved 2013-10-28. 
  28. 28.00 28.01 28.02 28.03 28.04 28.05 28.06 28.07 28.08 28.09 28.10 Jan Asle Olseth, Arvid Skartveit, Frank Cleveland, Tor de Lange, Tor-Villy Kangas (2004). Radiation Yearbook No. 39 (PDF). Bergen, Norway: Geophysical Institute, University of Bergen. p. 78. Retrieved 2013-10-28. 
  29. "pyrheliometer, In: Wiktionary". San Francisco, California: Wikimedia Foundation, Inc. October 7, 2013. Retrieved 2013-10-28. 
  30. "radiometer, In: Wiktionary". San Francisco, California: Wikimedia Foundation, Inc. October 8, 2013. Retrieved 2013-10-28. 

External links[edit]

{{Astronomy resources}}{{Charge ontology}}{{Technology resources}}

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