Solar rotation varies with
latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
. The
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
is not a solid body, but is composed of a gaseous
plasma. Different latitudes rotate at different periods. The source of this differential rotation is an area of current research in solar astronomy. The rate of surface
rotation is observed to be the fastest at the
equator (latitude ) and to decrease as latitude increases. The solar
rotation period is 24.47 days at the equator and almost 38 days at the
poles
Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, who share a common history, culture, the Polish language and are identified with the country of Poland in C ...
. The average rotation is 28 days.
Surface rotation as an equation
The
differential rotation
Differential rotation is seen when different parts of a rotating object move with different angular velocities (rates of rotation) at different latitudes and/or depths of the body and/or in time. This indicates that the object is not solid. In fl ...
rate is usually described by the equation:
:
where
is the angular velocity in degrees per day,
is the solar latitude, A is angular velocity at the equator, and B, C are constants controlling the decrease in velocity with increasing latitude. The values of A, B, and C differ depending on the techniques used to make the measurement, as well as the time period studied. A current set of accepted average values is:
:A= 14.713 ± 0.0491 °/day
:B= -2.396 ± 0.188 °/day
:C= -1.787 ± 0.253 °/day
Sidereal rotation
At the equator, the solar rotation period is 24.47 days. This is called the
sidereal rotation period, and should not be confused with the
synodic rotation period of 26.24 days, which is the time for a fixed feature on the Sun to rotate to the same apparent position as viewed from
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. The synodic period is longer because the Sun must rotate for a sidereal period plus an extra amount due to the orbital motion of Earth around the Sun. Note that astrophysical literature does not typically use the equatorial rotation period, but instead often uses the definition of a
Carrington rotation: a synodic rotation period of 27.2753 days or a sidereal period of 25.38 days. This chosen period roughly corresponds to the
prograde rotation at a latitude of 26° north or south, which is consistent with the typical latitude of
sunspots and corresponding periodic solar activity. When the Sun is viewed from the "north" (above Earth's north pole), solar rotation is counterclockwise (eastward). To a person standing on the
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...
, sunspots would appear to move from left to right across the Sun's face.
The Sun's rotation is counterclockwise as viewed looking down from an imaginary point above the Sun's North Pole. When viewed from an imaginary point above the Earth's
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...
, sunspots are seen to move from left to right across the Sun's face. In Stonyhurst heliographic coordinates, the left side of the Sun's face is called East, and the right side of the Sun's face is call West. Therefore, sunspots are said to move across the Sun's face from east to west.
Bartels' Rotation Number
Bartels' Rotation Number is a serial count that numbers the apparent rotations of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
as viewed from Earth, and is used to track certain recurring or shifting patterns of solar activity. For this purpose, each rotation has a length of exactly 27 days, close to the synodic Carrington rotation rate.
Julius Bartels
Julius Bartels (17 August 1899, Magdeburg – 6 March 1964) was a German geophysicist and statistician who made notable contributions to the physics of the Sun and Moon; to geomagnetism and meteorology; and to the physics of the ionosphere. He al ...
arbitrarily assigned rotation one day one to 8 February 1832. The
serial number serves as a kind of calendar to mark the recurrence periods of solar and
geophysical
Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' some ...
parameters.
Carrington rotation
The Carrington rotation is a system for comparing locations on the Sun over a period of time, allowing the following of
sunspot groups or reappearance of eruptions at a later time.
Because solar rotation is variable with latitude, depth and time, any such system is necessarily arbitrary and only makes comparison meaningful over moderate periods of time. Solar rotation is taken to be 27.2753 days (see below) for the purpose of Carrington rotations. Each rotation of the Sun under this scheme is given a unique number called the Carrington Rotation Number, starting from November 9, 1853. (The Bartels Rotation Number is a similar numbering scheme that uses a period of exactly 27 days and starts from February 8, 1832.)
The heliographic longitude of a solar feature conventionally refers to its angular distance relative to the central meridian crossed by the Sun-Earth radial line.
The "Carrington longitude" of the same feature refers to an arbitrary fixed reference point of an imagined rigid rotation, as defined originally by Richard Christopher Carrington.
Carrington determined the solar rotation rate from low latitude sunspots in the 1850s and arrived at 25.38 days for the sidereal rotation period. Sidereal rotation is measured relative to the stars, but because the Earth is orbiting the Sun, we see this period as 27.2753 days.
It is possible to construct a diagram with the longitude of sunspots horizontally and time vertically. The longitude is measured by the time of crossing the central meridian and based on the Carrington rotations. In each rotation, plotted under the preceding ones, most sunspots or other phenomena will reappear directly below the same phenomenon on the previous rotation. There may be slight drifts left or right over longer periods of time.
The
Bartels "musical diagram" or the Condegram spiral plot are other techniques for expressing the approximate 27-day periodicity of various phenomena originating at the solar surface.
Using sunspots to measure rotation
The rotation constants have been measured by measuring the motion of various features ("tracers") on the solar surface. The first and most widely used tracers are
sunspots. Though sunspots had been observed since ancient times, it was only when the telescope came into use that they were observed to turn with the Sun, and thus the period of the solar rotation could be defined. The English scholar
Thomas Harriot
Thomas Harriot (; – 2 July 1621), also spelled Harriott, Hariot or Heriot, was an English astronomer, mathematician, ethnographer and translator to whom the theory of refraction is attributed. Thomas Harriot was also recognized for his con ...
was probably the first to observe sunspots telescopically as evidenced by a drawing in his notebook dated December 8, 1610, and the first published observations (June 1611) entitled “De Maculis in Sole Observatis, et Apparente earum cum Sole Conversione Narratio” ("Narration on Spots Observed on the Sun and their Apparent Rotation with the Sun") were by
Johannes Fabricius
Johann Goldsmid, better known by his Latinisation of names, Latinized name Johann(es) Fabricius (8 January 1587 – 19 March 1616), eldest son of David Fabricius (1564–1617), was a Frisians, Frisian/Germans, German astronomer and a discoverer of ...
who had been systematically observing the spots for a few months and had noted also their movement across the solar disc. This can be considered the first observational evidence of the solar rotation.
Christoph Scheiner
Christoph Scheiner SJ (25 July 1573 (or 1575) – 18 June 1650) was a Jesuit priest, physicist and astronomer in Ingolstadt.
Biography Augsburg/Dillingen: 1591–1605
Scheiner was born in Markt Wald near Mindelheim in Swabia, earlier markgrav ...
(“Rosa Ursine sive solis”, book 4, part 2, 1630) was the first to measure the equatorial rotation rate of the Sun and noticed that the rotation at higher latitudes is slower, so he can be considered the discoverer of solar differential rotation.
Each measurement gives a slightly different answer, yielding the above standard deviations (shown as +/-). St. John (1918) was perhaps the first to summarise the published solar rotation rates, and concluded that the differences in series measured in different years can hardly be attributed to personal observation or to local disturbances on the Sun, and are probably due to time variations in the rate of rotation, and Hubrecht (1915) was the first one to find that the two solar
hemispheres rotate differently. A study of magnetograph data showed a synodic period in agreement with other studies of 26.24 days at the equator and almost 38 days at the poles.
Internal solar rotation
![Tachocline](https://upload.wikimedia.org/wikipedia/commons/4/47/Tachocline.svg)
Until the advent of
helioseismology
Helioseismology, a term coined by Douglas Gough, is the study of the structure and dynamics of the Sun through its oscillations. These are principally caused by sound waves that are continuously driven and damped by convection near the Sun's sur ...
, the study of wave oscillations in the Sun, very little was known about the internal rotation of the Sun. The differential profile of the surface was thought to extend into the solar interior as rotating cylinders of constant angular momentum. Through helioseismology this is now known not to be the case and the rotation profile of the Sun has been found. On the surface, the Sun rotates slowly at the poles and quickly at the equator. This profile extends on roughly radial lines through the
solar convection zone to the interior. At the
tachocline the rotation abruptly changes to solid-body rotation in the
solar radiation zone.
See also
*
Differential rotation in stars
Differential rotation is seen when different parts of a rotating object move with different angular velocities (rates of rotation) at different latitudes and/or depths of the body and/or in time. This indicates that the object is not solid. In flu ...
*
Magnetohydrodynamics
Magnetohydrodynamics (MHD; also called magneto-fluid dynamics or hydromagnetics) is the study of the magnetic properties and behaviour of electrically conducting fluids. Examples of such magnetofluids include plasmas, liquid metals, ...
*
Orbital period
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
*
Tachocline
References
{{Reflist, 30em
*Cox, Arthur N., Ed. "Allen's Astrophysical Quantities", 4th Ed, Springer, 1999.
*Javaraiah, J., 2003. Long-Term Variations in the Solar Differential Rotation. Solar Phys., 212 (1): 23-49.
*St. John, C., 1918. The present condition of the problem of solar rotation, Publications of the Astronomical Society of the Pacific, V.30, No. 178, 319-325.
External links
Carrington Rotation Commencement Dates 1853-2016Carrington Rotation Start and Stop Times
Sun
Articles containing video clips