Heliographic coordinates

Heliographic coordinates on the solar sphere (orange arrows indicate the direction of increasing or decreasing latitude B or longitude L)

The system of heliographic coordinates is used to specify precise positions on the surface of the sun . The two spherical coordinates refer to the mean height of the photosphere (visible boundary of the sun's edge ) and are called

heliographic latitude and
called heliographic longitude .

They are defined analogously to the geographical latitude and longitude , the earth's surface corresponds to the mean elevation level of the photosphere. However, the heliographic coordinates (in contrast to the latitude and longitude information on Earth) do not refer to an ellipsoid , but to an exact sphere. In addition, unlike on Earth, the differential rotation , i. H. the different orbital times of a point on the sun's surface, depending on the parallel.

The term heliographic comes from the Greek for sun ( Hελios, Helios ) and to draw / describe (γραφειν, grafe · in) . It was introduced into astronomy by analogy with selenography , when the focus of solar research shifted from astrometry to solar physics and computational models of solar rotation became necessary.

Heliographic latitudes and longitudes

Axis of rotation, equator, central and prime meridian

The sun shows a rotation whose direction of rotation is similar to the direction of rotation of the earth around the sun, it rotates prograd . The two points at which the axis of rotation penetrates the solar sphere are the solar poles . The northern solar pole is the one that points towards the north celestial hemisphere when viewed from Earth . When viewed from the north solar pole, the sun rotates counterclockwise. West is in the direction of the twist, east in the opposite direction; In the case of the sun - unlike many other celestial bodies - degrees of longitude are counted in ascending order in the direction of rotation.

The solar equator runs perpendicular to the axis of rotation in a plane with the center of the solar sphere . It defines the heliographic latitude of zero. Towards the north, the heliographic latitude increases up to 90 ° at the North Pole, to the south it takes values of up to –90 ° at the South Pole. There are two approaches to defining longitudes: longitudes that rotate with the sun, or longitudes that are fixed relative to the observer.

Carrington coordinates

Carrington coordinates are defined in terms of merdians rotating with the sun. The determination of the prime meridian of the sun was arbitrary. The Carrington Prime Meridian , named after the British astronomer Richard Christopher Carrington , is the semicircle of longitude that passed through the ascending node of the solar equator - the intersection of the equator and the ecliptic in the direction of rotation - on January 1, 1854 at 12 noon universal time . On November 9, 1853, this prime meridian was equal to the central meridian , i.e. the longitude, which at that time (apparently) ran perpendicular to the solar equator through the center of the solar disk when viewed from the earth. Starting from this prime meridian, longitudes range from −180 ° in the east to + 180 ° in the west.

Stonyhurst coordinates

Stonyhurst disk on which the position of sunspots relative to the equator and central meridian is recorded

In the case of Stonyhurst coordinates, named after Stonyhurst College in London, the longitudes are based on the central meridian of the solar disk; they are fixed relative to the observer. A solar phenomenon that rotates with the sun has an increasing degree of longitude in this system.

In order to determine the Stonyhurst coordinates of a phenomenon observed on the solar disk, one must consider the inclination of the sun's axis of rotation to the ecliptic. The axis is inclined sideways by the position angle P ₀ and inclined to the observer by B ₀ ; P ₀ fluctuates between ± 26.3 °, B ₀ between ± 7.25 °. So-called Stonyhurst disks are graticules for different values of P ₀ and B ₀ . When placed over the solar disk, the coordinates of a phenomenon visible on the disk can be read from them.

When converting Stonyhurst to Carrington coordinates, the latitude remains the same, the longitude must be converted using the distance between the zero and central meridian, which depends on the time of observation.

Differential rotation and position determination

The rotation of the sun on the equator is slightly faster than in higher heliographic latitudes; it is a differential rotation . The sidereal rotation period is

at the equator: 25.03 days ( synodic , i.e. viewed from the rotating earth, 26.9 days)
near the poles: 30.875 days (synodically 33.708 days, about 20 percent slower)
mean, corresponding to ± 16 ° latitude: 25.38 days (synodically 27.2753 days).

For a rotation of the prime meridian, the mean value of 25.38 days is used. Accordingly, the zero and central meridian coincide approximately every 27.2753 days.

Because of the differential rotation, solar phenomena that are motionless in relation to the center of the photosphere and are not within ± 16 ° latitude will no longer lie exactly on the prime meridian after a rotation: they will be closer to the equator, but closer to the Poland, they will run after them.

Spherical shape of the sun

A second peculiarity of the heliographic compared to the geographical coordinates lies in the difference between the sun and the earth figure . The latter is approximately an ellipsoid , while the sun is almost exactly a sphere . Because a flattening of the sun is hardly detectable by measurement, ellipsoidal coordinates are not required. Therefore, a distinction does not have to be made between ellipsoidal latitude and (geo) centric latitude , but a latitude related to the central solar sphere is sufficient as a coordinate specification. The flattening of the sun, which has been sought for a long time, is very low and could only be approximately determined a few decades ago. The main problem here is the thermal influences during daytime observations .

Positions of solar phenomena

Information on the heliographic coordinates of the apparent center of the sun can be found in every more detailed Astronomical Yearbook , especially in the Astronomical Ephemeris . Among other things, they are required for the precise measurement of sunspots , flares and other phenomena in the photo- and chromosphere of the sun. The heliographic position of sunspots provides - in addition to rotation analyzes - further information on the astrophysics of the sun's interior and its convection processes . The mutual heliographic displacements of the sunspots gave the first indications of a differential solar rotation at the beginning of the 19th century (the corresponding laws of rotation were developed by the Englishman Richard Christopher Carrington and the German Gustav Spörer almost simultaneously), which soon became a research topic in gas dynamics . Spörer's law describes a connection between the course of the sunspot cycle and the mean heliographic latitude of the spots - see also the butterfly diagram .

literature

E. Junker: Position determination of solar phenomena . In: Günter D. Roth (Ed.): Handbook for Star Friends: Volume 2: Observation and Practice . Springer, 2013, ISBN 978-3-662-35380-6 , pp. 53-69 .
Arnold Hanslmeier: Introduction to Astronomy and Astrophysics . 3. Edition. Springer, 2014, ISBN 978-3-642-37700-6 , 7. The sun - 7.1 Basic data and coordinates, p. 228-229 , doi : 10.1007 / 978-3-642-37700-6 .

Web links

The Sun - Introduction to Positioning. Vereinigung der Sternfreunde , Fachgruppe Sonne, 1st edition 2017.

Individual evidence

↑ Oliver Montenbruck: Basics of the ephemeris calculation . 7th edition. 2009, ISBN 978-3-8274-2292-7 , pp. 109-110 .
↑ ^a ^b Carrington heliographic coordinates . In: Oxford Reference . doi : 10.1093 / oi / authority.20110803095551605 ( oxfordreference.com ).
↑ ^a ^b ^c ^d ^e E. Junker: Position determination of solar phenomena . In: Günter D. Roth (Ed.): Handbook for Star Friends: Volume 2: Observation and Practice . Springer, 2013, ISBN 978-3-662-35380-6 , pp. 53-69 .
^ Stonyhurst heliographic coordinates . In: Oxford Reference . doi : 10.1093 / oi / authority.20110803100534821 ( oxfordreference.com ).
↑ Hans-Ulrich Keller: Compendium of Astronomy: Introduction to the Science of the Universe . Kosmos, 2019, ISBN 978-3-440-16631-4 , pp. 96 .
↑ Arnab Rai Choudhuri: Nature's Third Cycle: A Story of sunspots . Oxford University Press, 2015, ISBN 978-0-19-967475-6 , pp. 2-4, 28-32 , doi : 10.1093 / acprof: oso / 9780199674756.001.0001 .

[1] Oliver Montenbruck: Basics of the ephemeris calculation . 7th edition. 2009, ISBN 978-3-8274-2292-7 , pp. 109-110 .

[oxfordReCarrington-2] Carrington heliographic coordinates . In: Oxford Reference . doi : 10.1093 / oi / authority.20110803095551605 ( oxfordreference.com ).

[junker2013-3] E. Junker: Position determination of solar phenomena . In: Günter D. Roth (Ed.): Handbook for Star Friends: Volume 2: Observation and Practice . Springer, 2013, ISBN 978-3-662-35380-6 , pp. 53-69 .

[4] Stonyhurst heliographic coordinates . In: Oxford Reference . doi : 10.1093 / oi / authority.20110803100534821 ( oxfordreference.com ).

[5] Hans-Ulrich Keller: Compendium of Astronomy: Introduction to the Science of the Universe . Kosmos, 2019, ISBN 978-3-440-16631-4 , pp. 96 .

[choudhuri2015-6] Arnab Rai Choudhuri: Nature's Third Cycle: A Story of sunspots . Oxford University Press, 2015, ISBN 978-0-19-967475-6 , pp. 2-4, 28-32 , doi : 10.1093 / acprof: oso / 9780199674756.001.0001 .