Solar coordinate systems#Heliographic

{{Further|Solar rotation}}

The Sun is a rotating sphere of plasma at the center of the Solar System. It lacks a solid or liquid surface, so the interface separating its interior and its exterior is usually defined as the boundary where plasma becomes opaque to visible light, the photosphere. Since plasma is gaseous in nature, this surface has no permanent demarcated points that can be used for reference. Furthermore, its rate of rotation varies with latitude, rotating faster at the equator than at the poles.{{cite journal |last1=Thompson |first1=W. T. |title=Coordinate systems for solar image data |journal=Astronomy & Astrophysics |date=April 2006 |volume=449 |issue=2 |pages=791–803 |doi=10.1051/0004-6361:20054262|bibcode=2006A&A...449..791T |url=https://www.aanda.org/10.1051/0004-6361:20054262/pdf }}{{cite journal |last1=Ulrich |first1=Roger K. |last2=Boyden |first2=John E. |title=Carrington Coordinates and Solar Maps |journal=Solar Physics |date=May 2006 |volume=235 |issue=1–2 |pages=17–29 |doi=10.1007/s11207-006-0041-5|bibcode=2006SoPh..235...17U }}

{{See also|Cardinal directions#In astronomy}}

File:BBSO H-alpha solar disk 2002-07-24.jpg, the cardinal directions are indicated by a compass rose in the top-left corner: north is up (↑), west is right (→), south is down (↓), and east is left (←).]]

In observations of the solar disk, cardinal directions are typically defined so that the Sun's northern and southern hemispheres point toward Earth's northern and southern celestial poles, respectively, and the Sun's eastern and western hemispheres point toward Earth's eastern and western horizons, respectively. In this scheme, clockwise from north at 90° intervals one encounters west, south, and east, and the direction of solar rotation is from east to west.{{cite news |last1=Young |first1=C. Alex |last2=Cortés |first2=Raúl |title=Why are east and west on the sun reversed? |url=https://earthsky.org/sun/east-and-west-on-the-sun-reversed/ |access-date=17 May 2024 |publisher=EarthSky |date=3 May 2022}}{{cite book |last1=Jenkins |first1=Jamey L. |title=Observing the Sun: A Pocket Field Guide |date=2013 |publisher=Springer |location=New York, NY |isbn=978-1-4614-8015-0 |pages=128–129}}

Heliographic coordinate systems are used to identify locations on the Sun's surface. The two most commonly used systems are the Stonyhurst and Carrington systems. They both define latitude as the angular distance from the solar equator, but differ in how they define longitude. In Stonyhurst coordinates, the longitude is fixed for an observer on Earth, and, in Carrington coordinates, the longitude is fixed for the Sun's rotation.{{cite encyclopedia |editor-last=Ridpath |editor-first=Ian |title=heliographic coordinates |encyclopedia=A Dictionary of Astronomy |edition=3rd |year=2018 |publisher=Oxford University Press |doi=10.1093/acref/9780191851193.001.0001 |isbn=978-0-19-185119-3 |url=https://www.oxfordreference.com/display/10.1093/acref/9780191851193.001.0001/acref-9780191851193-e-1660}}{{cite journal |last1=Sánchez-Bajo |first1=F. |last2=Vaquero |first2=J. M. |title=Measuring solar rotation from digital camera images |journal=European Journal of Physics |date=1 May 2013 |volume=34 |issue=3 |pages=527–536 |doi=10.1088/0143-0807/34/3/527|bibcode=2013EJPh...34..527S }}{{cite book |last1=Stix |first1=Michael |title=The Sun: An Introduction |series=Astronomy and Astrophysics Library |date=2002 |publisher=Springer |location=Berlin, Heidelberg |isbn=978-3-642-56042-2 |edition=2nd |doi=10.1007/978-3-642-56042-2 |url=https://link.springer.com/book/10.1007/978-3-642-56042-2 |url-access=subscription}}{{cite journal |last1=Çakmak |first1=H. |title=Computer-aided measurement of the heliographic coordinates of sunspot groups |journal=Experimental Astronomy |date=November 2014 |volume=38 |issue=1–2 |pages=77–89 |doi=10.1007/s10686-014-9410-5 |arxiv=1407.1626|bibcode=2014ExA....38...77C }}

The Stonyhurst heliographic coordinate system, developed at Stonyhurst College in the 1800s, has its origin (where longitude and latitude are both 0°) at the point where the solar equator intersects the central solar meridian as seen from Earth. Longitude in this system is therefore fixed for observers on Earth.

The Carrington heliographic coordinate system, established by Richard C. Carrington in 1863, rotates with the Sun at a fixed rate based on the observed rotation of low-latitude sunspots. It rotates with a sidereal period of exactly 25.38 days, which corresponds to a mean synodic period of 27.2753 days.{{cite book |last1=Carrington |first1=R. C. |title=Observations of the spots on the sun |date=1863 |publisher=Williams and Norgate |location=London |url=https://archive.org/details/observationsofsp00carr}}{{rp|221}}

Whenever the Carrington prime meridian (the line of 0° Carrington longitude) passes the Sun's central meridian as seen from Earth, a new Carrington rotation begins. These rotations are numbered sequentially, with Carrington rotation number 1 starting on 9 November 1853.{{cite web |title=Synoptic Maps |date=10 July 2018 |url=https://nso.edu/data/nisp-data/synoptic-maps/ |publisher=National Solar Observatory |access-date=21 December 2023}}{{cite web |title=Solar-Terrestrial Coordinate Systems |url=http://wso.stanford.edu/words/Coordinates.html |publisher=Wilcox Solar Observatory |access-date=21 December 2023}}{{cite journal |last1=Thompson |first1=W. T. |last2=Wei |first2=K. |title=Use of the FITS World Coordinate System by STEREO/SECCHI |journal=Solar Physics |date=January 2010 |volume=261 |issue=1 |pages=215–222 |doi=10.1007/s11207-009-9476-9|bibcode=2010SoPh..261..215T }}{{rp|278}}

{{further|Equatorial coordinate system#Heliocentric}}

Heliocentric coordinate systems measure spatial positions relative to an origin at the Sun's center. There are four systems in use: the heliocentric inertial (HCI) system, the heliocentric Aries ecliptic (HAE) system, the heliocentric Earth ecliptic (HEE) system, and the heliocentric Earth equatorial (HEEQ) system. They are summarized in the following table. The third axis not presented in the table completes a right-handed Cartesian triad.{{cite web |last1=Hapgood |first1=Mike |title=Heliocentric coordinate systems |url=https://www.mssl.ucl.ac.uk/grid/iau/extra/local_copy/SP_coords/heliosys.htm |publisher=Mullard Space Science Laboratory |access-date=21 December 2023 |date=July 1997}}{{cite journal |last1=Hapgood |first1=M. A. |title=Space physics coordinate transformations: A user guide |journal=Planetary and Space Science |date=May 1992 |volume=40 |issue=5 |pages=711–717 |doi=10.1016/0032-0633(92)90012-D |bibcode=1992P&SS...40..711H |url=http://www.igpp.ucla.edu/public/vassilis/ESS261/Lecture03/Hapgood_sdarticle.pdf}}{{cite journal |last1=Fränz |first1=M. |last2=Harper |first2=D. |title=Heliospheric coordinate systems |journal=Planetary and Space Science |date=February 2002 |volume=50 |issue=2 |pages=217–233 |doi=10.1016/S0032-0633(01)00119-2 |bibcode=2002P&SS...50..217F |url=http://www2.mps.mpg.de/homes/fraenz/systems/systems2art.pdf}}

• Astronomical coordinate system
• Ecliptic coordinate system

{{Ref-list}}

• [https://docs.sunpy.org/en/stable/reference/coordinates/index.html {{codett|sunpy.coordinates}}], a sub-package of SunPy used to handle solar coordinates
• [https://space.umd.edu/pm/crn/ MTOF/PM Data by Carrington Rotation], a table of Carrington rotation start and stop times

Category:Sun

Category:Astronomical coordinate systems