Solar coordinate systems

Short description: Methods to identify locations on the Sun

"Heliographic" redirects here. For other uses, see Heliograph (disambiguation).

In solar observation and imaging, coordinate systems are used to identify and communicate locations on and around the Sun. The Sun is made of plasma, so there are no permanent demarcated points that can be referenced.

## Contents

* 1 Background
* 2 Cardinal directions
* 3 Heliographic
* 3.1 Stonyhurst system
* 3.2 Carrington system
* 4 Heliocentric
* 5 See also
* 6 References
* 7 External links

## Background

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.[1][2]

## Cardinal directions

In this image of the Sun's chromosphere, 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.[3][4]

## Heliographic

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.[5][6][7][8]

### Stonyhurst system

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.[8][5]

### Carrington system

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.[9]: 221 [1][2][5]

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.[10][11][12][7]: 278

## 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.[1][13][14][15]

Common heliocentric coordinate systems  Name | Abbreviation | +X-axis | +Z-axis

|
|
|

Heliocentric inertial | HCI | Solar ascending node on ecliptic | Solar rotational axis
Heliocentric Aries ecliptic | HAE | First point of Aries | Ecliptic north pole
Heliocentric Earth ecliptic | HEE | Sun–Earth line | Ecliptic north pole
Heliocentric Earth equator | HEEQ | Intersection between solar equator and solar central meridian as seen from Earth | Solar rotational axis

## See also

* Astronomical coordinate system
* Ecliptic coordinate system

## References

1. ↑ 1.0 1.1 1.2 Thompson, W. T. (April 2006). "Coordinate systems for solar image data". Astronomy & Astrophysics 449 (2): 791–803. doi:10.1051/0004-6361:20054262. Bibcode: 2006A&A...449..791T. https://www.aanda.org/10.1051/0004-6361:20054262/pdf.
2. ↑ 2.0 2.1 Ulrich, Roger K.; Boyden, John E. (May 2006). "Carrington Coordinates and Solar Maps". Solar Physics 235 (1–2): 17–29. doi:10.1007/s11207-006-0041-5. Bibcode: 2006SoPh..235...17U.
3. ↑ Young, C. Alex; Cortés, Raúl (3 May 2022). "Why are east and west on the sun reversed?". EarthSky. https://earthsky.org/sun/east-and-west-on-the-sun-reversed/.
4. ↑ Jenkins, Jamey L. (2013). Observing the Sun: A Pocket Field Guide. New York, NY: Springer. pp. 128–129. ISBN 978-1-4614-8015-0.
5. ↑ 5.0 5.1 5.2 Ridpath, Ian, ed (2018). "heliographic coordinates". A Dictionary of Astronomy (3rd ed.). Oxford University Press. doi:10.1093/acref/9780191851193.001.0001. ISBN 978-0-19-185119-3. https://www.oxfordreference.com/display/10.1093/acref/9780191851193.001.0001/acref-9780191851193-e-1660.
6. ↑ Sánchez-Bajo, F.; Vaquero, J. M. (1 May 2013). "Measuring solar rotation from digital camera images". European Journal of Physics 34 (3): 527–536. doi:10.1088/0143-0807/34/3/527. Bibcode: 2013EJPh...34..527S.
7. ↑ 7.0 7.1 Stix, Michael (2002). The Sun: An Introduction. Astronomy and Astrophysics Library (2nd ed.). Berlin, Heidelberg: Springer. doi:10.1007/978-3-642-56042-2. ISBN 978-3-642-56042-2. https://link.springer.com/book/10.1007/978-3-642-56042-2.
8. ↑ 8.0 8.1 Çakmak, H. (November 2014). "Computer-aided measurement of the heliographic coordinates of sunspot groups". Experimental Astronomy 38 (1–2): 77–89. doi:10.1007/s10686-014-9410-5. Bibcode: 2014ExA....38...77C.
9. ↑ Carrington, R. C. (1863). Observations of the spots on the sun. London: Williams and Norgate. https://archive.org/details/observationsofsp00carr.
10. ↑ "Synoptic Maps". National Solar Observatory. 10 July 2018. https://nso.edu/data/nisp-data/synoptic-maps/.
11. ↑ "Solar-Terrestrial Coordinate Systems". Wilcox Solar Observatory. http://wso.stanford.edu/words/Coordinates.html.
12. ↑ Thompson, W. T.; Wei, K. (January 2010). "Use of the FITS World Coordinate System by STEREO/SECCHI". Solar Physics 261 (1): 215–222. doi:10.1007/s11207-009-9476-9. Bibcode: 2010SoPh..261..215T.
13. ↑ Hapgood, Mike (July 1997). "Heliocentric coordinate systems". Mullard Space Science Laboratory. https://www.mssl.ucl.ac.uk/grid/iau/extra/local_copy/SP_coords/heliosys.htm.
14. ↑ Hapgood, M. A. (May 1992). "Space physics coordinate transformations: A user guide". Planetary and Space Science 40 (5): 711–717. doi:10.1016/0032-0633(92)90012-D. Bibcode: 1992P&SS...40..711H. http://www.igpp.ucla.edu/public/vassilis/ESS261/Lecture03/Hapgood_sdarticle.pdf.
15. ↑ Fränz, M.; Harper, D. (February 2002). "Heliospheric coordinate systems". Planetary and Space Science 50 (2): 217–233. doi:10.1016/S0032-0633(01)00119-2. Bibcode: 2002P&SS...50..217F. http://www2.mps.mpg.de/homes/fraenz/systems/systems2art.pdf.

## External links

* sunpy.coordinates, a sub-package of SunPy used to handle solar coordinates
* MTOF/PM Data by Carrington Rotation, a table of Carrington rotation start and stop times

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