L0

sunpy.coordinates.sun.L0(time='now', light_travel_time_correction=True, nearest_point=True, aberration_correction=False)[source]

Return the L0 angle for the Sun at a specified time, which is the apparent Carrington longitude of the Sun-disk center as seen from Earth.

Observer corrections can be disabled, and then this function will instead return the true Carrington longitude.

Parameters
Returns

Longitude – The Carrington longitude

Notes

This longitude is calculated using current IAU values (Seidelmann et al. 2007 and later), which do not include the effects of light travel time and aberration due to Earth’s motion (see that paper’s Appendix). This function then, by default, applies the light-travel-time correction for the nearest point on the Sun’s surface, but does not apply the stellar-aberration correction due to Earth’s motion.

We do not apply the stellar-abberation correction by default because it should not be applied for purposes such as co-aligning images that are each referenced to Sun-disk center. Stellar aberration does not shift the apparent positions of solar features relative to the Sun-disk center.

The Astronomical Almanac applies the stellar-aberration correction in their printed published L0 values (see also Urban & Kaplan 2007). Applying the stellar-aberration correction due to Earth’s motion decreases the apparent Carrington longitude by ~20.5 arcseconds.

References

  • Seidelmann et al. (2007), “Report of the IAU/IAG Working Group on cartographic coordinates and rotational elements: 2006” (link)

  • Urban & Kaplan (2007), “Investigation of Change in the Computational Technique of the Sun’s Physical Ephemeris in The Astronomical Almanac” (link)