L0#
- sunpy.coordinates.sun.L0(
- time='now',
- light_travel_time_correction=True,
- nearest_point=True,
- aberration_correction=False,
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:
time (
tuple
,list
,str
,pandas.Timestamp
,pandas.Series
,pandas.DatetimeIndex
,datetime.datetime
,datetime.date
,numpy.datetime64
,numpy.ndarray
,astropy.time.Time
) – Time to use in a parse_time-compatible formatlight_travel_time_correction (
bool
) – If True, apply the correction for light travel time from Sun to Earth. Defaults to True.nearest_point (
bool
) – If True, calculate the light travel time to the nearest point on the Sun’s surface rather than the light travel time to the center of the Sun (i.e., a difference of the solar radius). Defaults to True.aberration_correction (
bool
) – If True, apply the stellar-aberration correction due to Earth’s motion. Defaults to False.
- Returns:
Longitude
– The Carrington longitude
Notes
This longitude is calculated using current IAU values (Seidelmann et al. [SAAhearn+07] 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-aberration 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
Urban & Kaplan (2007), “Investigation of Change in the Computational Technique of the Sun’s Physical Ephemeris in The Astronomical Almanac” (link)