get_heliocentric_angle#

sunpy.coordinates.utils.get_heliocentric_angle(coordinate_on_solar_disk)[source]#

Returns the heliocentric angle, the angle between the observer look direction for a point on the surface of the Sun and the local vertical for that point.

If a point is on the visible side of the Sun for the observer, the angle ranges between 0 \(^\circ\) (at disk center) and 90 \(^\circ\) (at the solar limb). A point on the far side of the Sun has to be provided as a 3D coordinate, and then the angle ranges from 90 \(^\circ\) to 180 \(^\circ\).

The heliocentric angle is related to the parameter \(\mu\) commonly used for limb-darkening calculations:

\[\mu = \cos(heliocentric\_angle)\]

Parameters:: coordinate_on_solar_disk (astropy.coordinates.SkyCoord) – A coordinate on the solar disk, requires the observer and obstime to be set.
Returns:: heliocentric_angle (Quantity) – The angle between the local solar vertical and the line of sight from the observer to a point on the solar disk.

Examples

>>> import numpy as np

>>> import astropy.units as u
>>> from astropy.coordinates import SkyCoord

>>> from sunpy.coordinates.utils import get_heliocentric_angle

>>> # At the center of the solar disk
>>> hpc_coord_center = SkyCoord(0*u.arcsec, 0*u.arcsec, frame='helioprojective', observer="earth", obstime="2017-07-26")
>>> get_heliocentric_angle(hpc_coord_center)
<Quantity 0. deg>
>>> # mu
>>> np.cos(get_heliocentric_angle(hpc_coord_center))
<Quantity 1.>

>>> # Almost at the limb
>>> hpc_coord_limb = SkyCoord(944.35*u.arcsec, 0*u.arcsec, frame='helioprojective', observer="earth", obstime="2017-07-26")
>>> get_heliocentric_angle(hpc_coord_limb)
<Quantity 89.26429919 deg>
>>> # mu
>>> np.cos(get_heliocentric_angle(hpc_coord_limb))
<Quantity 0.01284005>