Sun

The satkit.sun module provides functions for computing the sun's position and related quantities such as sunrise/sunset times and satellite shadow status. The Sun position is Vallado (2013) Algorithm 29 (§5.1.1, accurate to ~0.01° over 1950–2050), sunrise/sunset is Algorithm 30 (§5.3.1), and the shadow function is the conical umbra/penumbra model of Montenbruck & Gill (2000), §3.4.2.

sun

Astrodynamic calculations related to the sun

rise_set(time, coord, sigma=90.0 + 50.0 / 60.0)

Sunrise and sunset times on the day given by input time and at the given location.

Time is a "date" at location, and should have hours, minutes, and seconds set to zero

Vallado Algorithm 30

Parameters:

Name	Type	Description	Default
time	time	date for which to compute sunrise & sunset	required
coord	itrfcoord	location for which to compute sunrise & sunset	required
sigma	float	angle in degrees between noon & rise/set. Common Values: "Standard": 90 deg, 50 arcmin (90.0+50.0/60.0) "Civil Twilight": 96 deg "Nautical Twilight": 102 deg "Astronomical Twilight": 108 deg If not passed in, "Standard" is used (90.0 + 50.0/60.0)	90.0 + 50.0 / 60.0

Returns:

Type	Description
tuple[time, time]	tuple[satkit.time, satkit.time]: (sunrise, sunset)

Example

coord = satkit.itrfcoord(latitude_deg=42.36, longitude_deg=-71.06, altitude=0)
t = satkit.time.from_date(2024, 6, 21)
sunrise, sunset = satkit.sun.rise_set(t, coord)
print(f"Sunrise: {sunrise}")
print(f"Sunset:  {sunset}")

shadowfunc(sunpos, satpos)

Is satellite in Earth shadow given sun position

Parameters:

Name	Type	Description	Default
sunpos	NDArray[float64]	geocentric Sun position, meters	required
satpos	NDArray[float64]	geocentric satellite position, meters	required

Notes

• See algorithm in Section 3.4.2 of Montenbruck and Gill for calculation

Returns:

Name	Type	Description
float	float	number in range [0,1] indicating no sun or full sun (no occlusion) hitting satellite

Example

t = satkit.time(2024, 1, 1)
sun = satkit.sun.pos_gcrf(t)
sat_pos = np.array([6.781e6, 0, 0])  # satellite position, GCRF, meters
shadow = satkit.sun.shadowfunc(sun, sat_pos)
if shadow < 0.01:
    print("Satellite is in Earth shadow")
else:
    print(f"Illumination fraction: {shadow:.2f}")