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Python: Find rise/set/culm of user-defined stars.
Added Python support for user-defined stars. Defined new StateVector methods: Position and Velocity. Defined division operator: Vector / float. Bumped version number to 2.1.12.
This commit is contained in:
Don Cross
@@ -198,6 +198,13 @@ However, because Python defines the [angular conversion functions](https://docs.
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---
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<a name="AU_PER_LY"></a>
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### `AU_PER_LY = 63241.07708807546`
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**The number of astronomical units in one light-year.**
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---
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<a name="CALLISTO_RADIUS_KM"></a>
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### `CALLISTO_RADIUS_KM = 2410.3`
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@@ -881,6 +888,18 @@ The state vector also includes a time stamp.
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| `float` | `vz` | The z-component of the velocity, measured in AU/day. |
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| [`Time`](#Time) | `t` | The date and time at which the position and velocity vectors are valid. |
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#### member functions
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<a name="StateVector.Position"></a>
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### StateVector.Position(self)
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Extracts a position vector from this state vector.
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<a name="StateVector.Velocity"></a>
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### StateVector.Velocity(self)
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Extracts a velocity vector from this state vector.
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---
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<a name="Time"></a>
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@@ -1086,6 +1105,14 @@ two cases applies to a particular apsis event.
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| `Moon` | The Earth's moon. |
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| `EMB` | The Earth/Moon Barycenter. |
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| `SSB` | The Solar System Barycenter. |
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| `Star1` | User-defined star 1. |
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| `Star2` | User-defined star 2. |
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| `Star3` | User-defined star 3. |
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| `Star4` | User-defined star 4. |
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| `Star5` | User-defined star 5. |
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| `Star6` | User-defined star 6. |
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| `Star7` | User-defined star 7. |
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| `Star8` | User-defined star 8. |
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---
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@@ -1421,6 +1448,29 @@ body to arrive at the observer at the observation time.
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---
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<a name="DefineStar"></a>
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### DefineStar(body, ra, dec, distanceLightYears)
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**Assign equatorial coordinates to a user-defined star.**
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Some Astronomy Engine functions allow their `body` parameter to
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be a user-defined fixed point in the sky, loosely called a "star".
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This function assigns a right ascension, declination, and distance
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to one of the eight user-defined stars `Body.Star1`..`Body.Star8`.
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A star that has not been defined through a call to `DefineStar`
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defaults to the coordinates RA=0, DEC=0 and a heliocentric distance of 1 light-year.
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Once defined, the star keeps the given coordinates until
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a subsequent call to `DefineStar` replaces the coordinates with new values.
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| Type | Parameter | Description |
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| --- | --- | --- |
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| [`Body`](#Body) | `body` | One of the eight user-defined star identifiers: `Body.Star1`, `Body.Star2`, ..., `Body.Star8`. |
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| `float` | `ra` | The right ascension to be assigned to the star, expressed in J2000 equatorial coordinates (EQJ). The value is in units of sidereal hours, and must be within the half-open range [0, 24). |
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| `float` | `dec` | The right ascension to be assigned to the star, expressed in J2000 equatorial coordinates (EQJ). The value is in units of degrees north (positive) or south (negative) of the J2000 equator, and must be within the closed range [-90, +90]. |
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| `float` | `distanceLightYears` | The distance between the star and the Sun, expressed in light-years. This value is used to calculate the tiny parallax shift as seen by an observer on Earth. If you don't know the distance to the star, using a large value like 1000 will generally work well. The minimum allowed distance is 1 light-year, which is required to provide certain internal optimizations. |
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---
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<a name="DeltaT_EspenakMeeus"></a>
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### DeltaT_EspenakMeeus(ut)
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@@ -44,6 +44,7 @@ def _cbrt(x):
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KM_PER_AU = 1.4959787069098932e+8 #<const> The number of kilometers per astronomical unit.
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C_AUDAY = 173.1446326846693 #<const> The speed of light expressed in astronomical units per day.
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AU_PER_LY = 63241.07708807546 #<const> The number of astronomical units in one light-year.
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# Jupiter radius data are nominal values obtained from:
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# https://www.iau.org/static/resolutions/IAU2015_English.pdf
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@@ -223,6 +224,9 @@ class Vector:
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def __neg__(self):
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return Vector(-self.x, -self.y, -self.z, self.t)
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def __truediv__(self, scalar):
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return Vector(self.x/scalar, self.y/scalar, self.z/scalar, self.t)
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def format(self, coord_format):
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"""Returns a custom format string representation of the vector."""
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layout = '({:' + coord_format + '}, {:' + coord_format + '}, {:' + coord_format + '}, {})'
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@@ -290,6 +294,14 @@ class StateVector:
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self.t
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)
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def Position(self):
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"""Extracts a position vector from this state vector."""
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return Vector(self.x, self.y, self.z, self.t)
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def Velocity(self):
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"""Extracts a velocity vector from this state vector."""
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return Vector(self.vx, self.vy, self.vz, self.t)
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@enum.unique
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class Body(enum.Enum):
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"""The celestial bodies supported by Astronomy Engine calculations.
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@@ -310,6 +322,14 @@ class Body(enum.Enum):
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Moon: The Earth's moon.
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EMB: The Earth/Moon Barycenter.
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SSB: The Solar System Barycenter.
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Star1: User-defined star 1.
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Star2: User-defined star 2.
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Star3: User-defined star 3.
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Star4: User-defined star 4.
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Star5: User-defined star 5.
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Star6: User-defined star 6.
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Star7: User-defined star 7.
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Star8: User-defined star 8.
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"""
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Invalid = -1
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Mercury = 0
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@@ -325,6 +345,78 @@ class Body(enum.Enum):
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Moon = 10
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EMB = 11
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SSB = 12
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Star1 = 101
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Star2 = 102
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Star3 = 103
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Star4 = 104
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Star5 = 105
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Star6 = 106
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Star7 = 107
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Star8 = 108
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class _StarDef:
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def __init__(self, ra, dec, dist):
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self.ra = ra
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self.dec = dec
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self.dist = dist
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_StarTable = [
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_StarDef(0.0, 0.0, AU_PER_LY), # Star1
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_StarDef(0.0, 0.0, AU_PER_LY), # Star2
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_StarDef(0.0, 0.0, AU_PER_LY), # Star3
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_StarDef(0.0, 0.0, AU_PER_LY), # Star4
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_StarDef(0.0, 0.0, AU_PER_LY), # Star5
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_StarDef(0.0, 0.0, AU_PER_LY), # Star6
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_StarDef(0.0, 0.0, AU_PER_LY), # Star7
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_StarDef(0.0, 0.0, AU_PER_LY), # Star8
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]
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def _IsUserDefinedStar(body):
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return Body.Star1.value <= body.value <= Body.Star8.value
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def _GetUserDefinedStar(body):
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return _StarTable[body.value - Body.Star1.value] if _IsUserDefinedStar(body) else None
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def DefineStar(body, ra, dec, distanceLightYears):
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"""Assign equatorial coordinates to a user-defined star.
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Some Astronomy Engine functions allow their `body` parameter to
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be a user-defined fixed point in the sky, loosely called a "star".
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This function assigns a right ascension, declination, and distance
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to one of the eight user-defined stars `Body.Star1`..`Body.Star8`.
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A star that has not been defined through a call to `DefineStar`
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defaults to the coordinates RA=0, DEC=0 and a heliocentric distance of 1 light-year.
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Once defined, the star keeps the given coordinates until
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a subsequent call to `DefineStar` replaces the coordinates with new values.
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Parameters
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----------
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body: Body
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One of the eight user-defined star identifiers: `Body.Star1`, `Body.Star2`, ..., `Body.Star8`.
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ra: float
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The right ascension to be assigned to the star, expressed in J2000 equatorial coordinates (EQJ).
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The value is in units of sidereal hours, and must be within the half-open range [0, 24).
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dec: float
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The right ascension to be assigned to the star, expressed in J2000 equatorial coordinates (EQJ).
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The value is in units of degrees north (positive) or south (negative) of the J2000 equator,
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and must be within the closed range [-90, +90].
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distanceLightYears: float
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The distance between the star and the Sun, expressed in light-years.
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This value is used to calculate the tiny parallax shift as seen by an observer on Earth.
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If you don't know the distance to the star, using a large value like 1000 will generally work well.
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The minimum allowed distance is 1 light-year, which is required to provide certain internal optimizations.
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"""
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star = _GetUserDefinedStar(body)
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if star is None:
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raise Error('Body must be a user-defined star, not {}.'.format(body))
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if not (0.0 <= ra < 24.0):
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raise Error('Invalid right ascension: {}'.format(ra))
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if not (-90.0 <= dec <= +90.0):
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raise Error('Invalid declination: {}'.format(dec))
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star.ra = ra
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star.dec = dec
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star.dist = distanceLightYears * AU_PER_LY
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def BodyCode(name):
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"""Finds the Body enumeration value, given the name of a body.
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@@ -4444,6 +4536,10 @@ def HelioVector(body, time):
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if body == Body.SSB:
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return _CalcSolarSystemBarycenter(time)
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star = _GetUserDefinedStar(body)
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if star is not None:
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return VectorFromSphere(Spherical(star.dec, 15.0*star.ra, star.dist), time)
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raise InvalidBodyError(body)
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@@ -4630,6 +4726,24 @@ def BackdatePosition(time, observerBody, targetBody, aberration):
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Its `t` field holds the time that light left the observed
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body to arrive at the observer at the observation time.
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"""
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if _IsUserDefinedStar(targetBody):
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# This is a user-defined star, which must be treated as a special case.
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# First, we assume its heliocentric position does not change with time.
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# Second, we assume its heliocentric position has already been corrected
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# for light-travel time, its coordinates given as it appears on Earth at the present.
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# Therefore, no backdating is applied.
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tvec = HelioVector(targetBody, time)
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if aberration:
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# (Observer velocity) - (light vector) = (Aberration-corrected direction to target body).
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# Note that this is an approximation, because technically the light vector should
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# be measured in barycentric coordinates, not heliocentric. The error is very small.
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ostate = HelioState(observerBody, time)
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rvec = tvec - ostate.Position()
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s = C_AUDAY / rvec.Length() # conversion factor from relative distance to speed of light
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return rvec + ostate.Velocity()/s
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# No correction is needed. Simply return the star's current position as seen from the observer.
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return tvec - HelioVector(observerBody, time)
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if aberration:
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# With aberration, `BackdatePosition` will calculate `observerPos` at different times.
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# Therefore, do not waste time calculating it now.
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@@ -6548,6 +6662,13 @@ def _MaxAltitudeSlope(body, latitude):
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elif body in [Body.Jupiter, Body.Saturn, Body.Uranus, Body.Neptune, Body.Pluto]:
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deriv_ra = -0.2
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deriv_dec = +0.2
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elif _IsUserDefinedStar(body):
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# The minimum allowed heliocentric distance of a user-defined star
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# is one light-year. This can cause a tiny amount of parallax (about 0.001 degrees).
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# Also, including stellar aberration (22 arcsec = 0.006 degrees), we provide a
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# generous safety buffer of 0.008 degrees.
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deriv_ra = -0.008
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deriv_dec = +0.008
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elif body == Body.Earth:
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raise EarthNotAllowedError()
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else:
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@@ -6,7 +6,7 @@ def _LoadFile(filename):
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setup(
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name='astronomy-engine',
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version='2.1.11',
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version='2.1.12',
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description='Astronomy calculation for Sun, Moon, and planets.',
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long_description=_LoadFile('README.md'),
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long_description_content_type='text/markdown',
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