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Full support for geocentric and barycentric EMB.
Now the Python version of Astronomy Engine supports calculating the Earth/Moon Barycenter (EMB) state vector (position and velocity) relative to the Earth's center (geocentric) or relative to the Solar System Barycenter (SSB). This completes support for this feature across C, C#, JavaScript, and Python.
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Don Cross
@@ -1229,6 +1229,25 @@ Angular coordinates expressed in the same equatorial system as `vec`.
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---
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<a name="GeoEmbState"></a>
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### GeoEmbState(time)
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**Calculates the geocentric position and velocity of the Earth/Moon barycenter.**
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Given a time of observation, calculates the geocentric position and velocity vectors
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of the Earth/Moon barycenter (EMB).
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The position (x, y, z) components are expressed in AU (astronomical units).
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The velocity (vx, vy, vz) components are expressed in AU/day.
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| Type | Parameter | Description |
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| --- | --- | --- |
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| [`Time`](#Time) | `time` | The date and time for which to calculate the EMB's geocentric state. |
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### Returns: [`StateVector`](#StateVector)
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The EMB's position and velocity vectors in J2000 equatorial coordinates.
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---
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<a name="GeoMoon"></a>
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### GeoMoon(time)
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@@ -1252,6 +1271,27 @@ The Moon's position as a vector in J2000 Cartesian equatorial coordinates.
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---
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<a name="GeoMoonState"></a>
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### GeoMoonState(time)
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**Calculates the geocentric position and velocity of the Moon at a given time.**
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Given a time of observation, calculates the Moon's position and velocity vectors.
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The position and velocity are of the Moon's center relative to the Earth's center.
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The position (x, y, z) components are expressed in AU (astronomical units).
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The velocity (vx, vy, vz) components are expressed in AU/day.
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If you need the Moon's position only, and not its velocity,
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it is much more efficient to use [`GeoMoon`](#GeoMoon) instead.
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| Type | Parameter | Description |
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| --- | --- | --- |
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| [`Time`](#Time) | `time` | The date and time for which to calculate the Moon's position and velocity. |
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### Returns: [`StateVector`](#StateVector)
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The Moon's position and velocity vectors in J2000 equatorial coordinates.
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---
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<a name="GeoVector"></a>
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### GeoVector(body, time, aberration)
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@@ -2487,7 +2487,6 @@ def GeoMoon(time):
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-------
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Vector
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The Moon's position as a vector in J2000 Cartesian equatorial coordinates.
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"""
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m = _CalcMoon(time)
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@@ -2506,6 +2505,69 @@ def GeoMoon(time):
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mpos2 = _precession(mpos1, time, _PrecessDir.Into2000)
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return Vector(mpos2[0], mpos2[1], mpos2[2], time)
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def GeoMoonState(time):
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"""Calculates the geocentric position and velocity of the Moon at a given time.
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Given a time of observation, calculates the Moon's position and velocity vectors.
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The position and velocity are of the Moon's center relative to the Earth's center.
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The position (x, y, z) components are expressed in AU (astronomical units).
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The velocity (vx, vy, vz) components are expressed in AU/day.
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If you need the Moon's position only, and not its velocity,
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it is much more efficient to use #GeoMoon instead.
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Parameters
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----------
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time : Time
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The date and time for which to calculate the Moon's position and velocity.
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Returns
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-------
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StateVector
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The Moon's position and velocity vectors in J2000 equatorial coordinates.
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"""
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# This is a hack, because trying to figure out how to derive a time
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# derivative for CalcMoon() would be extremely painful!
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# Calculate just before and just after the given time.
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# Average to find position, subtract to find velocity.
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dt = 1.0e-5 # 0.864 seconds
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t1 = time.AddDays(-dt)
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t2 = time.AddDays(+dt)
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r1 = GeoMoon(t1)
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r2 = GeoMoon(t2)
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return StateVector(
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(r1.x + r2.x) / 2,
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(r1.y + r2.y) / 2,
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(r1.z + r2.z) / 2,
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(r2.x - r1.x) / (2 * dt),
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(r2.y - r1.y) / (2 * dt),
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(r2.z - r1.z) / (2 * dt),
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time
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)
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def GeoEmbState(time):
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"""Calculates the geocentric position and velocity of the Earth/Moon barycenter.
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Given a time of observation, calculates the geocentric position and velocity vectors
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of the Earth/Moon barycenter (EMB).
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The position (x, y, z) components are expressed in AU (astronomical units).
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The velocity (vx, vy, vz) components are expressed in AU/day.
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Parameters
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----------
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time : Time
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The date and time for which to calculate the EMB's geocentric state.
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Returns
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-------
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StateVector
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The EMB's position and velocity vectors in J2000 equatorial coordinates.
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"""
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s = GeoMoonState(time)
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d = 1.0 + _EARTH_MOON_MASS_RATIO
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return StateVector(s.x/d, s.y/d, s.z/d, s.vx/d, s.vy/d, s.vz/d, time)
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# END CalcMoon
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#----------------------------------------------------------------------------
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# BEGIN VSOP
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@@ -4292,12 +4354,21 @@ def BaryState(body, time):
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if body == Body.Neptune:
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return _ExportState(bary.Neptune, time)
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if body in [Body.Moon, Body.EMB]:
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earth = _CalcVsopPosVel(_vsop[Body.Earth.value], time.tt)
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state = GeoMoonState(time) if body == Body.Moon else GeoEmbState(time)
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return StateVector(
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state.x + bary.Sun.r.x + earth.r.x,
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state.y + bary.Sun.r.y + earth.r.y,
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state.z + bary.Sun.r.z + earth.r.z,
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state.vx + bary.Sun.v.x + earth.v.x,
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state.vy + bary.Sun.v.y + earth.v.y,
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state.vz + bary.Sun.v.z + earth.v.z,
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time
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)
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if 0 <= body.value < len(_vsop):
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# Handle the remaining VSOP bodies: Mercury, Venus, Earth, Mars.
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# Calculate the heliocentric state of the given body
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# and add the Sun's heliocentric state to obtain the
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# body's barycentric state.
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# BarySun + HelioBody = BaryBody
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planet = _CalcVsopPosVel(_vsop[body.value], time.tt)
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return StateVector(
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bary.Sun.r.x + planet.r.x,
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@@ -4309,7 +4380,6 @@ def BaryState(body, time):
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time
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)
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# FIXFIXFIX: later, we can add support for Moon, EMB, etc.
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raise InvalidBodyError()
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