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Python: generalized light-travel correction.

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Don Cross
2022-05-30 21:37:19 -04:00
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@@ -763,6 +763,36 @@ to report information about the center of the Moon passing through the ecliptic
---
<a name="PositionFunction"></a>
### class PositionFunction
**A function for which to solve a light-travel time problem.**
This abstract class defines the contract for wrapping a
position vector as a function of time. A class derived from
`PositionFunction` must define a `Position` method that
returns a position vector for a given time.
The function [`CorrectLightTravel`](#CorrectLightTravel) solves a generalized
problem of deducing how far in the past light must have
left a target object to be seen by an observer at a
specified time. It is passed an instance of `PositionFunction`
that expresses a relative position vector function.
#### member functions
<a name="PositionFunction.Position"></a>
### PositionFunction.Position(self, time)
**Returns a relative position vector for a given time.**
| Type | Parameter | Description |
| --- | --- | --- |
| [`Time`](#Time) | `time` | The time at which to evaluate a relative position vector. |
**Returns**: [`Vector`](#Vector)
---
<a name="RotationMatrix"></a>
### class RotationMatrix
@@ -1218,6 +1248,33 @@ and the specified body as seen from the center of the Earth.
---
<a name="BackdatePosition"></a>
### BackdatePosition(time, observerBody, targetBody, aberration)
**Solve for light travel time correction of apparent position.**
When observing a distant object, for example Jupiter as seen from Earth,
the amount of time it takes for light to travel from the object to the
observer can significantly affect the object's apparent position.
This function solves the light travel time correction for the apparent
relative position vector of a target body as seen by an observer body
at a given observation time.
For a more generalized light travel correction solver, see [`CorrectLightTravel`](#CorrectLightTravel).
| Type | Parameter | Description |
| --- | --- | --- |
| [`Time`](#Time) | `time` | The time of observation. |
| [`Body`](#Body) | `observerBody` | The body to be used as the observation location. |
| [`Body`](#Body) | `targetBody` | The body to be observed. |
| `bool` | `aberration` | `True` to correct for aberration, or `False` to leave uncorrected. |
**Returns**: [`Vector`](#Vector)
The position vector at the solved backdated time.
Its `t` field holds the time that light left the observed
body to arrive at the observer at the observation time.
---
<a name="BaryState"></a>
### BaryState(body, time)
@@ -1296,6 +1353,39 @@ the converted B1875 (ra,dec) for that point.
---
<a name="CorrectLightTravel"></a>
### CorrectLightTravel(func, time)
**Solve for light travel time of a vector function.**
When observing a distant object, for example Jupiter as seen from Earth,
the amount of time it takes for light to travel from the object to the
observer can significantly affect the object's apparent position.
This function is a generic solver that figures out how long in the
past light must have left the observed object to reach the observer
at the specified observation time. It uses [`PositionFunction`](#PositionFunction)
to express an arbitrary position vector as a function of time.
This function repeatedly calls `func.Position`, passing a series of time
estimates in the past. Then `func.Position` must return a relative state vector between
the observer and the target. `CorrectLightTravel` keeps calling
`func.Position` with more and more refined estimates of the time light must have
left the target to arrive at the observer.
For common use cases, it is simpler to use [`BackdatePosition`](#BackdatePosition)
for calculating the light travel time correction of one body observing another body.
time : Time
The observation time for which to solve for light travel delay.
| Type | Parameter | Description |
| --- | --- | --- |
| [`PositionFunction`](#PositionFunction) | `func` | An arbitrary position vector as a function of time. |
**Returns**: [`Vector`](#Vector)
The position vector at the solved backdated time.
The `t` field holds the time that light left the observed
body to arrive at the observer at the observation time.
---
<a name="DeltaT_EspenakMeeus"></a>
### DeltaT_EspenakMeeus(ut)
@@ -1526,7 +1616,7 @@ using the center of the Earth as the origin. The result is expressed as a Carte
vector in the J2000 equatorial system: the coordinates are based on the mean equator
of the Earth at noon UTC on 1 January 2000.
If given an invalid value for `body`, this function will raise an exception.
Unlike [`HelioVector`](#HelioVector), this function always corrects for light travel time.
Unlike [`HelioVector`](#HelioVector), this function corrects for light travel time.
This means the position of the body is "back-dated" by the amount of time it takes
light to travel from that body to an observer on the Earth.
Also, the position can optionally be corrected for