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Famous EclipseGPS Satellites

Lunar Occultations

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Occultation of Spica

Occasionally the Moon passes in front of a planet or a star. Precise timing of such an event event can give information about the orbit of the Moon, and also information about the planet.

An occultation of the star Spica occurred on 1995 January 23.

date = EclipseBegin[Earth, Moon, Spica,
{1995,1,1}];

The event is visible from some places in North America.

EclipseTrackPlot[Earth, Moon, Spica,
date];1;

Let's set our location to a place on the track.

SetLocation[GeoLongitude -> -90*Degree,
GeoLatitude -> 24*Degree,
TimeZone -> -6];

When the separation is less than 0.25 degree, then Spica occluded by Moon.

Table[{m, Separation[Spica, Moon,
{1995,1,23,4,m,0},
ViewPoint ->
TopoCentric]},
{m,30,150,10}]

The occultation begins at about {1995,1,23,4,40,0}.

d1 = {1995,1,23,4,40,0};

And ends 95 minutes later at about {1995,1,23,6,15,0}.

d2 = {1995,1,23,6,15,0};

You can determine these times using the EclipseBegin and EclipseEnd functions.

d1 = EclipseBegin[
Spica, Moon, TopoCentric, {1995,1,1}]

d2 = EclipseEnd[
Spica, Moon, TopoCentric, {1995,1,1}]

To generate a graphic showing the Moon, its correct phase and orientation to the horizon, plus the background stars is somewhat tricky.

You can do it by first drawing a RadialStarChart with a small RadalAngle and aligned so the Zenith is vertical. Also generate a PlanetPlot3D graphic of the Moon at the same instant, and superimpose it on the RadialStarChart with Epilog.

It is very important to have the Moon in the center of the RadialStarChart graphic, otherwise this trick won't work. Also make sure you use ViewVertical->TopoCentric in both RadialStarChart and PlanetPlot3D. The occultation is only visible from certain locations on Earth.

Show stars near the Moon at start of occultation.

RadialStarChart[Moon, d1,
ViewPoint -> TopoCentric,
ViewVertical -> Zenith,
RadialAngle -> 0.5*Degree,
MagnitudeScale -> 0.2,
StarLabels -> True,
StarColors -> True,
Epilog ->
PlanetPlot3D[Moon, d1,
ViewPoint -> TopoCentric,
ViewVertical -> Zenith,
DisplayFunction -> Identity][[1]]];

Similarly you can do a plot at the end of the occultation.

RadialStarChart[Moon, d2,
ViewPoint -> TopoCentric,
ViewVertical -> Zenith,
RadialAngle -> 0.5*Degree,
MagnitudeScale -> 0.2,
StarLabels -> True,
StarColors -> True,
Epilog ->
PlanetPlot3D[Moon, d2,
ViewPoint -> TopoCentric,
ViewVertical -> Zenith,
DisplayFunction -> Identity][[1]]];

You can plot the track as follows.

EclipseTrackPlot[
Spica, Moon, TopoCentric, d1];

The graphic is aligned so that the vertical points to the north celestial pole.

Famous EclipseGPS Satellites



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