Geometry and visibility

Two families of tools cover this:

• Scalar at one epoch — Observation geometry: angular separation, angular size, light time, range rate, sub-observer point, phase angle, illumination fraction
• Windows over a range — Geometry finder: find intervals where a relational constraint is satisfied

When is the Moon visible above 10° from my site?

{
  "searchWindow": {
    "start": { "dateTime": "2024-01-01T00:00:00", "kind": "UTC" },
    "end":   { "dateTime": "2024-01-08T00:00:00", "kind": "UTC" }
  },
  "body": "EARTH",
  "latitude": 0.7853981633974483,
  "longitude": 0.0,
  "altitude": 0.0,
  "target": "MOON",
  "relationalOperator": "Greater",
  "value": 0.17453292519943295,
  "aberration": "LT+S"
}

Tool: FindElevationFromSiteConstraint. Returns a list of { start, end } windows.

For named DSN ground stations, use the DSN site tools instead.

When does Earth pass between the Moon and the Sun (a lunar eclipse)?

{
  "searchWindow": {
    "start": { "dateTime": "2024-01-01T00:00:00", "kind": "UTC" },
    "end":   { "dateTime": "2024-12-31T00:00:00", "kind": "UTC" }
  },
  "observerName": "MOON",
  "occultedBody": "SUN",
  "occultingBody": "EARTH",
  "occultingKind": "Any",
  "aberrationCorrection": "None"
}

Tool: FindOccultingConstraint. The same tool covers solar eclipses (observer=EARTH, occulted=SUN, occulting=MOON) and any other occultation between SPICE-backed objects.

Phase angle and illumination

{
  "observer": "EARTH",
  "target": "MOON",
  "illuminator": "SUN",
  "datetime": "2024-06-21T00:00:00",
  "aberration": "LT+S"
}

Tools: ComputePhaseAngle (radians), ComputeIlluminationFraction (0…1). For a "find the new moon / full moon" sweep over a range, use FindPhaseAngleConstraint.

Distance constraint

For approach analysis or close-encounter windows, FindDistanceConstraint is the cheapest tool. Example — find every time the Moon is closer than 380000 km from Earth in 2024 (perigee passes):

{
  "searchWindow": {
    "start": { "dateTime": "2024-01-01T00:00:00", "kind": "UTC" },
    "end":   { "dateTime": "2025-01-01T00:00:00", "kind": "UTC" }
  },
  "observerName": "EARTH",
  "targetName": "MOON",
  "relationalOperator": "Lower",
  "value": 380000000,
  "aberrationCorrection": "None"
}

Step-size tip

All Find*Constraint tools accept an optional stepSizeInSeconds. The default is searchWindow.duration / 3000. Smaller is more accurate but slower; for a yearly sweep 3600 (one hour) is usually enough; for a closer look at a single day, drop to 60–600.