Edge-On Coordinate System

The ring's edge-on coordinate frame is a useful way to describe the geometry of nearly edge-on views of the rings.

We construct a cylindrical coordinate frame (r, θ, z), where:

r is the radial distance from the planet's rotation axis (Edge-On Ring Radius),

θ is the inertial longitude (Edge-On Intercept Longitude),

z is the elevation distance above the equatorial plane (Edge-On Ring Elevation).

We define the "intercept point" as the point along a given line of sight where r is minimized; this location is closely associated with what we typically call the ring's "ansa." For example, in an edge-on view of the rings, the tip of Saturn's A ring would typically fall at r ~ 136,780 km, z = 0. See figure.

Here is an example search for the core of the F ring (r ~ 140,219 km) using the "Edge-On" system:

Edge-On Ring Radius (km)         min = 140,000       max = 140,450

Edge-On Ring Elevation (km)       min =   -1,000       max =     1,000

Edge-On Definitions

• Intercept Point

  The point along a given line of sight where r is minimized; this location is closely associated with what we typically call the ring's "ansa."

• Edge-On Ring Radius (r)

  The radius, or distance from the planet's rotation axis, of the intercept point in the edge-on ring geometry frame. This would commonly be called the radius at the observed ring ansa in a nearly edge-on view.

• Edge-On Ring Elevation (z)

  The distance from the ring plane to the intercept point as measured in the edge-on ring geometry frame. It typically represents the projected distance from the ring plane in nearly edge-on views of the rings. Positive values are measured in the direction of positive rotation; this is IAU-defined north for Jupiter, Saturn and Neptune, but IAU-defined south for Uranus.

• Edge-on Intercept Longitude (θ)

  The inertial longitude at the intercept point in the edge-on ring geometry frame.

• Edge-On Point Range (km)

  The distance from the observer to the intercept point in the edge-on ring geometry frame. It is the distance to the ring ansa for situations where the rings are viewed nearly edge-on.

• Edge-On Ring Resolution

  The spatial resolution (km/pixel or km/sample) at the intercept point in the edge-on ring geometry frame. It would commonly be called the radial resolution at the observed ring ansa in a nearly edge-on view.

• Edge-On Solar Hour Angle

  The Edge-On Solar Hour Angle is the solar hour angle at the intercept point in the edge-on ring geometry frame.

  • The solar hour angle measures the spacecraft-planet-antisolar direction as projected into the ring plane, in degrees. It can range from 0 to 360. It is measured in the direction of ring rotation so SHA=90 places the spacecraft above the morning quadrant of the rings (where the ring material has just emerged from shadow) and SHA=270 places the spacecraft above the evening quadrant (where ring material is heading into the shadow).

• Edge-On J2000 Longitude

  The J2000 inertial longitude at the inertial longitude in the edge-on ring geometry frame.

  • This J2000 longitude is measured from the First Point of Aries (right ascension = 0) along the Earth's equator to the ascending node of the planet's ring plane, and thence along the ring plane. (This will be a component of future capabilities to search for moving targets in ring data.)