Greg,

You've been saying some strange things recently about image geometries and "reflections" so I thought I'd go through this example and check our mutual understanding of the terms used and their implications.

Greg
martian polar branches
Sat Jun 2 10:21:25 2001

Here is a similar photo to the previous

http://barsoom.msss.com/moc_gallery/m13_m18/images/M14/M1401838.html

This new photo has a similar "fault".

http://barsoom.msss.com/moc_gallery/m07_m12/images/M09/M0906354.html

Longitude of image center: 7.22°W
- Self explanatory

Latitude of image center: 79.52°S
- ditto

Scaled pixel width: 2.76 meters
- on average each pixel covers this distance on the ground

Scaled image width: 1.42 km
- Total image width

Scaled image height: 7.15 km
- Total image length

Solar longitude (Ls): 252.60°
- time of year on Mars. Spring equinox in the northern hemisphere is zero. The Martian year is divided into 360 degrees of Solar longitude, each lasting about 2 Martian days. Here, we are in the southern polecap approaching midsummer (Ls=270). The sun is in the sky all day and night.

Local True Solar Time: 1.06 decimal hours
- just past 1 a.m., yet the midnight sun illuminates the scene

Emission angle: 0.19°
- This vital information is the difference between PERPENDICULAR and the look angle of the spacecraft. i.e. it is looking down at 89.81 degrees. Damn near vertical. This is typical MOC geometry and means that NO REFLECTIONS will be seen from objects beside a horizontal surface such as a lake or skating rink, even if they overhang it. All you will see is the tops of the original objects. (And their physical shadows as cast by sunlight)

Incidence angle: 76.14°
- This is the angle of the Sun to the PERPENDICULAR. i.e. in this case an observer on the ground would have seen the Sun at an elevation of 13.86 degrees above the horizon

Phase angle: 76.09°
- This is the angle between the incoming Sunlight and the spacecraft look direction

North azimuth: 254.15°
Compared to left-right of the image, this is the direction of true north. At 270 degrees NA, the image would be aligned north-south along track. Here it is rotated 15.85 degrees from that.

Sun azimuth: 59.09°
Compared to left-right of the image, this is the direction of the Sun. Here, we have midnight Sun coming from the bottom-right of the image, over the pole. For completeness, we can compare NA and SA and get the true solar direction as seen by a ground observer. The sun azimuth can be determined relative to north by subtracting the north azimuth from the
sun azimuth, so here 59.09 minus 254.15 gives -194.94 degrees or +165.06, so the sun is coming from the SSE.

Spacecraft altitude: 371.39 km
- Self explanatory

Slant distance: 371.39 km
- Distance from the spacecraft to the image centre, measured in the oblique look direction. Since look angle differs by only 0.19 degrees from vertical, this is essentially identical to the spacecraft altitude.