Author Topic: Finding the sun position in LRO images  (Read 2752 times)

Geoff

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Finding the sun position in LRO images
« on: October 29, 2010, 02:02:45 pm »
When working with image strips from LRO it is sometimes difficult to tell which direction the sunlight is coming from. There is an image parameter called sub-solar azimuth, among the parameters stored with the image. The explanation of this parameter (found by googling) is quite baffling:

The angle in degrees relative to three o'clock positive clockwise at the centre of the image to the direction of the Sun at the time the image was acquired. It tells you which direction the Sun is coming from in the image.

Irene Ant supplied the following description of how to interpret this:
  • Imagine you are standing on a large clock face, where 12 o'clock is in front of you.
  • Now add a strip of LROC data, running from the 12 o'clock position to the 6 o'clock position, oriented so that North is at the 12 o'clock position (you will need to look at the Upper/Lower Lat values for this).
  • In your mind, draw a line from the zenith (directly above you) to the sun, and then extend this line to the horizon.
  • Point your left arm to the spot where this line connects with the horizon.
  • Next, extend your right arm directly to your right, towards the 3 o'clock position.
  • Measure the angle from your right arm, going in a clockwise direction, to your left arm.
This is the sub-solar azimuth.
So, if the sub-solar azimuth is zero, then the sun is shining from the right of your LROC image. If it is 180 degrees, the sun is coming from the left. If it is 90 degrees, the sun is shining from the south. And so on.