|Fig. 1 Moon at two different moments: during the penumbral eclipse (October 18 2013) and just after the eclipse end.|
|Fig. 2 Equipment used during the eclipse of Fig. 1.|
Instead of rotating the image, we extract a sample of it along a line or "cut" of both images and compare them. We chose two well distinguished and point-like features in the image as shown in Fig. 3. The first point is crater Aristarcus as shown. The second spot is denoted point "B" (Reichenbach, Stevinus, ref. 1).
Fig. 3 By carefully choosing two bright features and tracing a line joining them, we can extract an image sample for photometric estimation. Point 'B' may correspond to craters Reichenbach and Stevinus (ref. 1).
|Fig. 4 Traced lines corresponding to image sections containing data to be analyzed. The moon at left is eclipsed, while the image at right is the "reference" moon.|
To check if the extraction is ok, we can calculate the distance between the two chosen points on each image. For the reference moon, this distance is equal to 275,1 pixels and for the eclipsed moon 276,3 pixels.
|Fig. 5 Photometric responses of the reference (black line) and eclipsed (blue line) moons. The lower curve is the difference between the black and the blue lines and represents the degree of darkening caused by subtle illumation changes.|
|Fig. 6 Application of a non-linear transformation to the eclipsed moon image enhances contrast and reveals the darkening in an artificial way.|