Geometry of HiRISE observations is on Talk!
Good news: our wonderful development team has added new feature that many of our volunteers have asked for! Now you can see north azimuth, sub-solar azimuth, phase angle, and emission angle on the Talk pages directly. You can see an example here. These angles give you information about how HiRISE took the image and where the Sun was at that moment.
To understand what those angles are, here is an illustration for you:
You see how the MRO spacecraft flies over the surface while HiRISE makes an image. The Sun illuminates the surface .
Consider a point on the martian surface P.
Emission angle: HiRISE does not necessarily look at point P straight down, i.e. the line connecting point P and HiRISE has some deviation from vertical line – it is noted as angle e on the sketch. This is emission angle. It tells you much we tilted spacecraft to the side to make the image.
Phase angle: Because all the images you see in our project is from polar areas, the Sun is often low in the sky when HiRISE observes. To get an idea on how low, we use phase angle – it is the angle between the line from Sun to the point P and line from point P to the HiRISE. It is noted φ in the sketch. The larger phase angle is, the lower the Sun in the sky, the longer are the shadows on the surface.
Sub-solar azimuth: To understand what is the direction towards the Sun in the frame of HiRISE image, we use sub-solar azimuth. In any frame that you see on our project it is an angle between horizontal line from the center of the frame towards right and the Sun direction. It is counted clock-wise. The notation for it in the sketch is a.
North azimuth: The orbit of MRO spacecraft defines orientations of HiRISE images. North azimuth tells us direction to the Martian north pole. In the frame of an image it’s counted same as sub-solar azimuth, i.e. from the horizontal line connecting center of the frame and its right edge in the clock-wise direction.
I hope this helps you enjoy exploring Mars with HiRISE!
Anya
Planet Four Blog 
Thanks Anya for the nice explanation review. Christina’s diagram was nicely done also.
Unrelated; by any chance does anyone recall the link which shows how many images we have worked? It would be nice (fun) to see who has done the most
and their total as well as who made (if any) discovery(ies).
Hi Bob,
You can check through your profile or on your Zooniverse projects page.
Cheers,
~Meg
Thanks Meg! “Projects” was the word I could not recall!
Nice. However, I can I derive light direction for map projected HiRISEs?
Hi Simone! Only by checking the label (or image webpage at Arizona). It has solar azimuth entry.
Hi Anya thank you very much. However, is not as simple as the SSazimuth is for non-projected images. I’ve tried this method that shoud work: I have donwloaded the lev1 image (not-map projected) and imported to Illustrator. I drawn an arrow from the center of the image to the right. I’ve rotated the arrow according to the sub solar azimuth. Then I’ve downloaded the mapprojected (lev2) image. I’ve rotated the lev1 together with the arrow to match the lev2 image. The arrow is now poinitng to the sub solar point that should be the position of the sun over the image. However, light is coming from and not poinitng to the sub solar point. So I’ve further rotated the arrow 180° to have the direction the sunlight is coming from. There should be a more clever way for doing it for sure (probably by looking at the kernels), but this should also work (at least I guess). Thank you gain.
Simone
Hi Simone! I am not sure what is the confusion. Your first comment asked for map-projected images, right? Sub-solar azimuth in HiRISE is counted clock-wise from 3pm line. If you did all you wrote in that definition, non-map projected and map-projected images should fit. if for some image it’s wrong, we need to investigate.