As you know from having searched for impact craters on images here at CosmoQuest, some craters are are easier to spot and measure than others. One of the things that makes a crater easier to identify is a prominent shadow cast from the rim. Such shadows are created when the source of illumination (the sun) is nearer the horizon. Scientists would say that’s a “high angle of incidence.” Let’s explore what that means in this post, and then we’ll talk about the importance of “incidence angle” for crater identification in the next post in this series.

(But if you’d like a head start understanding about “angle of illumination” and the optical illusions it can cause, then check out some sites here at CQ! First is to revisit the Moon Mappers tutorial section on lighting effects, and the second is to check out a post about “Illumination and Optical Illusions.”)

This diagram shows how scientists define “incidence angle” or “angle of incidence.” When the Sun is nearly overhead, the arc shown in the diagram is small. When the Sun is near the horizon, the arc is big. That big arc is a “high” angle of incidence. It can be easy to get confused! When the Sun is “high” in the sky, that’s a “low” angle of incidence. Diagram credit, Jennifer Grier

For us to even see a crater in a visible light image there has to be some source of illumination – something lighting up the ground. This is usually the Sun. As we know from our everyday lives, the rotation of the Earth means sometimes (from our personal perspective) the Sun is directly overhead, and sometimes it is on the horizon, rising or setting. Note of course that the same time it is noon for us, somewhere else in the world it is sunset.

This is of course also true for the Moon. Here is an image of a “first quarter” Moon as seen from the perspective of the Earth. If we were astronauts standing at point “A” we would see the Sun directly overhead. If we were standing at point “C” we would see the sun on the horizon. At point “B” the sun would be about halfway up in the sky from our perspective.

A first quarter moon as imagined from Earth. The sun is off to the right in this image. Note that the sun shown here is FAR smaller than the real sun, and this image is just here for context. Credit for quarter Moon image: NASA Scientific Visualization Studio

So why do we care about this effect when doing planetary image analysis? Take another look at the image of the Moon at first quarter. Look at the surface of the Moon near the letter “A” and compare it to the way the surface looks at point “C.” At point “C” we can easily identify mountains, crater rims, and other topography. At point “A” we can’t really see the mountains or crater rims at all, but what we can see really well is the difference between dark and light areas (albedo). This difference between “A” and “C” is something scientists are always aware of when they look at images and when they design spacecraft to take pictures in visible light. If they want to study the differences in light/dark of a surface, they more often use images where the illumination is right overhead (low incidence angle, as measured from a line pointing straight up). If they want to study the mountains and other topography, they will more often use images where the illumination is coming from the side (high incidence angle as measured from a line pointing straight up).

You, and scientists, can’t always choose the perfect illumination angle. Sometimes one image of an area is all we have, and we have to do a variety of studies on it regardless. Sometimes scientists can do corrections for this effect, and sometimes not. Then it becomes even more important to be alert for any observational bias you might have based on where the sun is in the sky. So why is this important for craters? We’ll look more closely at that in the next post in this series. Until then, check out those other CQ links, and you’ll be on your way!