THE SKY GUIDE CHALLENGE
Making the most of the ever-changing shadow effects near the Moon’s terminator
At an average 384,400km away, the Moon is close enough for us to see a variety of geological features on its surface. These are made more dramatic by the way sunlight interacts with them and an interesting challenge is to devise a way to illustrate how these shadowy effects appears to change over time.
The phases of the Moon repeat after one synodic month, or approximately 29.5 days. Over the course of one Earth day the Sun appears to move 360/29.5=12.2° across the Moon’s sky. With patience over an extended observing session it’s possible to see subtle changes in the way the shadows are being cast, especially close to the terminator, the constantly progressing longitudinal division between lunar night and lunar day. Here sunlight shines obliquely across the lunar surface while the slow yet unrelenting movement of the Sun creates the most extreme changes in the length of the shadows. Watching the top of a high mountain or elevated crater rim shining against dark, shadowed surroundings is similarly mesmerising.
Revealing this motion with a camera requires you to take still images over a period of time. Arranging the results in a grid or – better still – animating them in sequence, reveals the motion in the lunar shadows. The results vary depending on the image-scale (magnification) and techniques used. A high frame-rate camera can be used to capture short movie sequences, which can then be registered and stacked into detailed highresolution images that you can turn into an animated time-lapse sequence. The extra detail such images produce will reveal even more intricate and intriguing changes in the shadows. The larger the image scale, the less time you need between stills to show motion.
An effective technique is to centre on a recognisable feature that’s close to the terminator. We would recommend a telescope focal length of over 2m. Aim to capture an image every five minutes. If using a high frame rate camera, restrict captures to less than 60s repeated every five minutes. If you have one, a red or infrared pass filter used in conjunction with a mono high frame-rate camera will help reduce some effects of seeing.
Continue your capture run for at least an hour. Load each result in time order into a separate layer using a layer-based editor. Load the earliest result at the bottom. Make all but the bottom two images invisible and align the upper image with the base image. Repeat, working your way up the layer stack carefully aligning each image with the base layer. Once done, crop to remove any edge gaps revealed during alignment. Arrange in a grid or, if your software provides animation facilities, animate the sequence to show the shadows creeping across the lunar surface.