“HDR” stands for “high dynamic range,” a photo editing technique used to combat the increased contrast that all standard photo methods are prone to – see a greater explanation here. Sometimes it’s used to produce unrealistic images with light levels that really can’t exist naturally, but other times it’s an effort to present more what we see with our highly adaptable vision. This is one of the latter cases – for the moment, anyway.
Tonight, a first-quarter moon (or “half,” don’t ask me why astronomers have this thing about “quarters”) was accompanied by a very close companion, easily visible even with the humidity of the evening. The brilliant blue star Spica was overcoming the typical glare that makes it hard to see stars close to a bright moon – this is because it’s one of the brighter stars in the sky, and among the brightest along the path that the moon takes in its orbit. Out of curiosity, I checked with Stellarium to see just how close they would pass (or if I’d missed it already) and found that they would appear within the moon’s diameter of one another – if I was a few time zones further west. The moon would be well set here in North Carolina as it occurred.
That image above is a composite. Even as bright as Spica is (magnitude .9 or so,) there really isn’t any useful way of capturing the wide disparity of light between it and the moon. The exposure to capture Spica was 1/2 second; the moon’s was 1/20 second. In the world of photographic exposures, that translates to the Spica exposure being 3 1/3 stops greater than the moon’s, or about ten times the light level (it progresses exponentially, halving or doubling every stop, so increasing one stop doubles the light, two stops is four times, three is eight.) In the original image that provided the bright Spica used here, the moon was blown out into a featureless white semi-circle. As I said long ago, even getting clouds in the image requires special conditions.
What’s also difficult is finding a way to identify the features you see in moon images such as this. Partially lit moons are more interesting than full in many ways, because the lower light angles throw shadows from the terrain and enhance the geography. [Momentary side note: I paused just now to wonder if “geography” was a technically appropriate word to use for the moon, and realized that it was probably okay, while “terrain” wasn’t.] But the distinct craters visible here along the terminator (the border between light and shadow) are much harder to make out when lit directly from above, as is the case with a full moon and virtually every method of displaying the moon’s features that can be found. I was trying to determine if I’d captured the prominent crater Tycho, right on the shadow line near the bottom, and this took a lot more effort than you might have thought. The crater, by the way, isn’t Tycho; it’s close neighbor Maginus. To find this out more distinctly, I resorted to a sneaky trick that produced the following image.
While, in certain conditions, it is possible to see something vaguely like this, it’s rather shamelessly a Photoshop job. The shadowed portion of the moon really does get some illumination from sunlight bouncing off of the daylight portions of the Earth, known as earthshine, but it’s nowhere near this distinct. I did, in fact, try to capture it tonight, but it took exposures so long that the ambient light from the sunlit side was scattering through the lens, like fog, and the moon was showing visible movement anyway. So this is actually a portion of a full moon image from years ago, patched into the shadow area and tweaked to look a little more realistic. It also shows that Tycho was receiving just the first contact of sunlight with its eastern rim – you can just make out the rays pointing to the dark circle in near-contact with Maginus on the border. In another few hours Tycho would be seeing sunrise, and this would take a while, too, since days on the moon are two weeks long. It’s all part of that “tidal orbit” thing which keeps the same side of the moon facing us all the time. We see the moon rise and set each night, because the Earth rotates, but the moon goes through phases because of its own orbit, which is also its rotation – one orbit every 27 days, always facing Earth. That means a day/night cycle on the moon is the same amount of time. Luckily the astronauts did not have to adapt to this schedule while there…
If you think that the editing job wasn’t perfect and the moon seems a bit oblong or football-shaped, you’re right. This is because the tidal orbit is not perfect and the face of the moon wobbles as seen from Earth, over the period of its orbit. Since the pics were taken at different phases, the moon was actually facing different directions, and matching the terminator required shifting things a bit.
The full moon image I used is actually the same one appearing here, which has another connection to all of this. That post mentions how it was believed that Tycho and the dinosaur-extincting impact here on Earth might have been caused by the same source, an asteroid called Baptistina that broke up 160 million years ago. A few months after posting that (but which I did not find myself until just tonight,) NASA nixed this theory, since their Wide-field Infrared Survey Explorer (WISE) mission determined that the remnants of the Baptistina asteroid are too young to fit the bill; the chances of it breaking up and making it from the asteroid belt to Earth/Moon orbit in that brief time are minuscule. “Brief time,” in this case, means only a few million years – stuff does not fly around the solar system the way that we might imagine. While it remains possible that both Tycho and Chicxulub have a common origin, it is probably not Baptistina. Two craters once again in search of their parents…
Just for complete confusion, I have to mention that there’s still some question about the Chicxulub impact being solely responsible for the KT extinction event. “KT” used to stand for “Cretaceous/Tertiary,” the two periods separated by the event, but is now being referred to as the “Cretaceous/Paleogene,” or K-Pg. I’ll go into all of this in another post someday, but if you can’t wait, search on those terms and throw in “Deccan Traps” too.