Occultation of Aldebaran – The Video


Such as it is ... a better title might be "What I Did Wrong Trying to Record the Occultation of Aldebaran." But there's always some science to be gained, even from failed experiments. So that's what I tried to do with this video. There's three sequences: disappearance of Aldebaran behind the Moon, real-time motion of the Moon through the sky, and reappearance of Aldebaran. The video is 25 Mb, so it might take awhile to load. Patience grasshopper.

If I wanted to do serious astrovideography I'd get a CCD imaging system for my telescope, or maybe a GoPro camcorder. For this video I used a Sony TVR-310 Digital-8 camcorder — 15 year old technology. And I used it at maximum zoom (20X optical). The CCD is ¼ inch with about 300,000 pixels, and not nearly as sensitive as current chips. It still works great for daytime terrestrial action, but really wasn't designed for astrovideography. I already knew that. I was just curious how well the camcorder would perform. </disclaimer>

The first sequence begins with the tiny dot of Aldebaran clearly visible as it catches up to the Moon. Alas, the Moon is overexposed, but I had to set the exposure high enough to make Aldebaran visible. As the moment of "contact" approached I turned the exposure down in several steps to try capturing more detail on the Moon. Unfortunately, somewhere along the way, I lost Aldebaran and did not capture the moment of occultation.

My camcorder was on a sturdy tripod, but my exposure adjustments while recording created vibrations in the image. I removed some of that using the Deshaker filter in VirtualDub, but it has a hard time working with a black background. Registering the frames individually was out of the question — there are 900 of them at 30 fps. I considered using RegiStax to remove the shaking, but given the video quality it just wasn't worth the effort. The same thing happened in the third sequence.

The second sequence was better. Exposure was good and no adjustments were required. Aldebaran is now behind the Moon and we're killing time waiting for it to reappear. Here's where we can learn some science. The camera is on a non-tracking mount, so you can see the real-time motion of the Moon through the sky. We've all watched the Moon rise and set, when its motion relative to the horizon is obvious, but when it's high up in the sky that motion is less noticeable.

The Moon moves at an apparent speed of 14.5°/hr. We follow the Moon for the amount of time it takes to move its own diameter (0.5°). That time would be: t = d/v = (0.5°) / (14.5°/hr) = 0.0345 hr = 2.07 minutes = 124 seconds. Aldebaran spent 29 minutes behind the Moon as seen from my location in Arizona, during which time the Moon moved 14 diameters.

Allow me to clarify what I mean by apparent speed. Most of the motion we see in the sky comes from the 15°/hr rotation of Earth. But the Moon is also in motion around the Earth, circling it once every 29.5 days. When you combine these two motions you get 14.5°/hr. Stars, by comparison, have an apparent speed of 15.04°/hr (which is why Aldebaran can catch up to the Moon). The apparent motion of a star is a combination of Earth's rotation plus its revolution around the Sun.

The final sequence shows the reappearance of Aldebaran from behind the Moon. Whereas the disappearance was on the illuminated limb of the Moon, reappearance happens on the "dark side." This lets you more easily spot Aldebaran when it first appears. The video clearly shows that reappearance. Stars are very small compared to the Moon (in apparent size), so it doesn't take very long for them to "rise." The time between when it first appears as a point of light, to when it's completely beyond the limb of the Moon, is typically less than 0.1 second. You can blink and miss it.

Next Week in Sky Lights ⇒ Why the ISS Orbit Appears Curved

November 2 Quadruple Conjunction
Q&A: Why the ISS Orbit Appears Curved