Missed It By That Much

One of my favorite astrophoto-ops is catching a saguaro cactus or other interesting flora silhouetted by the Full Moon. There’s a nice one in my June 23, 2014 post. Unfortunately, as the slideshow reveals, I got zilch on this attempt. But when I analyzed my fail I learned something that might be of interest, especially to photographers.

Knowing when and where to look for these rare alignments is a matter of having the right astronomical software. I use the open source Cartes du Ciel. It can show me the sky in any direction, at any time and date, from any location on Earth. So when it told me there was a good chance to catch the rising Moon behind one of the many large saguaros on Gavilan Peak, a mountain half a kilometer to the east, I started planning.

On the appointed date I was ready with my Canon DSLR + 200 mm lens on a sturdy tripod. My east yard is open and flat and provides an area of about 200 m2 for deployment. Where exactly in the yard should I place the tripod? An effect called parallax makes that an important decision. The graphic below shows the geometry:

From one position along the baseline the saguaro is perfectly aligned with the Moon. From the other position it misses entirely. The diagram is obviously not to scale, so let’s put some numbers on it …

Google Earth clearly shows that ridge with all the saguaros, so I know distance D = 495 m. My east yard provides a photography baseline of B = 20 m. We can use geometry to calculate the parallax caused by shifting from one end of that baseline to the other:

Θ = tan-1(B/D) = tan-1(20 m / 495 m) = tan-1(0.04) = 2.3°

2.3° might not seem like much of a shift in line of sight, but the Moon has an angular size of only about 0.5°, so moving from one end of the baseline to the other amounts to 5 Moon-diameters of shift. That leaves a lot of room for error with alignment.

In practice, Cartes du Ciel tells me the exact azimuth (compass heading) at which the Moon will rise. So what I have to do is ensure my camera is pointing in that direction and see if there are any good saguaro in the FOV. If not, I shift my tripod left or right (maintaining the same azimuth) until I see what I want. This is much easier to do when using a telescope, where setting the azimuth is a matter of reading a digital or engraved scale. But with a basic tripod you have to use a magnetic compass and eyeball the direction.

The Moon moves its own diameter (0.5°) through the sky in 2 minutes. The glow of the Moon extends for several degrees around it, even on a clear night. So there’s a brief period of time when you can see the glow of the not-yet-risen Moon, estimate where it’s going to appear, and adjust the camera position accordingly — maybe 2–3 minutes. I’ve done it on occasion, but it’s always a frantic race to relocate and level the tripod, aim the camera, check the focus, and trip over the dog.

You can work the geometry backwards and see that my miss of about 0.4° could have been corrected by shifting the tripod a mere 3.5 m to the left. But I thought from the glow (image 2/4 in the slideshow) I surely had this one nailed. Alas, in the words of the inimitable Maxwell Smart — missed it by that much.

Next Week in Sky Lights ⇒ Asymmetric Moon Cratering

Q&A: Crossing Contrails
Q&A: Asymmetric Moon Cratering