They Move!

Last week a FB friend of mine forwarded a list of December sky happenings and I noticed that the planet Mars happens to be passing the outermost planet Neptune (that is if you believe in the demotion of Pluto as a planet). Shown at left is the plot of the characters in this scene. Mars is relatively close to the earth, and Neptune very distant But from our vantage point Mars appears to be zipping past Neptune. At best, Neptune is visible in a very good pair of binoculars or small telescope, but I've been watching reddish Mars for a few months in the evening sky - now almost due south just after sunset. I really didn't have any interest in actually SEEING Neptune, I've seen it many times in a telescope - appears as a featureless tiny bluish disk. But what I was HOPING to record, was its motion over a day or two. All planets move around the sun, and of course, the earth's motion also contributes to their apparent motion in the sky.



Monday night when I was photographing windmills, I took a set of exposures of Mars with my 200mm lens, using my little tracking platform. I took a half dozen exposures of 20 seconds each that I stacked to minimize the noise in a single image. As shown at right in the stacked image (also some cropping), Mars is the reddish brilliant object below Lambda Aquarii (seen in the inset in right diagram). Oh - hopefully Neptune is in there too!




We've had a stretch of cloudy weather since, but Wednesday night there was a brief break of clear sky between storm systems, and I headed west of town in hopes of providing a second "epoch" to demonstrate 48 hours of motion. I again took a series of 20 second identical exposures. After completion I headed back to town and later in the evening stacked those exposures too. Putting the two data sets together shows the motion over the time gap between them. Now realize that Mars was about 1 AU (astronomical unit - mean distance of sun to earth, about 93 million miles) from us, and Neptune was 30 AU away - 2.7 BILLION miles away! As a result Neptune moves pretty slowly, and I didn't expect much of a shift for Neptune, but I was in luck - it was clearly seen - do you see it at left? Clearly seen in this cropped image is the doubled image of brilliant Mars, with its considerable motion over the 48 hours. But Neptune is harder to spot...

But the right image shows the labeled position and the doubled position of Neptune over the 48 hour gap. Neptune moved a tiny, but definite amount. Well it takes 165 years to orbit the sun, so it isn't in much of a hurry...