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Highlands Astronomical Society

Seeing Stars - Inverness Courier, Fri 3rd Dec 2010

Spots and Wrinkles - Antony McEwan, Highlands Astronomical Society

Spots and Wrinkles

December is a fine time to explore a wealth of Deep Sky Objects: nebulae, galaxies, supernova remnants, open and globular clusters, etc, but this month I’m going to concentrate on two much brighter, much easier to find objects. Jupiter and The Moon, both of which are in Pisces this month.

Jupiter is undergoing some rebuilding work at the moment. When looking at the planet through even a small telescope, there are usually several coloured bands visible, with two particularly obvious ones standing out against the pale cream disc. One of these is the Northern Equatorial Belt (NEB) and the other is the Southern Equatorial Belt (SEB). The SEB is not a permanent feature though, as it can disappear semi-regularly, only to be recreated a short while later on. It can fade away every three to fifteen years or so, and last disappeared in the early 90s.

When it has gone, observers are left with a view of Jupiter that can take a while to get used to! Thankfully though, after a while the belt is reformed, and that process appears to have already started. In November, a small white spot has been photographed several times in the vicinity of where the SEB once was. This ‘spot’ has already begun to emit tendrils of material outwards, and if it follows the familiar pattern it will spread out, joining up with other spots in the same latitude until a new belt is formed.

For amateur astronomers it provides us with a chance to watch something very dynamic taking place over the course of several weeks or months, right in front of our eyes. The spots are atmospheric features, at high altitude in Jupiter’s clouds, and once they start joining together it will be fascinating to see the new belt forming, with its tumultuous structures, swirls and storms, all within reach of telescopes of moderate aperture (4-inches or even less).

While waiting for this wonder of atmospheric recycling to unfold, we still have the regular dance of the Galilean satellites to follow as well. The moons Io, Europa, Ganymede and Callisto all orbit Jupiter in such a way that it is possible for us to observe the moons passing in front of, or behind, the Jovian disc. Often we can even observe the shadow cast by one of these moons transiting the disc. In fact, the shadow is often easier to detect than the moon itself, as it appears darker against the pale creamy tones of the planet’s clouds than the moon itself does.

It is possible to predict the times of the various orbiting events, and you can see what is going to happen at any given time on any night by accessing the Sky & Telescope website’s Observing page and running the Jupiter’s Moons JavaScript utility. There is also a utility that will predict the transit times (times of crossing) of the planet’s Great Red Spot (GRS), which actually varies in colour quite considerably from year to year. Salmon pink best describes the tone at the moment, but the Great Salmon-Pink Spot doesn’t roll off the tongue so easily.

Bright object number 2 is The Moon. It astounds me how little some people seem to know about the Moon. I recently was asked if a “New Moon” was when it broke up and reformed every month! However, there are actually many things that scientists have discovered about our closest celestial neighbour that the general population is not perhaps aware of.  For example, its size and distance from the Earth make it exactly right for providing us with total solar eclipses. We are here at just the right time in history to be able to appreciate this remarkable phenomenon. If the Moon were further away, it would be too small relative to the Sun’s disc. If it were closer, too large for us to see the fantastic “Ring of Fire” effect or “Diamond Ring”. Coincidence? Who knows…

 Notwithstanding its current serendipitous distance from us, the Moon is receding from Earth at the rate of 3.8cm per year (Data from NASA). How do we know this? Well you remember those Moon landings that some people say didn’t happen? Well while the Moonwalking astronauts weren’t up there they also didn’t install a series of laser ranging retroreflectors. Those retroreflectors obviously haven’t been returning lasers directed at them from ground-based emitters for the last 31 years, nor have the timings been analysed to see that it is taking gradually longer for the beams to be reflected back at Earth. If they really had done this, that would have been really clever.

Now how about this: the Moon is shrinking! Images taken by a NASA orbiter have revealed wrinkles on the lunar surface. Some of these contraction marks are several miles long and rise tens of metres above the lunar surface. Contraction of a planetary body is usually associated with the cooling of a hot core, so maybe the Moon is not completely cold as was recently believed, but in actual fact is still in the process of cooling?

You see: more unanswered questions! Ones to ponder while pointing your telescope at that bright white orb hanging in the frosty December sky. Sadly the laser beam retroreflectors are too small to be seen by your telescope, and you won’t see the wrinkles either, nor will you be able to detect the slow ponderous 3.8cm per year drift with your telescope, no matter how careful your observations! It is still remarkable though to think that our greatest minds combined with the latest technology in this ‘space age’ (well ok, cameras and mirrors then) are still having to think long and hard about the Moon.

From an amateur’s point of view the Moon is a fine luxury. We can observe it for pleasure or we can observe it for data. We can try to observe such events as the recent deliberate crashing of a probe into the lunar surface, or we can simply count the craterlets visible to us in the grand dark-floored crater Plato. We can make careful observations of the features revealed to us by the Moon’s libration (the rocking back and forth of the Moon as it orbits us, so that we can actually observe 59% of the lunar surface rather than the expected 50%), or we can cruise the terminator looking for solitary sunlit peaks standing out in stark contrast against the dark unlit portion of the disc.

Or, to really get the most out the Moon, we can simply stand on a blanket of fresh snow and gaze up at it in awe (telescope optional), ideally with a loved one to share the view, perhaps while listening for sleigh bells on a particularly magical night in December.

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