Eclipses and comets are the most amazing celestial events, visible to the naked eye (unless you are lucky to be watching, when a nearby supernova erupts!).
Foreknowledge of obscurations, formerly had high strategic value. The explorer Christopher Columbus, used a looming eclipse to dupe native Americans out of needed “essential commodities”.
Long ago too, an eclipse decided the outcome of a major war in the Near East, while NASA reports that two ancient Chinese astrologers lost their heads for failing to predict an obscuration.
Thanks to modern education and the public media, aesthetic appeal and intellectual appreciation of this celestial shadow-play is gradually replacing fear, dead and superstition.
Happily, the same is true of comets which, in pre-industrial cultures, were also a cause of consternation and supernatural concern.
Justifiably so. Because the appearance of these apparitions over the horizon–with their brightly glowing heads and long luminous tails– are emotionally stirring, even to the experienced sky-watcher.
I can attest to this unequivocally, having observed the historic Hale-Bopp comet on two successive evenings, in 1997.
In reality, comets are small icy bodies, averaging less than ten km across. They originate from the outer fringes of the solar system and hurdle sunward in highly eccentric orbits.
Frozen into cometary ices, are tonnes of pebbles and dust. As the object approaches and careens around the Sun, its volatile ices heat up—with fabulous consequences, for sky-watchers.
Volatiles start to evaporate, spewing out jets of hydrogen gas and dust particles, which form an expansive and reflective cloud around the comet’s nucleus (the solid mass).
The “coma,” as astronomers call it, can extend several hundred thousand km from the nucleus, imbuing the miniscule visitor with a spectacular visual persona.
“As the comet gets closer to the Sun,” NASA explains, “the coma grows. The solar winds push the dust and gas away from the coma, causing them to stream off into space to form the comet’s tail”.
NASA notes further, that “The tails of comets can reach 150 million kilometers in length!” Imagine seeing a luminous configuration like this, trailing across the firmament at twilight or midnight!
Well, it’s possible—because there is a continuous procession comets, into the inner solar system. The problem is, most of them visit by stealth, secretly spiriting around the Sun, just out of naked eye range.
A bit of technical talk, may help here: “Naked eye” objects, are those that a human with 20/20 vision can perceive, on a cloudless and moonless night, away from city lights (without the use of a viewing aid).
In assigning visibility values to celestial bodies, astronomers use a “magnitude scale” that is based on how luminous an object appears from Earth’s surface, rather than its actual (absolute) brightness.
Keep in mind too, that smaller numbers denote brighter celestial bodies, while progressively larger numerical values are ascribed to dimmer and dimmer objects.
In terms of brightness, the magnitude scale goes beyond “0,” so that very luminous objects, such as the Sun, the full Moon and Venus, for example, have negative values— “-26,” “-13” and “-4”.
Hence naked-eye visibility runs roughly from anything minus (-) to around five (+5 or simply 5). You’ll need binoculars, to view bodies brighter than “10” but dimmer than “5”.
There are, “at any given time,” International Comet Quarterly reports, two or more comets within this magnitude range.
The British meteorologist, Jonathan Shanklin, predicts 66 arrivals in 2017, mostly of binocular visibility. But naked-eye observers shouldn’t despair: Nature may still have us in mind!
To be continued.