2. What do comets look like in the sky?
In the previous section, the root of the word comets was given as the Greek kometes, long-haired (star). Well, that's basically what a comet looks like in the sky -- a fuzzy spot with streamers of light flowing behind. Early drawings usually show a circle or dot with lines behind it. Early Chinese astronomers were careful observers and made sketches, that while crude, showed that they noticed differences in the appearance of comets -- some had broad fan-like tails while others had narrow, spiky tails.
One resembles the narrow spikey-tailed comets depicted in Mawangdui silk and the other resembles the broader tail depictions.
Fig 1: The Mawangdui silk, a 'textbook' of cometary forms and
the various disasters associated with them, was compiled
sometime around 300 BC, but the knowledge it
encompasses is believed to date as far back as 1500 BC.
In the 1700's, the telescope was a new instrument that allowed people to see the heavens as they had never before. It became fashionable to search for comets and basically discover them before they were visible to the eye alone. Today, we know that these observers were usually seeing a comet when it was further from the sun. These more distant comets tended to be more like fuzzballs rather than dramatic objects with sweeping tails in the sky. However, not all objects that were dim and fuzzy in the telescope view turned out to be comets. Some never moved, so some of the observers began cataloguing these 'nuisances.' One such list, the Messier List, was compiled by Charles Messier. Today, his list is one that many amateur astronomers use in their quest to see some of the showpiece clusters, nebulae and galaxies visible in small telescopes.
What will HArtley 2 look like?
[pre-encounter expectations] Although Hartley 2 has been observed at every appartition since discovery, the observing circumstances have never been ideal and so it was usually a faint comet. This year, the observing circumstances are the best that they can be. Observers are already getting images of the comet. By mid-October, it is expected to reach magnitude 6 and by encounter, it could be a naked-eye object.
What did HArtley 2 look like?
[post-encounter results] Too soon to say! Come back in December 2010!
What will Tempel 1 look like?
[pre-encounter expectations] Tempel 1 is not a very bright comet. At a normal perihelion, it typically reaches about magnitude 9. So in the days before the Deep Impact impactor hits Tempel 1, this comet will indeed be a very faint fuzzy visible mainly with telescopes. After the impact, there are different thoughts as to how bright the comet will become. It's not the brightness from the explosion but rather from the increased amount of gas and dust thrown into the comet's coma by the impact. If lots of dust is released then the comet could get quite bright, possibly as bright as magnitude 5, which believe it or not, is still a faint fuzzy, but bright enough to be seen with binoculars. We really won't know until it happens!
What did Tempel 1 look like?
[post-encounter results] Well, despite the fact that it was considered to be a good apparition (based on relative location to Earth), the comet was best observed in the southern hemisphere and low northern latitudes. It ended up being slightly fainter than expected. In addition, although the Deep Impact impactor excavated alot of fine dust, it did not brighten the coma as significantly as the DI team had hoped. Be sure to visit the AOP and STSP image galleries to see all of the images of Tempel 1 taken by other amateurs.