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I was intending to update you on the status of the de Havilland Mosquito project this week but things are afoot in the sky and there is something I believe is worth looking at and discussing.
Comets are still somewhat of a mystery and in the past have been viewed with superstition and irrationally and erroneously labeled as, unlucky, bad omens, harbingers of doom, attacks on terrestrial beings by heavenly creatures as well as a host of other even more absurd names. They could be harbingers of doom if they were to impact the Earth, as happened to Jupiter in 1993 when comet Shoemaker-Levy 9 broke apart and eventually impacted the planet. It is also postulated that the Tunguska Event in 1908 may have been a comet that disintegrated in the atmosphere in a cataclysmic aerial explosion. The name comet has a somewhat obscure derivation as it came to English from Latin where it in turn came from Ancient Greek. It is believed that Aristotle was the first to use the word komētēs to describe what he referred to as a star with hair.
For the most part we have gotten over the superstition and while we have learnt much about comets they are still very much an unknown and mysterious object. It is believed that they originate from the Ort Cloud and after being disturbed make fleeting visits to the inner solar system on highly elliptical orbits with periods that vary between a few years and several millennia. Once a comet begins its dive towards the inner solar system the ever increasing intensity of the sunlight causes them to start heating up and as a result spew gas and debris from their surface. It is this gas and debris that forms what Aristotle describes as its hair.  For some comets that have orbits that take them too close to the Sun their visit results in the disintegrating into a stream of debris. The gas and debris are then deflected by the solar wind forming the tails that point away from the Sun. As the gas is usually ionized it is often not affected by the solar wind in the same way as the debris and form separate tails. The animation on the right shows the gaseous tail in cyan and dust tail in grey. Another point worth noting is the directions of the tails, since it is the solar wind rather than the comet's motion that has the greatest effect on the tails they are always on the side that is away from the Sun.
Comets have orbits that are often affected by their close encounters with the planets as well as the jets of gas spewing from their surface making their orbits and reappearances difficult to accurately calculate and predict. They are also notoriously fragile and easily broken apart by tidal forces if they approach to close to the planets or Sun.
We will discuss comets in more detail in later threads but for the moment we will leave it there and get onto the real subject of this thread which is comet P17 Holmes. However, before we do so it's worth looking at the way you are likely to see dates and times represented in the links I have supplied. The most common format is yyyymmdd.tttt where yyyy is the year in numerical format, mm is the month in either numerical or name format, dd is the day of the month and tttt time represented as a decimal representation of the time normalized to 1. For example, 20071117.500 represents 12 noon on 17th November 2007.
Comet 17P was first observed on 6th November 1892 by Edwin Holmes from his observatory in London England. It took some time to calculate its orbit to any degree of accuracy partially due to its initial discovery being 5 months after perihelion and its perihelion distance of 2AU. AU stands for Astronomic Unit which is currently defined as 149,597,870,691 ± 30 metres but for all practical purposes it is the distance the Earth is from the Sun.
None the less after considerable work by the astronomers Heinrich Kreutz and George Mary Searle its orbit was calculated and its return in 1899 and 1906 was correctly predicted. Comet 17P turned out to be more mysterious than usual. On several occasions and for unknown reasons comet 17P suddenly and dramatically increased in brightness over very short periods of time. In fact it was only first noticed in 1892 after one of these flare ups
Comet 17P next visits to the inner solar system was predicted for 1913 but it was never seen and by 1947 it had been declared lost and unlikely to be ever seen again. However, with the advent of computers and software capable of calculating and predicting orbits it was once again located by Elizabeth Roemer on 16th July 1964 and has been observed on each subsequent visit.
Comet 17P is currently some 244.Gm from Earth and 377 Gm from the Sun and had a magnitude of 17 making it only visible to observers using larger telescopes. However, on the 23rd of October comet 17P had one of its flare ups increasing to a brightness of 2.5 in the space of a few hours and making it clearly visible to the naked eye. Nobody is certain of the actual mechanism that causes these flare ups, but a more than 600,000 fold increase in brightness would tend to indicate that something dramatic had happened.
 Unfortunately my observation point makes observing comet 17P difficult as it only gets around 5° above the horizon and there is considerable light pollution in that direction. Since its latest flare up it has dramatically increased in size making it somewhat dimmer but it should still be clearly visible to observers in the northern hemisphere with the unaided eye provided you are away form the light pollution of the major cities. The animation and image below and on the right will give you an idea of where to look.
Comet 17P Holmes is an example of how the night sky is not the completely static and unchanging thing many believe it to be. It is always worth spending time whenever you have a few minutes to spare just looking at the night sky. You will not only  become familiar with it but you will be astounded with how dynamic it can actually be. At least we no longer need to be worried about being placed under house arrest like Galileo Galilei or burnt at the steak like Giordano Bruno for telling others what we have seen in the night sky.
As usual I have included the links below that will give you further information on comet 17P Holmes. I would highly recommend visiting Gary Kronk's Cometography page as it has some interesting information about comet 17P Holmes.
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_07914C2F-03E6-EA6A-A1B74455BF257290.jpg "MaSu on Machines, Meteors & Mozzies"){kind=logo}
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