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A Great Year for the Geminids!

Geminds in 2015 by Ian SharpActive from December 6-17, but with a slow rise to maximum, the Geminids are currently the richest of the regular annual meteor showers, producing an abundance of bright meteors, with rates outstripping those of the August Perseids for a 24-hour interval centred on their 14 December maximum – a real treat for observers prepared to brave the winter winds, cold and damp.

The other good news is that Geminid maximum this year occurs just before new Moon, so there will be no interference by moonlight, enabling many fainter meteors to be seen in addition to the brightest members of the shower. This year, the time of Geminid maximum is especially favourable for observers in Europe, with peak activity expected at about 02h on Thursday, December 14, when the ZHR may again reach 100 to120 meteors per hour.

In recent years, from the UK, the Geminids have shown typical peak observed rates of 70-80 meteors per hour in good skies. The maximum is quite broad, however, and respectable Geminid rates may be expected throughout the nights of December 13/14 and 14/15. Past observations have shown that bright Geminids become more numerous some hours after the rates have peaked, a consequence of particle-sorting in the meteoroid stream.

Geminid radiantThe Geminid shower radiant (at RA 07h 33m, Dec +32o, just north of the first magnitude star Castor) rises early in the evening and reaches a respectable elevation above the horizon (> 40o) well before midnight, so observers who are unable to stay up late can still contribute very useful watches. However, the early morning hours of Thursday, 14th December are likely to see the greatest Geminid activity, when the radiant is high in the sky.

As with any meteor shower, when observing the Geminids it is best to look at an altitude of 50o (about the same altitude as the Pole Star from southern parts of the UK) and 40-50o to either side of shower radiant, rather than looking directly at the radiant itself, although Geminid meteors may appear in any part of the sky. December nights can be quite chilly, especially in the early morning hours, so wrap up well with plenty of layers of warm, dry clothing and make sure that you wear a hat, gloves, thick socks and sensible waterproof footwear.

One surprising aspect of the Geminid shower is that it was unknown 200 years ago. Although there is evidence that the Geminids were active as early as December 1833, with further reports in 1836, the ‘official’ discovery of the shower was made by Robert Greg of Manchester in 1861 and the following year by Benjamin Marsh and Alex Twining from the USA. The shower has been recorded each year since that time, and observations by members of the BAA Meteor Section since 1890 have indicated steadily increasing activity. The peak Zenithal Hourly Rate (ZHR = the number of meteors a single observer would see in an hour under very clear, dark skies with the radiant at the zenith) was typically 20-60 meteors per hour in the early twentieth century, rising to 80 meteors per hour by the 1980s. In recent years, the peak ZHR has usually exceeded 100 meteors per hour, even approaching 120 meteors per hour (two meteors per minute) on several occasions.

The majority of the annual meteor showers are associated with known periodic comets, yet there is no very short period comet that matches the orbit of the Geminid meteoroid stream. Instead, the orbit of the Geminids is occupied by an object called 3200 Phaethon, which looks remarkably like a rocky asteroid. It was discovered in 1983 by NASA's IRAS satellite. The problem was that 3200 Phaethon appeared to shed very little dusty debris – not nearly enough to explain the amount of material in the Geminid stream. However, the eccentric orbit of 3200 Phaethon brings it well inside the orbit of Mercury every 1.43 years. The rocky body consequently receives a regular blast of solar heating that might somehow trigger the release of dust to enrich the Geminid stream.

To test whether this might be the answer, a group of astronomers led by David Jewitt and Jing Ki of UCLA used NASA’s STEREO-A spacecraft to take a closer look at 3200 Phaethon when it passed closest to the Sun in June 2009. The resulting photometry showed a doubling of Phaethon's brightness as it approached the Sun, as if sunlight were shining through a cloud of dust around the asteroid. The observers began to suspect 3200 Phaethon was something new – a "rock comet" which is, essentially, an asteroid that approaches so close to the Sun that solar heating scorches dusty debris right off its rocky surface forming a tail of rocky grains. Seeing 3200 Phaethon sprout a tail, even a small one, provides some confidence that Phaethon is indeed the source of the Geminids, but a major problem remains: the amount of dust 3200 Phaethon ejected during its 2009 Sun-encounter added a mere 0.01 per cent to the mass of the Geminid stream – not nearly enough to keep the stream replenished over time. Perhaps the rock comet was more active in the past? Only time and further continued observations may provide an answer.

Geminid meteors enter the atmosphere at a relatively slow 35 kilometres per second, and thanks to their robust (presumably more rocky than dusty) nature tend to last longer than most in luminous flight. Unlike swift Perseid or Orionid meteors, which last only a couple of tenths of a second, Geminids may be visible for a second or longer, sometimes appearing to fragment into a train of ‘blobs’. Their low speed and abundance of bright events makes the Geminids a prime target for imaging.

The Geminid shower has grown in intensity over the past 50 years as a result of the stream orbit being dragged gradually outwards across that of the Earth. A consequence is that we currently encounter the most densely-populated parts of the stream. This happy situation is unfortunately only temporary – in a few more decades, Geminid displays can be expected to diminish in intensity. Here we have an excellent opportunity to follow, year on year, the evolution of a meteoroid stream.

The BAA’s visual meteor report forms, available as downloads in both pdf and Excel formats, enable observers to record the details of each meteor seen. These include: time of appearance (UT); apparent magnitude (brightness); type (shower member, or random, ‘background’ sporadic); constellation in which seen; presence and duration of any persistent train. Other notes may mention flaring or fragmentation in flight, or marked colour. Watches should ideally be of an hour’s duration or longer (in multiples of 30 minutes). Observers are reminded to carefully record the observing conditions and the stellar limiting magnitude. Wrap up warmly and enjoy what should be a great show!

By whatever means you observe the Geminids this year, please submit your results to the BAA Meteor Section via meteor@britastro.org.


Dr John Mason
Director, BAA Meteor Section
email: docjohn@dircon.co.uk

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Stargazers' Almanac 2018

Stargazers' Almanac 2018

A monthly guide to the stars and planets

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Sky Notes

BAA Observing Sections Comet

174P Echeclus in outburst

The unusual Centaur comet 174P Echeclus is currently undergoing one of its brightest ever outbursts. Also known as minor planet 60558 this object moves in an orbit which has a perihelion of 5.8 AU, an aphelion of 15.5 AU and a period of 34.9 years. It last came to perihelion in 2015 April and it is currently moving outwards. At the start of December it was 7.3 au from the sun and around mag 17.

On December 8th Brian Skiff reported that images he had taken on the previous day showed the comet to be at least 4 magnitudes brighter than expected. Jean-Francois Soulier and Richard Miles confirmed the outburst with 0.20m and 2.0m telescopes respectively. At first the object was stellar but it is now showing a distinct coma which can be detected in small instruments as shown in this image from Tim Haymes. Tim has used Astrometrica to measure the Point Spread Function (PSF) of the comet and of a nearby star and the comet is clearly "fuzzier" than the star. The magnitude of the comet is now around mag 13 or so.

The comet is conveniently placed in the early evening sky and the Comet Section would be pleased to receive your images and photometry of this unusual object.

The BAA image archive for 174P is here. Compare an image taken before the outburst with one taken just afterwards.

An observation of the comet by Tim Haymes showing that it has a broader PDF than a star.

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Patterns in the sky

Figure 1 The night sky is divided up into areas known as constellations. (Stellarium)Go out on a clear, dark moonless night, well away from any bright artificial lights and you will see stars, lots of stars. Typically, people overestimate the number of individual stars they see at any one time. In reality the average number is around three thousand under the best conditions.

This article will discuss how the stars are divided up into constellations and how they are named.

The constellations

The human eye-brain combination is good at seeing patterns and joining up the dots. Millennia ago our ancestors did just this, forming what we now call constellations (Figure 1). Different civilisations around the globe created their own patterns in the sky to reflect local myths and legends.

The constellations that we now recognise are generally believed to have had their earliest origins in Mesopotamia (approximately modern day Iraq) around 3200BC with more and more added up to roughly 500BC.

Some constellations may have had an even earlier genesis. For example Ursa Major, the Great Bear, is recognised as such by many different cultures around the northern hemisphere even though it does not really resemble a bear; this leads us to wonder why so many seemingly un-related cultures hold such a similar belief. The suggestion is that as humans moved across the northern hemisphere they were sharing their ideas and beliefs, including the concept that this particular collection of stars represented a great bear in the night sky.

Starting in Eurasia, as people moved eastward they carried the tradition with them. During the ice age when sea levels were lower they crossed the Bering Straits and into North America where the bear is recognised as such by something like fifty percent of the Native American tribes. The implication of this is that the origins of the Great Bear go back at least 14,000 years to the last ice age. Even earlier than that, there are suggestions that markings in the famous Upper Paleolithic cave paintings of Lascaux in France show parts of the night sky.

However they originated, the constellations visible from the classical world came down to us from Mesopotamia via ancient Greece. There seems to have been a major influx of Mesopotamian constellations into Greece by 500BC. The Greeks adopted, added to and adapted these patterns to their own myths and heroes. In the second century AD the Alexandrian astronomer Ptolemy, in a work that we now call The Almagest, compiled a list of 48 constellations that forms the basis of what we use today.

These constellations reflect the myths of the ancient world showcasing its heroes and monsters. There is Orion the mighty hunter, the legendary Hercules and many more. Some of these are woven from complete stories. We have Andromeda the daughter of king Cepheus and queen Cassiopeia. Chained to a rock as a sacrifice to the sea monster Cetus, she is rescued by Perseus arriving, in some versions of the myth, on the winged steed Pegasus. All of these characters are now constellations in the sky.

The stars of the southern skies had to wait until the voyages of discovery from the 15th century onwards to be charted by Europeans though many of the peoples of the southern hemisphere would have had their own constellations long before this. Many of the new constellations formed in the southern sky, rather than representing myths and legends from the ancient world, reflect the discoveries and ideas of the age, so we find a telescope, a microscope and a number of exotic birds amongst others.

As time went by more and more constellations were added with almost every new map of the sky. This was not just with the southern stars but new constellations were also carved from existing ones. Eventually the system became chaotic and unwieldy; time was ripe for order to be brought forth from chaos.

In the first part of the twentieth century the newly formed International Astronomical Union (IAU) formally defined the eighty-eight constellations marking down their boundaries in the sky and establishing the global standard we use today. The IAU continues to be the internationally recognised authority for assigning designations related to celestial bodies.

Below is a list of the eighty-eight constellations which are formally recognised today. The official names are in Latin and these are what are generally used. Beside them I have provided an English translation and the standard abbreviation. The last column is the Latin 'genitive form' of which more later.

Latin nameEnglish translationAbbr.Genitive
Andromeda The chained maiden And Andromedae
Antlia Air pump Ant Antliae
Apus Bird of paradise Aps Apodis
Aquarius Water bearer Aqr Aquarii
Aquila Eagle Aql Aquilae
Ara Altar Ara Arae
Aries Ram Ari Arietis
Auriga Charioteer Aur Aurigae
Boötes Herdsman Boo Boötis
Caelum Engraving tool Cae Caeli
Camelopardalis Giraffe Cam Camelopardalis
Cancer Crab Cnc Cancri
Canes Venatici Hunting dogs CVn Canum Venaticorum
Canis Major Great dog Cma Canis Majoris
Canis Minor Lesser dog Cmi Canis Minoris
Capricornus Sea goat Cap Capricorni
Carina Keel Car Carinae
Cassiopeia The Seated queen Cas Cassiopeiae
Centaurus Centaur Cen Centauri
Cepheus The King Cep Cephei
Cetus Sea monster Cet Ceti
Chamaeleon Chameleon Cha Chamaeleontis
Circinus Compass Cir Circini
Columba Dove Col Columbae
Coma Berenices Berenice's hair Com Comae Berenices
Corona Australis Southern crown CrA Coronae Australis
Corona Borealis Northern crown CrB Coronae Borealis
Corvus Crow Crv Corvi
Crater Cup Crt Crateris
Crux Southern cross Cru Crucis
Cygnus Swan Cyg Cygni
Delphinus Dolphin Del Delphini
Dorado Swordfish Dor Doradus
Draco Dragon Dra Draconis
Equuleus Little horse Equ Equulei
Eridanus River Eridanus Eri Eridani
Fornax Furnace For Fornacis
Gemini Twins Gem Geminorum
Grus Crane Gru Gruis
Hercules Hercules Her Herculis
Horologium Clock Hor Horologii
Hydra Female water Snake Hya Hydrae
Hydrus Male water snake Hyi Hydri
Indus Indian Ind Indi
Lacerta Lizard Lac Lacertae
Leo Lion Leo Leonis
Leo Minor Lesser lion Lmi Leonis Minoris
Lepus Hare Lep Leporis
Libra Scales Lib Librae
Lupus Wolf Lup Lupi
Lynx Lynx Lyn Lyncis
Lyra Lyre Lyr Lyrae
Mensa Table Mountain Men Mensae
Microscopium Microscope Mic Microscopii
Monoceros Unicorn Mon Monocerotis
Musca Fly Mus Muscae
Norma Carpenter's square Nor Normae
Octans Octant Oct Octantis
Ophiuchus Serpent bearer Oph Ophiuchi
Orion The hunter Ori Orionis
Pavo Peacock Pav Pavonis
Pegasus The winged horse Peg Pegasi
Perseus The hero Per Persei
Phoenix Phoenix Phe Phoenicis
Pictor Painter’s easel Pic Pictoris
Pisces Fishes Psc Piscium
Piscis Austrinus Southern fish PsA Piscis Austrini
Puppis Stern (of a ship) Pup Puppis
Pyxis Compass Pyx Pyxidis
Reticulum Reticle Ret Reticuli
Sagitta Arrow Sge Sagittae
Sagittarius Archer Sgr Sagittarii
Scorpius Scorpion Sco Scorpii
Sculptor Sculptor Scl Sculptoris
Scutum Shield Sct Scuti
Serpens Serpent Ser Serpentis
Sextans Sextant Sex Sextantis
Taurus Bull Tau Tauri
Telescopium Telescope Tel Telescopii
Triangulum Triangle Tri Trianguli
Triangulum Australe Southern triangle TrA Trianguli Australis
Tucana Toucan Tuc Tucanae
Ursa Major Great bear Uma Ursae Majoris
Ursa Minor Little Bear Umi Ursae Minoris
Vela Sails Vel Velorum
Virgo Virgin Vir Virginis
Volans Flying fish Vol Volantis
Vulpecula Fox Vul Vulpeculae

It is worth noting a few facts about some of these constellations and their names:

  • As noted above, you should generally use the Latin names when referring to constellations. However some English versions have entered common usage such as the Great Bear and Southern Cross although these are very much the exception.
  • The English translations in the above list are from the official IAU website. Several of the constellations represent named mythological characters and these are usually referred to by their Latin name, not the translation. For example Andromeda is invariably referred to as Andromeda, if you call her the chained maiden you will likely get some strange looks. The same applies to Cassiopeia, Cepheus, Hercules, Orion, Pegasus and Perseus.
  • Apus: This is sometimes rendered in English as The Bee. This is incorrect, the error is believed to have been caused, at some point, by a misspelling or translation error since the Latin for bee is Apis.
  • Carina, Puppis and Vela were once the parts of a much larger constellation, Argo Navis, the ship Argo. This was one of Ptolemy’s original 48 constellations but because of its enormous size it was split into these three separate constellations.
  • Cetus: This is often translated as the whale rather than sea monster.
  • Reticulum: A reticle is a set of wires or cross hairs in a telescope eyepiece used for measurement purposes.
  • Serpens: Unlike all the other constellations, Serpens is split into two entirely separate parts, one on either side of Ophiuchus the Serpent Bearer. The westernmost part is referred to as Serpens Caput (the serpent’s head) and the easternmost, Serpens Cauda (the serpent’s tail).
Asterisms

In addition to constellations there are also more informal patterns known as asterisms. Perhaps the most famous of these is the Plough or Big Dipper. Many people think of this as a constellation in its own right. In reality it is only part of the larger constellation of Ursa Major (Figure 2A).

Some asterisms are formed from parts of several constellations rather than just one. For example the so-called Summer Triangle is formed from the brightest stars of Cygnus, Lyra and Aquila. Cygnus itself contains the well-known asterism, The Northern Cross (Figure 2B).

Figure 2A (left) The asterism of the Plough or Big Dipper. Figure 2B (right) The Summer Triangle (red) and the Northern Cross (green). (Stellarium)

Joining the dots

For many centuries the constellations were viewed as pictures of the objects they represented as shown in Figure 3A. Many of the early star atlases and globes were things of great artistic beauty but they did not make it easy to translate the pictures into the star patterns in the sky. 

Figure 3A (left) The constellations represented as mythological figures. Figure 3B (right) The same constellations shown as stick figures. (Stellarium)

Nowadays we create simple stick figures by joining up the brightest stars as in Figure 3B. You may be wondering if there are standard, official representations for these patterns. The answer is no, there are many variations.

In 1952 an American author, H. A. Rey, wrote a book entitled “The Stars: A New Way to See Them”. In this he redrew the star patterns so that they were more representative of what the constellations were meant to represent. These layouts, sometimes called ‘modern layouts’ are quite popular but there are a couple of issues.  Firstly, Rey often used faint stars to create his patterns. In today's light polluted skies these may well not be visible. Secondly, he sometimes goes against the classical representations, for example what is traditionally shown as the head of Cetus the whale or sea monster Rey depicts as its tail.

Naming names

So far we have grouped the stars into constellations but how do we then identify individual stars? Some of the brighter stars have names to identify them so we have Sirius, Rigel, Vega, Deneb, Polaris and so on. However with around three thousand stars visible at any one time in a dark sky this method soon ceases to be practical.

A better way is to identify a star within its parent constellation. This has been done in a number of ways over the centuries.  The German astronomer Johann Bayer who in 1603 published a new star atlas entitled Uranometria first defined the method in use today. To each plotted star he assigned a lower case Greek letter, with alpha being usually, but not always, the brightest star in the constellation, beta the second brightest and so on. He suffixed this with the genitive version of the constellation. So Sirius is Alpha Canis Majoris.

The genitive version is the ‘of’ or possessive form of the name. Alpha Canis Majoris means the alpha star of Canis Major and would normally be abbreviated Alpha CMa. Where there were so many stars in a constellation that the Greek letters ran out he carried on by using the letters of the normal alphabet we use every day. Bear in mind that Bayer’s star atlas was published before the invention of the telescope so he did not have to worry about faint stars below the naked eye limit.

Successive catalogues have added to this scheme, for example by assigning a number to each star. There are special schemes for stars that vary in brightness and so on. Generally speaking for the naked eye Bayer’s scheme is all that is needed.

Below is a table of the lower case Greek alphabet.

Alpha α Iota ι Rho ρ
Beta β Kappa κ Sigma σ
Gamma γ Lambda λ Tau τ
Delta δ Mu μ Upsilon υ
Epsilon ε Nu ν Phi φ
Zeta ζ Xi ξ Chi χ
Eta η Omicron ο Psi ψ
Theta θ Pi π Omega ω

Do you need to learn the Greek alphabet? The answer is it depends on what you want to do with your astronomy. Generally when reading articles about the stars an object will have the Greek character spelt out alongside the actual letter itself. An example would be alpha (α) CMa or α (alpha) CMa. Where it is really useful is in reading star charts, here you will only find the Greek character; quite simply there is often no room for both that and the full spelling. My advice would be to try it and see how you get on; you may find that you pick up the symbols through practice and regular use without having to formally learn them.

In conclusion

The sky we look out upon today can seem very different from that seen by our ancestors when they created the first constellations. Whereas they had clear pristine skies, we are plagued in many areas of the world by man made light pollution. This robs us of the night sky and can often obliterate many of the fainter stars that make up the constellations. The BAA is in the forefront of the fight against unnecessary lighting and you can read about the problems and solutions here.

A future article will discuss how to discover which constellations are visible at any particular time and how to find your way around them.

In the meantime, whatever your skies, go out at night, look up and enjoy what there is to see. Contemplate that in doing so you are reaching out into the depths of space and that the light entering your eyes has been travelling tens, hundreds and in some cases thousands of years to reach you and impact on your retinas.

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David Basey is an amateur astronomer living in semi-rural East Anglia. Primarily a visual observer he has been scanning the skies for nearly fifty years. He is embarrassed to admit that even after nearly half a century he has still not identified all of the faintest constellations visible from where he lives.


 

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Comets 67P and 74P

Image
About this observation
Observer
Damian Peach
Time of observation
24/11/2017 - 23:30
Object
Comets 62P and 76P
Observing location
New Mexico, USA
Equipment
17 inch CDK
FLI camera
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Comets 62P and 74P recently passed very close to each other. Damian's image shows them at closest approach on Nov 24th.  74P is some magnitudes fainter but does show a short dust tail.

Copyright of all images and other observations submitted to the BAA remains with the owner of the work. Reproduction of the work by third-parties is expressly forbidden without the consent of the copyright holder. For more information, please contact the webmaster.

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