Comets are a fascinating subject to study as they are varied in their appearance and are sometimes very unpredictable in their behaviour. They are always in need of observation as they evolve and decay over time. So any observation of a comet may prove to be of value to comet researchers. Of course, do casual observations of comets and admire them, but it is a rewarding pleasure if you take some empirical observations and send them to bodies such as the comet section of the British Astronomical Association and the Society for Popular Astronomy.

What are Comets?

Comets are believed to be the remnants of the primordial cloud of gas, ice and dust that made up the remainder of the solar system. The solar system was born 4.6 billion years ago out of this cloud, as the dust, gases and ices coalesced into the planets and asteroids we see today. Due to geological, mechanical and thermal processes, the material in the planets and asteroids have been changed with regards there composition while the comets have not changed. Therefore, any information we can glean from comets is actually giving us knowledge of the original stuff that the solar system was built from.

Where and When to observe comets

There are about 3,000 known comets and theory has it that there are millions out there. Most of these are far away in the cold dark depths of space, residing in two zones known as the Kuiper Belt and the Oort Cloud. The Kuiper is situated beyond the orbit of Neptune ( 30 au.) out to a distance of 55 au. Comets that have orbital periods of up to 200 years are believed to originate from this belt. Comets that have orbits greater than 200 years and up to tens of thousands of years are believed to originate from the Oort Cloud. The Oort Cloud is much further away at a distance of 50,000 au. Comets remain in these belts for billions of years until gravitational disturbances to these comets cause some of them to be pushed into the inner solar system, where we see them. The planets, especially Jupiter, can alter the orbits that these comet take and give them semi stable orbits that keep them within the inner zone of the solar system and we can follow these year after year as they orbit the Sun. It is the Sun that causes the icy comet to become visible. As the comet gets closer to the Sun, it gets heated up and the ice begins to evaporate, carrying away the volatile materials, along with dust to form a coma and sometimes a classic tail. Not all comets have tails. In fact, the majority of comets appear as faint fuzzy object, not unlike unresolved globular star clusters as seen in a small telescope.

Comets can appear in any part of the sky as they are not restricted to the ecliptic plane that we find the planets. There are now a hand full of dedicated, professional observatories that regularly scan and image the night sky, looking for asteroids and comets, with most new comet discoveries being picked up by computers analyzing these images. But occasionally, there are still new comets being first discovered by amateur astronomers who happen to be looking at the right place and time. One of the areas that is still free from the eyes of robotic telescopes are towards the evening and morning twilight skies. It may be worth scanning these areas with binoculars or wide field telescopes in the hope that a new comet has appeared. See further notes below for searching for new comets.

The positions of known comets are to be found in yearly periodicals such as “Yearbook of Astronomy” edited by Patrick Moore, The British Astronomical Association “Handbook”, Philip's “Stargazing” edited by Heather Couper and Nigel Henbest and popular magazine annuals by Sky and Telescope and Astronomy Now.

There are also many computer planetarium type software that can plot positions of comets for any date and time. Programs such as Guide, Starry Night and Redshift can be updated with the latest comet discoveries and orbit updates.

What do comets look like.

Most comets appear as faint fuzzy objects and the majority do not have the classic tail that most people associate with a comet. Tails are only produced when a comet is reasonably close to the heating influence of the Sun. The image depicted shows the main elements of the classic comet. The coma is the gas and dust cloud that surrounds the actual nucleus of the comet. The nucleus is never directly observed when a comet is active. The coma is usually circular in shape, but can be distorted from a perfect circular shape. Within the coma, some fine spiral or spike structures may be seen, especially at a high telescope magnification. These are dust and gas jets emanating from the actual comet. The material from these jets contribute matter to the comets coma and tail. Extending away from the coma a tail may be seen. Some comets display two tails. One is known as a dust tail and the second in the gas tail (or ion tail). The dust tail is normally white with some hint of yellow. The dust particles within the tail will follow in their own actual orbits that are different from the parent comet and depending on the aspect that the comet is viewed from, the tail may look curved. The gas tail is usually fainter than a dust tail and some hint of blue may be seen. The gas tail is made of charged gas particles that flow away from the comet in the direction of the magnetic field of the solar wind. The gas tail is normally in the form of a straight line.

What to Record in your observation

There are some basic details that you must record if you want your observation to be of scientific value.

Name of Comet (If Known) Examples: 1P / Halley, 2006 M4 SWAN, C/2007 E2 Lovejoy.

Date and Time (Year, Month, Date, Hours, Minutes) e.g. 20071105_20.05 UT.

Note that the time must be Universal Time NOT local time.

Optical equipment used with details: Examples: 80mm F/15 Refractor at 50x (24mm Plössl Eyepiece) , 7x50 Binoculars, Naked Eye.

Sky conditions. Include the “seeing” conditions which relate to the stability of the image. A good scale to use is the Antoniadi Scale. This has a range of I to V, which equates to the following: I - Exceptional steady image. II - Steady image with occasional slight wobbles in the image. III - Image is affected by large wobbles during steadier moments. IV - Image is constantly affected by atmospheric turbulence. V - Very poor image and making a sketch would not be possible. Also include an indication of how clear the sky is. A good scale would be 0 to 9 where 0 would equate to impossible to see through atmosphere and 9 would be a sky where the Milky Way is visible down to the horizon.

The magnitude of the comet. This is determined by comparing the comets coma with a out of focus image of a star. What you should do is first look at the comet coma with the optics in focus. Then bring the optics to an out of focus condition and find a de-focused star that looks similar in size and brightness as the in focus coma. You should find a star that is near the comet and at the same elevation in the sky.

Degree of Condensation. The coma will normally look brighter towards the center and gradually fade towards the outer edges. The amount of change in brightness of the central region to the outer edge is known as degree of condensation, abbreviated DC. Some comet comas have very star like central condensations while other have no definite brighter central regions. There are some comet comas that look like stars with very little fuzziness to its edges. A comet with a DC=0 will have no change in brightness from the center to the outer edge of the coma. DC=9 is assigned to a coma that looks stellar or disc like.

The Diameter of the comet coma can be measured from well executed drawings and compared to good quality star maps or computer planetarium type software. Similarly digital images can be measured as well.

Tail Length again can be measured from an accurate drawing and using scale as derived from the surrounding star field. When you do a sketch, ensure you accurately position some stars in the field sketch to make the measurements. You will also be able to work out the north south orientation and scale from the plotted stars.

Tail Position Angle. If there is a tail to the comet, then its length and Position Angle (P.A.) should be recorded. Note that some comets can have multiple tails.

Name and Address. This will be needed if someone needs some clarification of your observation and even more so if you wish to have a new comet named after you!

Searching for new comets

As previously mentioned, there is still an opportunity for amateur astronomers to search for new comets, even in this day when automated computer controlled telescopes scan the night skies. You should use a large aperture instrument with a low focal ratio. This will allow for a wide field of view and a bright crisp image to find faint fuzzy objects. An ideal instrument would be a mounted pair of 100 to 150mm binoculars that have a magnification of between 20 to 30x. Short focal length telescopes using a wide angled eyepiece would also be suitable. The technique requires you to sweep a systematic, overlapping scan of the sky. When you come across a faint object, take care to see if the object is not a faint group of stars that can sometimes be mistaken for a comet. If it still looks fuzzy, then use a larger instrument (telescope with higher magnification) to see if the object continues to remain a fuzzy object. You may have come across a globular star cluster and this may become obvious if a larger telescope is trained upon the fuzzy. If you still see it as a fuzzy object, then consult a good star atlas, such as Norton's, Uranometria. Computer generated planetarium software are also a good source to identify the object. If you find no such object in the observed position (after noting globular clusters, tight star clusters or galaxies) then make a sketch of the fuzzy along with accurately positioned star within the field of view. And then wait! A comet will betray its presence by the fact that in a couple of hours, its position will have changed. After confirming its movement, then you may need see if the new object is not a previously know or recently discovered comet. Once you're confident that the object is a comet, then notify relevant persons that may follow up and report the object to the Minor Planet Center in Cambridge, USA.

Further reading:

Observing Guide to Comets. Jonathan Shanklin. British Astronomical Association. 2002 Comet Carl Sagan and Ann Druyan Micheal Joseph Ltd. 1985 Great Comets Robert Burnham Cambridge University Press. 2000

Comet Observations by IoMAS Members

Below is a record of observations of members of the Isle of Man Astronomical Society. This part of web site will evolve over time as old and new observations are added. Please send your comet observations to Dave Storey for inclusion in this page.

Comet 17/P Holmes




Comet 46/P Wirtanen

Comet 65P/Gunn

78P /Gehrels

Comet C/2006 W3 Christensen

Comet C/2007 N3 Lulin

Comet C/2008C1 Chen Gao

Comet C/2008 R3 LINEAR