IAU Central Bureau for Astronomical Telegrams

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PRESS INFORMATION SHEET: Comet C/1995 O1 (Hale-Bopp)

Produced at the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, Massachusetts, U.S.A.

[Updated 2000 October 20.]

The discovery of Comet C/1995 O1 (Hale-Bopp) generated a great number of inquiries from the news media and the general public. This information sheet (which has been available at this website since shortly after the comet's discovery, and which has been updated numerous times) addresses the most commonly asked questions. (Please see the Appendix at the end for definitions of astronomical terms in this discussion.)

1) What's this I hear about a bright comet that was visible from late 1996 and into early 1997?

A comet was discovered in July 1995 that became bright enough in 1997 to be easily seen with the naked eye, even from urban sites, and remained easily visible to northern-hemisphere observers through April/May 1997. Comet C/1995 O1 (Hale-Bopp) orbits the sun, with closest approach (perihelion) having occurred on 1997 April 1. Ever since discovery, the comet has shown high activity and has been intrinsically very bright at relatively large distances from the sun; in fact, it is evidently the brightest comet *intrinsically* (NOT *apparently*!) whose orbit passes inside the earth's orbit in over 400 years (since the great comet of 1577). Its high rate of activity, and the fact that it was a naked-eye object for many months, is why much excitement has been present in the astronomical community.

C/1995 O1 (Hale-Bopp) became brighter than was C/1996 B2 (Hyakutake) in late March 1996. Comet C/1995 O1 was probably the best-observed comet in the history of mankind (not because its *apparent* brightness was so great, but because it was a relatively easy naked-eye object in the evening sky for northern-hemisphere observers for a solid two months, with plenty of media attention to drive people outdoors in the early evening to have a look).

2) What is a comet?

It is a small body in our solar system that orbits the sun much as do the earth and other planets. It has a "nucleus", or solid body, that is usually around 1-10 km across and is supposedly a "dirty snowball" consisting of ices and dust and rock. When far from the sun in the outer reaches of the solar system, there is very little activity coming off such a nucleus. However, when the comet nucleus gets closer to the sun, the sun's radiation warms the nucleus, causing the ices to sublimate (or "steam") outwards from the nucleus from various vents, carrying along various atoms and molecules that constitute different ices and dust and rock in the original nucleus. This venting outwards creates both the coma (or atmosphere) surrounding the nucleus --- out to thousands, hundreds of thousands, and sometimes millions of kilometers from the nucleus itself --- and also the tail of material that generally streams in the anti-sunward direction from the nucleus. Once this venting activity "turns on", the true nucleus is almost invariably invisible from Earth, as intense material in the inner coma then tends to mask the tiny nucleus.

Note that comets are NOT the same as meteors (or so-called "shooting stars"; see definition in Appendix below); while meteors typically streak through our atmosphere in a second or two and are sometimes much brighter than even a "bright" comet (when they are called "fireballs"), comets are much further away than the moon and move slowly with respect to the background stars from night to night, rising and setting each day just as do the sun, the moon, the planets, and the stars.

3) How was this comet discovered?

On 1995 July 23, two observers first each spotted the comet while looking at a cluster of stars known as Messier 70 (M70) in the constellation Sagittarius. Within minutes of each other, Alan Hale in New Mexico and Thomas Bopp in Arizona independently recognized a fuzzy object near M70 that was fainter than the star cluster itself, and both reported it to the worldwide clearinghouse for comet discoveries in Cambridge, Massachusetts, the Central Bureau for Astronomical Telegrams (CBAT). The Central Bureau, which is operated by the Smithsonian Astrophysical Observatory for the International Astronomical Union (IAU) then issued an IAU Circular announcing the discovery (IAUC 6187, 1995 July 23), as is the practice for new comet, nova, and supernova discoveries. [See question 11, below, for further information on IAU Circulars.]

3a) Who are the discoverers?

Alan Hale has a Ph.D. in astronomy from New Mexico State University and resides in Cloudcroft, NM. He is one of the world's most active visual observers of comets and has seen nearly 200 different comets over many years. Thomas Bopp, who lives in a suburb of Phoenix, AZ, is an amateur astronomer who was observing at a "star party" with other amateur astronomers in the desert about 90 miles south of his home.

3b) What is the proper name of this comet, and how did it get that name?

The proper designation as used by the International Astronomical Union on its IAU Circulars, and the formal usage of professional scientists in the refereed astronomical literature, is "Comet C/1995 O1 (Hale-Bopp)", though many people have referred to it popularly as "Comet Hale-Bopp". Comets are normally named for their discoverers, and this is done by consultation between the CBAT and a special committee of nine astronomers within the IAU. The designation "C/1995 O1" means that this was the first comet found in the second halfmonth of July (letter O plus number 1) in the year 1995; halfmonths are given as letters, with "A" covering Jan. 1-15, "B" covering Jan. 16-31, "C" covering Feb. 1-15, etc. ("I" being omitted and "Z" not needed); the "C/" indicates that this is a long-period comet (that is, one with a solar-orbiting period of more than 200 years). Similar to the system used for designating asteroids, this system was brought into use for comets on 1995 Jan. 1.

Tip: Whenever one comes across a reference to comet C/1995 O1 (Hale-Bopp) as "the Hale-Bopp comet", one can be pretty sure that the information attached to that phrase is compiled by somebody unfamiliar with comets and thus prone to error and misinformation. (People familiar with comets will either use their proper designations or refer to them as "comet Hale-Bopp", "Halley's Comet", or simply "Hale-Bopp", but never "the Halley comet" or "the Hale-Bopp comet"). [This might be analagous to one saying "game ballfoot" iustead of "football game", or "office post" instead of "post office", or "Church Catholic Roman" instead of "Roman Catholic Church" --- not strictly incorrect, perhaps, but obviously odd to those familiar with those subjects.]

4) How far away is the comet now, and how close will it come to Earth?

Comet C/1995 O1 (Hale-Bopp) was 7.16 Astronomical Units (the equivalent of 1.07 billion kilometers, or 666 million miles) from the sun at discovery, and 6.20 AU (or 930 million km, or 577 million miles) from the earth. In late April 1996, the comet was about 4.5 AU from both the earth and the sun, and in September 1996 the comet was about 3 AU from both the earth and the sun. On 1997 January 7, the comet was 2.47 AU from the earth and 1.68 AU from the sun; on February 1, the comet was 2.00 AU from the earth and 1.37 AU from the sun. The comet came no closer to us than about 1.315 AU (197 million km, or 122 million miles), and that happened around 1997 March 22. This means that the comet came no closer to the earth than 1.3 times the sun-earth distance. [Contrast this with the minimum earth-comet distance of 0.10 AU, or 9.3 million miles, for C/1996 B2 (Hyakutake) on 1996 March 25. But because C/1995 O1 is intrinsically much brighter than C/1996 B2, it was brighter at 1.3 AU from the earth than was C/1996 B2 at 0.10 AU.] Comet C/1995 O1 (Hale-Bopp) reached perihelion (closest approach to the sun) on 1997 April 1 at around 3h30m Greenwich Mean Time (which corresponds to the following times on March 31 in the United States: 10:30 p.m. EST; 9:30 p.m. CST; 7:30 p.m. PST) at a distance of 0.91 AU (136 million km, or 85 million miles) from the sun. (For comparison, the comet's aphelion distance, at which point it is furthest from the sun in its orbit, is around 372 AU.) The comet's velocity with respect to the sun in September 1996 (when it was about 3 AU from the sun) was about 2.09 million kilometers per day (1.3 million miles per day), or about 24 km per second (15 miles/sec). When the comet was closest to the sun (0.91 AU from the sun) in early April 1997, it was travelling at about 44 km/sec (27 miles/sec).

4a) How do we know these distances and where the comet will be at any given time?

Orbit computation became possible with the publication in 1687 of Sir Isaac Newton's Principia, in which he produced the classical laws of physical motion for objects in the solar system. Over the centuries, numerical techniques (and now powerful electronic computers) have greatly improved the manner in which orbits of solar-system objects are calculated. There are six so-called orbital elements that are unique (like a fingerprint) for each solar-system object orbiting the sun, defining such quantities as the object's time of perihelion passage (closest point to the sun), the minimum (perihelion) distance from the sun, and the orientation of the orbit in space. The orbital elements, which are refined by additional observations, can then be used to predict an object's future course (and astronomers produce so-called ephemerides that list the object's position on the sky, and predicted brightness and distances from the earth and sun, by date). Our web pages contain both orbital elements and ephemerides for all currently observable comets.

Early observations suggested that comet C/1995 O1 was quite far away from us because of very small parallax seen in near-simultaneous observations made by observers in Australia and Japan. As many positional (or astrometric) observations poured in to our offices from observers around the world, we were able to compute the comet's path, or orbit, about the sun. The orbit of C/1995 O1 (Hale-Bopp) is almost perpendicular to the earth's own orbital plane and takes the comet out quite far from the sun. A single, apparent image of the comet was found by astronomer Robert H. McNaught of the Anglo-Australian Observatory from a wide-field photographic plate taken in late April 1993 (when the comet was about 13 AU from both the sun and earth). This observation strengthened the early orbital calculations by greatly extending the arc of observation. The comet has an orbital period of a few thousand years and extends out to some ten times the distance of Neptune at its furthest point. Recent orbital calculations indicate that comet C/1995 O1 (Hale-Bopp) last passed through the inner solar system about 4210 years ago (or around 2214 BC), and that it will return again in about 2380 years from now (or around 4377 AD) -- its orbital period being greatly shortened at its current "apparition" due to gravitational perturbations by the major planets. There is an uncertainty of a few years in these orbital periods.

We do not know where comet C/1995 O1 (Hale-Bopp) originated, though the most widely accepted current theories have comets forming in the region of the outermost major planets (Uranus and Neptune) some billions of years ago and being scattered into both much larger and much larger orbits over time by gravitational interactions with the major planets (and perhaps occasional stars passing by).

5) Where in the sky is the comet now?

The comet moved southward and is observable now chiefly from southerly latitudes. In 2000, the comet has still be faintly visible in large amateur telescopes, as it continues to recede from the sun and earth and fades.

The comet was observable in the morning sky into late March or early April 1997 (becoming lower in the northeast sky as each day passes), and became better visible in the evening sky during April (and still from the northern hemisphere). Beginning around the middle of March 1997, as comet C/1995 O1 neared peak brightness (a plateau lasting several weeks into the end of April), the comet became better placed for evening viewing. This comet was not a southern-hemisphere object in 1997 until about June (after which time it will now be chiefly observable from the earth's southern hemisphere and not the northern hemisphere).

At discovery, and for a few months afterwards, the comet was moving slowly with respect to the background stars from night to night in the evening sky in Sagittarius. It was visible in small amateur telescopes (with mirrors or lenses 4-6 inches across) in the second half of 1995 at visual magnitude 10-11, before becoming lost in the sun's glare during December 1995 and January 1996; comet C/1995 O1 (Hale-Bopp) emerged from its proximity to the sun in February, and was visible all night long during mid-1996 as an object of total visual magnitude about 6, and the first naked-eye detection was reported by experienced observer S. J. O'Meara at Volcano, Hawaii, on May 18. The comet then became a relatively easy binocular object, as it continued to move slowly northward through Sagittarius during April, May, and June 1996, and into Scutum during July (when it passed opposition, the point opposite the sun in the sky, meaning that it rose around the time it gets dark in the evening was up all night). It gradually faded from binocular visibility during 1998, and is now only visible with relatively large telescopes. Finding ephemerides (giving the object's position on the sky as a function of date) have been published in the annual ICQ Comet Handbook, the Minor Planet Circulars and the IAU Circulars; an ephemeris is also available on the World Wide Web at http://www.cfa.harvard.edu/iau/Ephemerides/Comets/1995O1.html.

5a) Can members of the general public see the comet now?

Not easily, as explained above, and chiefly from the southern hemisphere. One would probably have to contact a local astronomy club, planetarium, or college observatory to find out about upcoming star parties or public observatory nights in which the comet will be shown to interested members of the public -- noting, however, that nothing has faint as C/1995 O1 now is should be considered visible from urban and suburban sites due to light pollution. But other fairly bright (meaning binocular-visibility) comets are present in our night sky usually for some weeks each year, and such objects can frequently be observed at public-open nights at observatories. [For example, the Center for Astrophysics in Cambridge, MA, holds monthly observatory nights for the public on the third Thursday of the month throughout the year; call 617-495-7461 for additional information; the CfA's Whipple Observatory near Tucson, Arizona, holds quarterly star parties (call 520-670-5707 for information there).]

6) So how bright did this comet become?

Comet C/1995 O1 (Hale-Bopp) reached peak brightness in March and April 1997 around total visual magnitude -1, which is rare for comets. This refers to the brightness of the comet's coma (or head or atmosphere). C/1995 O1 is not the brightest comet of the 20th century, though only a handful of comets have been brighter. Comet C/1995 O1 appears to be rather large as comets go, and astronomers have reported that its production rate has been running many times greater than that of Halley's comet at the same distance from the sun. This allowed the comet to give us the best naked-eye comet performance in 20 years [when comet C/1975 V1 (West), in the morning skies of March 1976, was as bright as Sirius (the brightest star)]. [Some observers who saw comet C/1996 B2 in a dark sky will argue that that comet put on a better naked-eye performance. Comet C/1996 B2 (Hyakutake) in March and April 1996 was a very nice comet to observers in a very dark sky, with a naked-eye tail extending a third or more of the way across the sky, but most urban viewers would not call that a "spectacular" comet because the tail had such low surface brightness that it appeared merely as a fuzzball.]

One concern of astronomers regarding earlier brightness predictions for this comet had involved the fact that no long-period comet with a perihelion distance near or inside the earth's orbit had been observed so far before perihelion passage; this means that we did not have previous such examples on which to base predictions. Also, comet C/1995 O1 exhibited much dust, carbon-monoxide (CO), and other gaseous emission during the first year of observation following discovery --- considerably more emission than is usually seen in comets at such large distances from the sun; most bright comets are thought to be fueled mostly by water-ice emissions, and theory suggests that in the vacuum of space, significant water-ice sublimation did not begin until the comet is within 3 AU of the sun (which did not occur until late September or October 1996). Emission from such molecules as the hydroxl radical (OH), diatomic carbon, and cyanogen (CN) were detected with the comet well over 4 AU from the sun. The unusual early strong detection of these emissions gave strong support to a good visual performance of this comet in early 1997. If this is a comet that is fueled more by CO sublimation (or by some other parent ices) than water sublimation, the peak brightness in 1997 might have been brighter or fainter than the projections, but this connection will be sorted out later. Just as important as the brightness figure for the general public's ability to see (or not to see) this comet as an impressive naked-eye comet in early 1997 was the development of a dust tail [see question 7, below]. Comet C/1995 O1 developed a very interesting pair of tails several degrees long, but they did not develop the high surface brightness that was hoped for.

Numerous comets have been observed completely to fall apart on approach to perihelion, as many comets are thought to have nuclei that are very loosely held together. This comet never appeared likely to fall apart. There were unsubstantiated reports of comet C/1995 O1 (Hale-Bopp) fragmenting (splitting); some observers have likely seen clumps of dust and/or gas coming off of the comet's nucleus, but there has been no confirmed breaking/fragmenting/splitting of the comet's nucleus.

6a) Was comet C/1995 O1 (Hale-Bopp) "the comet of the century"?

Probably not, either from a scientific standpoint or from a popular standpoint. The "comet of the century" in scientific terms was Halley's comet, the only comet to have been visited at close range by artificial imaging spacecraft. The "comet of the century" in popular terms would be up for debate, for many reasons, not the least of which is the fact that the world's communications/media industry/infrastructure is so much more advanced (and thus set up for rapid dissemination of results and images of any observed comets) in the 1990s than ever before; for this reason, comets such as C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp) have received more media attention than other comets (and the public possibly heard more about these comets than they did about Halley's comet in 1909-1910 or 1985-1986 simply because of the modern-day size and extent of the electronic and print media industry). What criteria make for a "comet of the century" must be first outlined, and depending on the specific criteria, one will probably get different answers. Numerous other comets in the 20th century certainly have been brighter than C/1995 O1 became; numerous other comets have had longer tail lengths than the maximum tail length of C/1995 O1; and while we've had a couple of years to talk about comet C/1995 O1 (Hale-Bopp), we've talked all century about Halley's comet and its last/next return(s). But few comets are visible from the northern hemisphere and remain as bright as (or brighter than) magnitude 0 or +1 for 2-3 months, as occurred for C/1995 O1 during March-May 1997. Because of this expected duration of significant brightness and because of widespread media attention, it is possible that a larger number of people on the planet saw comet C/1995 O1 (Hale-Bopp) than any other single comet in history. So the answer to this question is necessarily uncertain.

6b) What about that 'infamous' comet Kohoutek in 1973-1974?

Comet C/1973 E1 (Kohoutek) earned a bad reputation by "not living up to" the media hype, including stories about it being possibly the 'Comet of the Century'. [Some people said the same thing about 1P/Halley in 1985-86 and C/1996 B2 (Hyakutake) in March 1996.] It turns out that C/1973 E1 (Kohoutek) had a very stable light curve; what we did not know then was that light curves for comets that are thought to be entering the inner solar system for the first time (as apparently did C/1973 E1) very often have much slower rises in brightness than do comets that have made many journeys through the inner solar system. By comparison, C/1995 O1 (Hale-Bopp) has evidently made numerous trips through the inner solar system, every few thousand years, and there was reason to think that it would rise more rapidly in brightness than did Kohoutek. But other comets have very discontinuous light curves that make them totally unpredictable, such as comet C/1989 X1 (Austin), which first rose steeply in brightness and then leveled off to finish several magnitudes fainter than expected; and numerous comets such as the recent C/1999 S4 (LINEAR) break apart and fade from view (as C/1999 S4 did in July and August 2000).

7) Did this comet have a nice tail?

Not all comets have tails. There are two types of tails --- gas (or ion) tails and dust tails. Gas tails tend to be more common in comets, but they are also usually fainter than dust tails to the naked eye; this is because gas tails emit light by fluorescence, in which gas atoms emitted from the comet's nucleus interact with solar-wind radiation, and they re-transmit energy received from solar radiation at different wavelengths. This fluoresced light in comet tails is very blue, which is difficult for the human eye to perceive. Dust tails tend to become prominent in comets that travel inside the earth's orbit (i.e., less than 1 AU from the sun), in regions where the warming solar radiation more strongly interacts with ice in the comet's nucleus, causing much overall coma and tail activity.

Most of the so-called 'bright' comets of this past century displayed prominent naked-eye dust tails. Comet C/1996 B2 (Hyakutake) in March 1996 was an exception; its long, faintish gas tail was readily visible because that comet passed only 0.10 AU from the earth on March 25. One potential problem with C/1995 O1 (Hale-Bopp) was that its perihelion distance is much further out than most of the other "spectacular" comets of the 20th century (perihelion distance = 0.91 AU). The Great Comet of 1811 had an even greater perihelion distance (just outside the earth's orbit), but still displayed a tail at least 25 degrees long (or about a quarter of the way from the horizon to the zenith in the night sky). An intrinsically fainter comet, C/1983 H1 (IRAS-Araki-Alcock), became brighter than the stars of the Big Dipper in May 1983 but showed no dust tail, and thus appeared merely as a large fuzzy ball in the sky. We do know that comet C/1995 O1 is quite dusty as comets go, and its tail had higher surface brightness in 1997 than did the tail of C/1996 B2 in March 1996, being visible even from the largest cities on earth in March and April 1997. The tail of C/1995 O1 was tens of millions of miles long in space, corresponding to 10 or more degrees long to the naked eye in very-dark-sky conditions.

8) Do we know how large this comet is?

No, not really -- at least not in terms of the nucleus. Note that one must distinguish between the size of the nucleus (which has not yet been directly visible, due to significant "contamination" of light by the bright inner-coma material), the size of the coma or atmosphere surrounding the nucleus, and the size of the comet's tails. The coma size translated into well over 1 million km already in August 1995, though this size varies over time. This is very large for a comet this far from the sun, but we know that there is activity from sublimated carbon-monoxide (CO) ices (and probably from dust, as well). The activity varies, generally increasing (but sometimes decreasing) as a comet approaches the sun. The source of this activity is actually a much tinier nucleus, or solid, dirty snowball. Most comets have nucleus sizes around 1-10 km; comet 1P/Halley had an oblong nucleus of size 8x15 km. Because of the dense shroud of coma material around the nucleus, we cannot tell the size of the nucleus itself while the coma is active (without a close rendezvous by an artificial spacecraft, as with 1P/Halley). Astronomers have assumed that the large amount of coma activity (and corresponding total brightness) meant a larger-than-usual comet nucleus for comet C/1995 O1. But calculations by comet scientist Zdenek Sekanina of the NASA's Jet Propulsion Laboratory suggest that the activity need not require a nucleus larger than 10-15 km in size. The comet's tail length was tens of millions of miles long when the comet was nearest the sun. Some professional astronomers have estimated that the nucleus of C/1995 O1 has a diameter of 40 +/- 10 km, but it is possible that we will never know the true size of this comet's nucleus.

9) How frequently are comets discovered?

During 1990-1994, an average of about 12 comets per year were discovered (plus about one rediscovery per year of a "long-lost" short-period comet), with roughly four discovered by amateur astronomers. However, at the end of 1994, two major professional search programs for comets ceased at Palomar Mountain in southern California, and these programs had discovered four or more comets per year over the past 10-12 years. In 1995 there were five discoveries of previously-unknown comets, plus one discovery of a comet that had been lost for 150 years (122P/de Vico). In 1998, comet discoveries dramatically increased as a result of increasing coverage in scanning the night sky each month with dedicated, sensitive CCD cameras by programs in the southwestern United States such as LINEAR and LONEOS (which added to already-existing programs such as Spacewatch). Since 1998, the average number of new comet discoveries has been more than 30 per year (not counting hundreds of recently detected faint apparent comets close to the sun detected only by a spaceborn coronograph named SOHO, and not seen from the ground due to the sun's glare).

10) How frequently do 'spectacular' comets become visible?

It depends on your definition of "spectacular", but the range is roughly every 20 years or so (or a couple of times in a lifetime), especially if one defines "spectacular" as being as bright as the brightest planets or brighter. The increase in light pollution will make any comet such as C/1995 O1 (Hale-Bopp) harder to see for many people, regardless of its brightness. This, combined with a high standard for "spectacular" activities, could detract from public perception of this comet.

Be wary, then, that many members of the general public --- who are used to fireworks being spectacular (where fireworks are typically between the moon and sun in brightness) --- may not find anything fainter than a crescent moon (mag -8 or so) to be spectacular! Realize that there is a broad spectrum of listeners and readers out there! Light pollution is much bigger today than 20 or 30 years ago, and those stuck in a large city are perhaps unlikely to be impressed.

11) What are the IAU Circulars?

The Circulars are a publication of the Central Bureau for Astronomical Telegrams of the International Astronomical Union. The IAUCs are available both in paper form (by postal mail) and in electronic form via the CBAT Computer Service and via e-mail. IAUCs are the original source for discovery information regarding all new comets, novae, and supernovae. Newspapers and magazines, as well as libraries and professional and amateurs astronomers, subscribe to these useful astronomical news circulars. For subscription information either check out the on-line subscription information, send e-mail to iausubs@cfa.harvard.edu or send postal mail to:
     Central Bureau for Astronomical Telegrams
     Smithsonian Astrophysical Observatory
     60 Garden St.
     Cambridge, MA  02138; U.S.A.
And check out the Central Bureau's World Wide Web page with useful information at the following URL:


APPENDIX. Definitions of commonly-used terms. A larger glossary of comet terms is now on-line.

  • Arc minutes. There are 60 minutes of arc in 1 degree. In the sky, with an unobstructed horizon (as on the ocean), one can see about 180 degrees of sky at once, and there are 90 degrees from the true horizon to the zenith. The full moon is about 30 arc minutes across, or half a degree.
  • Astrometry. The careful, precise measurement of astronomical objects, usually made with respect to standard catalogues of star positions.
  • Astronomical Unit (AU). Approximately equal to the mean earth-sun distance, which is about 150,000,000 km or 93,000,000 miles.
  • AU. see Astronomical Unit.
  • Coma. A comet's atmosphere surrounding its nucleus. The coma is rather tenuous (except very close to the nucleus), and stars can be occasionally easily seen through it, shining from behind.
  • km. kilometer = 0.6 mile.
  • Light curves. Literally, a plot of a comet's brightness as a function of time. A comet never follows a smooth light curve in a strict sense, because there are many night-to-night and week-to-week fluctuations (sometimes on scales of several magnitudes); however, in general, the average brightness of most comets can be fairly well represented over intervals of months. Comet's light curves (or brightnesses) are complicated by the fact that, while they vary according to distance from the earth by the standard inverse-square law of physics (1 divided by the distance squared), they vary also in brightness as a function of distance from the sun that is usually an inverse-third-power, an inverse-fourth-power, or even an inverse-sixth-power. Long-period comets such as C/1995 O1 (Hale-Bopp) usually vary according to an inverse third or fourth power, but the exact power is unknown in advance of each comet's apparition (and astronomers usually assume an inverse-fourth-power law as an average among many comets).
  • Light pollution. The emission of stray light or glare from lighting fixtures in manners that counter the purpose of the light (which is to light what is below); also known as the waste of money and energy in the form of electric light, usually meant in the form of outdoor night lighting. Such light trespass causes severe safety problems for motorists, pedestrians, and cyclists at night from lighting that shines onto streets and highways and sidewalks from poorly-designed or poorly-mounted lighting. Such glare also imposes on privacy, by shining brightly into bedroom windows at night and into backyards where adults and children are trying to observe the night sky. While most people have accepted such bad, glare lighting without question and assumed that nothing could be done about it, dedicated groups of volunteers around the world are now showing that effective laws and guidelines can be instated at the local and regional levels of government (and in planning and engineering offices), which mean that proper outdoor night lighting can be a norm so that everybody benefits --- auto drivers, sleeping residents, government budgets, and skygazers alike. Laws mandating full-cutoff light fixtures are already in place in states such as Maine and Connecticut and are pending elsewhere. For more information on the Web, see URL http://www.cfa.harvard.edu/nelpag/nelpag.html .
  • Magnitude. The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, Sirius, shines at visual magnitude -1.5. Jupiter can get about as bright as visual magnitude -3 and Venus as bright as -4. The full moon is near magnitude -13, and the sun near mag -26. Comet C/1996 B2 (Hyakutake) reached magnitude about 0 in late March 1996. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness. In the case of comets, we speak of a magnitude that is "integrated" over an observed coma diameter of several arc minutes; thus, a 7th-magnitude comet is much harder to see than a 7th-magnitude star -- the latter having all its light in a pinpoint, and the former having the same amount of light spread out over a large area (imagine defocussing a 7th-magnitude star to the size of a diffuse comet). Typically, however, when comets become very bright, their apparent coma sizes shrink so that the majority of visible light is in a small, intense core of the comet's head (and the comet may appear starlike with a tail emanating from the comet's head).
  • Meteors. Small rocky and/or icy particles that are swept up by the earth in its orbit about the sun. Also called "shooting stars", they travel across the sky in a very short time, from less than a second to several seconds, and they do so because they are only a matter of tens of miles above the surface of the earth. Meteor showers are generally thought to be produced by the debris left by comets as the latter orbit the sun. (Comets, on the other hand, are not in our atmosphere but are much further away than is our own Moon; therefore, comets do not "streak" across the sky as do meteors -- a common misconception among the general public.)
  • Orbit. The path of one object about another (used here for an object orbiting the sun).
  • Parallax. the apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object (as from two different observing sites on earth).
  • Perihelion. The point where (and when) an object orbiting the sun is closest to the sun.
  • Sublimation. The change of a solid (such as ice) directly into a gaseous state (bypassing the liquid state). This happens in the vacuum of space with comets, as the heating effects of solar radiation cause ices in comets to "steam off" as gasses into space. The ice molecules present in the nucleus actually break up (or dissociate) into smaller atoms and molecules after leaving the nucleus in gas form.
  • Tail. see "Comet."
  • Zenith. The point directly overhead in the sky.

Contact: Daniel W. E. Green (Associate Director, Central Bureau for Astronomical Telegrams). E-mail dgreen@cfa.harvard.edu. Telephone 617-495-7440.

Written by D. W. E. Green, with input from B. G. Marsden, G. V. Williams, J. Hoskins, J. Corliss, and J. Cornell. [updated 2000 October 20]

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