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PRESS INFORMATION SHEET: Comet C/1995 O1 (Hale-Bopp)
Produced at the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge,
[Updated 1997 August 25.]
The discovery of Comet C/1995 O1 (Hale-Bopp) has generated a great number
of inquiries from the news media and the general public. This information
sheet 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 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 astronomers 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
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.
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
Early observations suggested that the comet 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 is not well placed for observation for northern-hemisphere observers
-- it has moved south and is observable chiefly from southerly latitudes.
Though the comet is still visible to the naked eye, a pair of binoculars
is advised to confirm the large, diffuse/condensed nature of the comet.
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 (which, incidentally will be a plateau
lasting several weeks into the end of April), the comet has become 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).
Finding ephemerides (giving the object's position on the sky as a function
of date) are published in the International
Comet Quarterly, the Minor
Planet Circulars and the IAU
Circulars; an ephemeris is also available on the World Wide Web at
5a) Can members of the general public see the comet now?
Yes, but chiefly from the southern hemisphere. The best way for an inexperienced
observer to see this comet now is 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. For example, the Center for Astrophysics in Cambridge, MA,
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) are/will receiving/receive more media attention
than other comets (and the public will possibly hear more about these comets
than they did about Halley's comet in 1909-1910 or 1985-1986). 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 at this time.
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.
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) is 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. The coma size has translated into well over 1 million km
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 may decrease or increase over the coming weeks and months.
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 been wondering if the large amount
of coma activity (and corresponding total brightness) might mean a larger-than-usual
comet nucleus for this comet. But calculations by comet scientist Zdenek
Sekanina of the NASA's Jet Propulsion Laboratory suggest that the current
activity need not require a nucleus larger than 10-15 km in size. The comet's
tail during February has been on the order of 15-30 million miles long
(or longer). Some professional astronomers have estimated that the nucleus
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). Comet C/1996 B2 (Hyakutake)
was the third new comet discovery of this year.
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 comet 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
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 email@example.com
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
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://cfa-www.harvard.edu/cfa/ps/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
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 firstname.lastname@example.org.
Written by D. W. E. Green, with input from B. G. Marsden, G. V. Williams,
J. Hoskins, J. Corliss, and J. Cornell. [updated 1997 August 25]
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