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    Re: Comet ISON
    From: Frank Reed
    Date: 2013 Dec 1, 12:32 -0800

    Bill, yep. Antares! And it makes a nice comparison for the comet's total magnitude just before it reaches perihelion (already fading to a level fainter than Antares at that point and thus clearly dying).

    Dave, nice job with the astrometry.net check (though a teeny bit of overkill)! And yes, that's good old M4 just to the right of Antares, easy to see with binoculars under a dark sky. If you do a google search for "globular cluster near ", it will fill in 'Antares' since this is apparently a common question. Here's a nice article with a very good finder chart:

    Lu, you wrote:
    "Local weather caster had the best I've heard: "comets and cats are alike: they both have tails and do whatever they please""

    Yep. He borrowed that, of course. That cat tale was running around the Internet on Thursday. Continuing the feline meme, one commentator called it "Schrödinger's comet" since, like the cat in the QM thought experiment, Comet ISON seemed to be both dead and alive depending on when it was observed. It turned out the "resurrection" was just an illusion.

    Lu, you asked:
    "Why, after passing the sun, does ISON's tail seem to go out perpendicular to the ISON's track rather than streaming directly away from the Sun as we're all taught in Junior HS science?"

    Short answer: "dust tail".
    Longer answer: first, there's no actual "comet" left after perihelion. You're looking at a rapidly expanding dust cloud. The comet melted down completely leaving this cloud of fine debris. The trajectories of dust particles from comets are determined by several factors: the times that they were released from the comet (most of the particles in the remnant cloud are believed to have been released within a couple of hours approaching perihelion), the relative velocities (both speeds and directions) they had as they were ejected from the comet, the Sun's gravity (which is still the dominant force for small solid objects even among all those impressive coronal streamers), and, if the particles are small enough, light pressure. Note that dust particles are NOT significantly affected by the solar wind, so any dust tail of a comet will not point radially away from the Sun. Speaking of what you learned in science classes, if you search out your memory a bit, you may recall that many comets have two tails: one points always directly away from the Sun. This is the "ion tail". It is strongly affected by the solar wind and the associated magnetic field environment which drives it directly away from the Sun. Many comets have a second tail. This is the dust tail which is usually curved and points in a direction between the orbital path and the radial ion tail. Since the dust has a very big range of sizes and since small particles are more affected by light pressure, the dust tail usually ends up fan-shaped. Finally, don't forget perspective. You're looking at a three-dimensional clouds tilted towards us. The radial ion tail normally "projects" to a radial tail on the sky, but the fan shape of the dust tail can even make it appear to point towards the Sun when projected onto the celestial sphere as seen by geocentric observers.

    Hooking back to navigation again, it's interesting to consider the angular rate of motion of the comet relative to the Sun. Could we use it as a "natural chronometer" as lunars were used in the late 18th/early 19th centuries? Its actual speed relative to the Sun was around 300km/sec. That's 0.1% of the speed of light, so it's really zipping along. But even at that maximum speed and even assuming the best possible case geometrically, this is only 0.4 minutes of arc per minute of time while the Moon's motion across the celestial sphere is 0.5 minutes of arc per minute of time. Even for an object with perihelion barely above the Sun's photosphere, that high actual speed is so diminished in angular terms by distance, that it still does not match the angular motion of the Moon across the sky. On the other hand, if this comet had passed the Earth at the Moon's distance as it was falling (almost on a straight line in) towards the Sun, then its angular speed would have been roughly 40 times higher than the Moon's angular velocity across the sky.


    PS: A telegram for you, sir:

    "Electronic Telegram No. 3731
    Central Bureau for Astronomical Telegrams
    CBAT Director: Daniel W. E. Green; Hoffman Lab 209; Harvard University;
    20 Oxford St.; Cambridge, MA 02138; U.S.A.
    Prepared using the Tamkin Foundation Computer Network

    The comet's nucleus apparently disrupted near perihelion, with the
    comet's head fading from perhaps a peak brightness of visual mag -2 some
    hours before perihelion to well below mag +1 before perihelion. M.
    Knight, Lowell Observatory, finds that the comet peaked around visual
    mag -2.0 around Nov. 28.1 UT, adding that the brightest feature in the
    coma of the comet faded steadily after perihelion from about mag 3.1 in
    a 95"-radius aperture when the comet first appeared from behind the SOHO
    coronagraph occulting disk on Nov. 28.92 to about mag 6.5 on Nov.
    29.98. K. Battams, Naval Research Laboratory, writes that, based on the
    most recent LASCO C3 images (Nov. 30.912 UT), there is no visible
    nucleus or central condensation; what remains is very diffuse, largely
    transparent to background stars, and fading; it appears that basically a
    cloud of dust remains from the nucleus. S. Nakano, Sumoto, Japan,
    writes that he measured the comet's total magnitude in a 27' photometric
    aperture from the SOHO C3 camera images to be as follows: Nov. 29.383,
    0.5; 29.755, 1.4; 30.013, 2.0; 30.496, 3.0; 30.883, 5.4.

    Z. Sekanina, Jet Propulsion Laboratory, reports that, from the
    position of the northeastern boundary of the comet's fan-shaped tail in
    three images taken with the C3 coronagraph onboard the SOHO spacecraft
    between 0.7 and 1.9 days after perihelion (Nov. 29.46 to 30.66 UT), he
    finds that the comet's production of dust terminated about 3 hours
    before perihelion. Although this result is preliminary, it is unlikely
    to be significantly in error, because the position angles of a
    perihelion emission are off in the three images by 14-22 deg, and those
    of post-perihelion emissions still more. The peak radiation-pressure
    accelerations derived from the tail boundary's angular lengths
    (estimated at 1.8-2.5 deg) are about 0.1-0.2 the solar gravitational
    acceleration, implying the presence of micron-sized particles. The
    estimated time of terminated activity is consistent with the absence of
    any feature that could be interpreted as a condensation around an active
    nucleus in the 20 or so images taken with the C2 coronagraph on Nov.
    28.8-29.0 UT (0.8 to 5.4 hr after perihelion) and with the appearance of
    a very sharp tip (replacing a rounded head) at the comet's sunward end
    in the C2 images starting about 4 hr before perihelion and continuing
    until its disappearance behind the occulting disc around Nov. 28.74 UT
    (or some 50 minutes before perihelion). The time of terminated activity
    is here interpreted as the end of nuclear fragmentation, a process that
    is likely to have begun shortly before a sudden surge of brightness that
    peaked nearly 12 hr prior to perihelion. Fine dust particles released
    before perihelion moved in hyperbolic orbits with perihelion distances
    greater than is the comet's, thus helping some of them survive. The
    post-perihelion tail's southern, sunward-pointing boundary consists of
    dust ejected during the pre-perihelion brightening. However, the
    streamer of massive grains ejected at extremely large heliocentric
    distances, so prominently seen trailing the nucleus along the orbit
    before perihelion (cf.CBET 3722), completely disappeared. The dust
    located inside the fan, between both boundaries, was released in
    intervening times, mostly during the last two days before perihelion.
    The strong forward-scattering effect (phase angles near 120-130 deg) has
    tempered the rate of post-perihelion fading of the comet, but the
    merciless inverse-square power law of increasing heliocentric distance
    is necessarily the dominant factor in the comet's forthcoming gradual

    H. Boehnhardt, J. B. Vincent, C. Chifu, B. Inhester, N. Oklay, B.
    Podlipnik, C. Snodgrass, and C. Tubiana, Max Planck Institute for Solar
    System Research, Katlenburg-Lindau, reports that two diffuse tail
    structures were analyzed in post-perihelion images obtained by the
    LASCO-C3 corongraph onboard the SOHO spacecraft between Nov. 29.60 and
    29.81 UT. The southward tail extended toward p.a. about 167 deg to
    about 0.4 deg distance from the central brightness peak. The eastward
    tail had an approximate position angle of 68 deg and extended to at
    least 1.2 deg distance. By Finson-Probstein simulations, the eastward
    tail can best be interpretated as being caused by a dust release about 1
    hr around perihelion. The maximum beta value in the eastward tail
    reaches values up to 1.5, typical for graphite or metallic grains of
    about 0.1 micron radius. No indications are found for a continuation of
    the release of similar dust after 2 hr post-perihelion. The shorter
    southward tail may be a relict of heavier grains released about 1-2 days
    before perihelion passage. Diffuse cometary material is noticeable in
    the p.a. range covered by the two dust tails. The match of the
    synchrone pattern for the eastward tail is not optimal, which may
    indicate secondary effects to the dust grains involved.
    NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
    superseded by text appearing later in the printed IAU Circulars.
    (C) Copyright 2013 CBAT
    2013 December 1 (CBET 3731) Daniel W. E. Green"

    PPS: Although this 'telegram' carries an explicit copyright, I consider this copying a legitimate use.

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