Get Ready for the Geminid Meteor Shower overnight this Friday/Saturday

As usual at this time of the year, the Earth is entering a stream of debris from rock comet 3200 Phaethon, which is the source of the annual Geminid meteor shower. Forecasters expect the shower to peak on Dec. 13-14 with as many as 120 meteors per hour.

This year the nearly full Moon will reduce the number of meteors you may see but it is still well worth a look. Expected to peak from about midnight Friday Australian Eastern Daylight Time (or 1300 UT) until 9pm (1000 UT) Saturday, this meteor shower will be visible in both hemispheres.

Though you do need to keep in mind that meteor showers often peak hours before or after predictions and for sure we certainly don’t know everything that a given meteor stream might have in store!

This shower is an interesting one though, with an equally interesting history and source. The Geminids were first identified as a distinct meteor shower by R.P. Greg of Manchester UK in 1862, and the estimated ZHR rose from about 20 to 80 through the 20th century. The parent source of this shower remained unknown until 1983, when astronomer Fred Whipple linked them to the strange “rock-comet” body 3200 Phaethon. This is an Apollo asteroid also thought to be a member of the Pallas family of asteroids, 3200 Phaethon seems to be shedding enough material to produce the annual Geminid meteor shower. This makes the annual shower rare as one not produced by a comet. It’s worth noting that 3200 Phaethon also passes extremely close – 0.14 AU – from the Sun at perihelion, and gets periodically “baked” during each 1.4 year passage.

In the 21st century, rates for the Geminids have stayed above a Zenith hourly rate (ZHR) of 120, now the highest of any annual shower. It’s worth noting that an extrapolated ZHR of almost 200 were seen in 2011 when the Moon was at an equally unfavorable waning gibbous phase! The Geminids always produce lots of fireballs, capable of being seen even under moonlit skies.

With our warmer nights down under it is a great time to get out and have a look! Jupiter is also looking good after about 10:30pm and Mars and Saturn are visible in the early dawn skies as well.

Tonight’s Southern Sky (December 1st, 2013)

Tonight we will have a nice dark sky as it is a waning crescent Moon showing only 5% of the visible lunar surface and it will have set by 17:55.

Tonight the Sun will set at 19:53 AEDT and nautical twilight will start at 20:54. This is when the Sun is at an angle of twelve degrees (12°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, it is dark for normal practical purposes. Astronomical twilight begins at 21:29 and is when the Sun is at an angle of eighteen degrees (18°) below an ideal horizon. At this time the illumination due to scattered light from the Sun is less than that from starlight and other natural light sources in the sky.

Planets visible tonight

Venus: Rises at 7:35 am and sets at 22:07 and is the brightest object in the Western Sky at present.

Jupiter: Rises at 21:42 and sets at 7:44am and is the brightest object in the western morning sky.

Mercury: rises at 3:49am and sets at 17:35pm so is a morning object in the eastern sky.

Mars: rises at 1:13am and sets at 13:04 and is easily visible as a bright orange/red star like object in the early morning sky.

Saturn: rises at 3:31 am and sets at 17:01 and again is only visible in the early morning eastern sky.

The old summer favourites of the Southern Cross and the Large and Small Magellanic Clouds are visible in the South.

The western sky has some of the more fainter constellations visible after sunset as Scorpius and Sagittarius set with Venus. These fainter and less known constellations include Cetus, Eridanus, Phoenix and Sculptor visible.

Looking towards the North and East, you get to see some of the more familiar constellations. During summer the Milky Way runs almost vertically from south to north. Orion has dominates the night sky along with Taurus, Gemini and Cancer.

Latest CBAT update on COMET C/2012 S1 (ISON)

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 disappearance.

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

An update on Comet ISON!

The comet had been visible in the Southern Hemisphere before passing the Sun but since the 19th November it has been very difficult to see as it has risen just before the Sun. After it had passed the Sun it would be rising just after the Sun rise and setting before the Sun set, in the southern hemisphere so hence we would not have been able to see it.

At around 6:44am our time this morning the comet reached perihelion (its closest approach to the Sun) where it broke up and then something continues on – it might just be gravel and dust or there might stay a chunk of rock big enough to stay comet like. But now only time will tell if it is big.  This goes to prove that although we certainly know a lot more about comets than we did before – there is a lot more that we do not know.  Many have pronounced Comet ISON as already being dead and it certainly will not reach the brightness and spectacular display that had been predicted – but as Mark Twain is often quoted: “Rumours of my demise are greatly exaggerated.”  Something emerged from the sun after Comet ISON made its closest approach today. Is it ISON? Both professional and amateur astronomers are analysing images from NASA satellites to learn more about comet’s fate. Northern ground based observers may have to wait until around the 9/10 of December now to see if there is anything to see. But they will not get the amazing views that we were all hoping for.

However, at every single opportunity it could find, Comet ISON has done completely the opposite of what was expected, and it certainly wouldn’t be out of character for this dynamic object to yet again do something remarkable. Even if the comet broke up, it offered a very rare opportunity to see how one of the oldest objects in the solar system interacted with the Sun’s magnetic field and its behaviour in the sun’s magnetic field will help scientists understand more about both comets and the Sun. This  was the first comet in recorded history which has come from so far away and passed so close to the sun, passing the sun at a distance of around 1.6 million kms that has been so well-studied and observed.

So we wait and see, this has been one of the most well observed, followed and commented in social media worldwide. A fleet of spacecraft watched ISON plunge toward the sun, including NASA’s STEREO satellite, the European Space Agency/NASA SOHO spacecraft and the Solar Dynamics Observatory. The Hubble Space Telescope should be able to take a close look in a couple of weeks if it did indeed survive.

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The photograph above is from the NASA SOHO Space Telescope’s LASCO C3 camera showing a fragment emerging from the other side of the Sun about 3 hours after perihelion.

The picture below is taken this at 00:42UT 1 December 2013 and shows the remnants of the comet as it leaves the SOHO LASCO C# camera’s field of view.