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 Post subject: ENB No. 373 April 6 2014
PostPosted: Sun Apr 27, 2014 8:38 am 
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Electronic News Bulletin No. 373 2014 April 6

Here is the latest round-up of news from the Society for Popular
Astronomy. The SPA is Britain's liveliest astronomical society, with
members all over the world. We accept subscription payments online
at our secure site and can take credit and debit cards. You can join
or renew via a secure server or just see how much we have to offer by

Space Telescope Science Institute
Comet C/2013 A1 Siding Spring is plunging towards the Sun along a
roughly 1-million-year orbit. The comet, discovered in 2013, was
within the radius of Jupiter's orbit when the Hubble Space Telescope
photographed it on 2014 March 11. Hubble resolves two jets of dust
coming from the solid icy nucleus. They seem to be persistent jets,
as they were first seen in Hubble pictures taken on 2013 October 29.
They should allow astronomers to determine the direction of the
nucleus's pole, and hence its rotation axis. The comet will come to
perihelion on October 25, at a distance of 130 million miles, well
outside the Earth's orbit.
On its inbound passage, Comet Siding Spring will pass within 84,000
miles of Mars on October 19, which is less than half the Moon's
distance from the Earth. The comet is not expected to become bright
enough to be seen with the unaided eye. A Hubble observation made on
January 21 this year caught the comet as the Earth was crossing its
orbital plane. That special geometry facilitates the determination of
the speed of the dust coming off the nucleus -- information that helps
astronomers to determine how likely and how much the dust grains in
the coma will impact Mars and Mars spacecraft.

Observations at many sites in South America have made the surprising
discovery that the remote asteroid Chariklo is surrounded by two dense
and narrow rings. It is by far the smallest object found to have
rings, and only the fifth body in the Solar System, after Jupiter,
Saturn, Uranus and Neptune. The rings may be the result of a
collision that created a disc of debris. The rings of Saturn are
among the most spectacular sights in the sky, and less prominent rings
have also been found around the other giant planets. Despite many
careful searches, no rings had been found around smaller objects in
the Solar System until now, when observations of the distant minor
planet (10199) Chariklo as it passed in front of a star have shown
that it has two fine rings.
Chariklo is the largest member of a class known as the Centaurs and it
orbits between Saturn and Uranus. Predictions had shown that it would
pass in front of the star UCAC4 248-108672 on 2013 June 3, as seen
from South America. Astronomers using telescopes at seven different
locations, including the 1.54-m Danish and TRAPPIST telescopes at La
Silla, and saw the star apparently vanish for a few seconds as its
light was occulted by Chariklo. But they found much more than they
were expecting. A few seconds before, and again a few seconds after,
the main occultation there were two further very short dips in the
star's apparent brightness. By comparing what was seen from different
sites the team could reconstruct not only the shape and size of the
object itself but also the shape, width, orientation and other
properties of the newly discovered rings. The team found that the
ring system consists of two sharply confined rings only seven and
three kilometres wide, separated by a clear gap of nine kilometres;
Chariklo itself has a diameter of about 250 km. Astronomers think
that such rings are likely to be formed from debris left over after a
collision. It must be confined into the two narrow rings by small
unseen satellites. So, as well as the rings, it is likely that
Chariklo has at least one small moon still waiting to be discovered.

Stanford University
New radar measurements of a large sea on Titan offer insights into
the weather patterns and landscape composition of the Saturnian moon.
The measurements, made in 2013 by the Cassini spacecraft, reveal that
the surface of Ligeia Mare, Titan's second-largest sea, possesses a
mirror-like smoothness, possibly owing to a lack of wind. Titan has
a dense, planet-like atmosphere and large seas made of methane and
ethane. Measuring roughly 420 by 350 km, Ligeia Mare is larger than
Lake Superior. Titan is the best analogue that we have in the Solar
System to a body like the Earth, because it is the only other body
that we know of that has a complex cycle of solid, liquid, and gas
constituents. Titan's thick cloud cover makes it difficult for
Cassini to obtain clear optical images of its surface, so scientists
must rely on radar, which can see through the clouds, instead of a
To paint a radar picture of Ligeia Mare, Cassini bounced radio waves
off the sea's surface and then analyzed the echo. The strength of the
reflected signal indicated how much wave action was happening on the
sea. The surface of Ligeia Mare seemed eerily still. One possible
explanation for the sea's calmness is that no winds happened to be
blowing across that region of the moon when Cassini made its fly-by.
Another possibility is that a thin layer of some material is
suppressing wave action. For example, on Earth, a layer of oil on top
of a sea suppresses small waves. Cassini also measured microwave
radiation emitted by the materials that make up Titan's surface.
Those measurements confirmed previous findings that the land around
Ligeia Mare is composed of solid organic material, probably the same
methane and ethane that make up the sea. Like water on the Earth,
methane on Titan can exist as a solid, a liquid, and a gas all at

By Geoff Elston, SPA Solar Section Director
Solar Cycle 24 continues to fascinate and confound us. It was thought
that we had reached sunspot maximum in late 2011/early 2012. The
expectation was that we would see a second slightly lower peak in
sunspots in 2013-14 (double peaks were seen in Cycles 22 and 23),
before we entered the gradual decline of sunspot activity thereafter.
That has not happened. While the intensity of Cycle 24 is not as high
as that of the previous two, the length of sustained activity has been
longer. The increase in sunspot activity in late 2013 and early 2014
(mostly in the southern hemisphere of the Sun) seems to have presented
us with a higher second peak in sunspot activity than the first peak
two years ago. We may only just have reached the maximum of Cycle 24,
some five years after sunspot minimum in 2008.
Rotation Nos. 2146/2147: Despite the varied and sometimes intense
level of sunspot activity in early- to mid-February, overall there was
a decrease in sunspot activity this month. The Mean Daily Frequency
went down to 4.42 and the Relative Sunspot Number to 75.25.
WHITE-LIGHT ACTIVITY: February began with a bang in the form of Active
Region 1967 (AR 1967) which was the second return of the sunspot group
designated AR 1944 on the previous appearance. AR 1967 was visually
stunning through a telescope with a full-aperture solar filter and was
immediately apparent to the (protected) unaided eye. On the 1st AR 1967
was seen midway between the E limb and the Central Meridian (CM) with
AR 1968 (another very complex sunspot group) just to the north.
AR 1967 is said to have spanned more than Jupiter's diameter at its
maximum extent. Both 1967 and 1968 were intensely flare-active, so
much so that they produced several Coronal Mass Ejections (CMEs),
leading to the appearance of aurora on the 7th. Both groups were
nearing the W limb by about the 8th to 10th, showing many bright
faculae because of limb-darkening.
From the 10th onwards three active groups remained visible -- AR 1975,
nearing the W limb, and 1974 and 1976, moving away from the E limb.
1974 developed as it passed over the CM and headed westwards, showing
a highly fractured structure of numerous spots and pores following a
developing larger pair of leader sunspots. As 1974 neared the W limb
there was still much to see in 1976, and in AR 1977 and 1980 as they
lay stretched out just west of the CM. In the last week of February,
another complex group appeared over the E limb, designated AR 1981,
1982 and 1984. That trail of spots was almost over the CM on the
22nd. On the 25th the former AR 1967 reappeared on the E limb and was
re-designated AR 1990 for this appearance. It produced a massively
strong flare (one of the strongest in the current cycle) leading to a
large and a bright CME at the solar limb. The result was an extensive
auroral display that was seen over many parts of the UK and Europe as
night fell on the 28th.
In the first few days of February, both AR 1967 and 1968 showed a
significant number of plages around them. 1967 also displayed some
small filaments, and a dark long convoluted filament was seen on the
SW quarter of the solar disc with some fine prominences along the E
limb. As AR 1971 came over the E limb on or around the 5th, filaments
were also visible, noticeably a long snake-like one extending away
from the limb to the south of 1971. Many prominences were seen
around the limb. On the 6th the whole disc was busy with filaments
and plages, and the NE limb was quite heavily populated with
prominences. By the 7th AR 1967 and 1968 were nearing the W limb and
still putting on a fine display of filaments and plages. Elsewhere
many filaments were on view, but notably there was a long dark
convoluted one across the SE quarter. On the S limb a fine example
of a 'smoking chimney' prominence was visible, as were two typical
'hedgerow'-type prominences on the W and SW limbs.
Filaments were still very evident on the 9th. On the 10th, with
AR 1973, 1974, 1975 and 1976 visible right across the disc, many
plages and filaments were also apparent. The N, S and SW limbs showed
the brightest prominences. On the 13th the area surrounding AR 1974
and towards the SW limb was busy with plages and filaments. The same
was true of 1976 and 1977 which showed much filament and plage
activity. The SE and SW limbs displayed some fine low-lying and
arch-type prominences. A long line of sunspots (comprising AR 1974,
1976, 1980 and 1977) with plages and filaments was on view on the 16th.
In the last week of the month, AR 1981 and 1982 were accompanied by
several filaments spread across the southern half of the disc.
Meanwhile, AR 1987 and 1989 brought their own plage and filament
activity with them as they emerged from the E limb. A magnificent
large prominence was visible on the E limb on the 25th, just south
from the returned sunspot group AR 1990. The SW limb was also active
with prominences too. A broad dark filament was visible to the south
of AR 1987, which was by then over the CM, on the 27th. The filament
seemed to have broadened and become less well-defined by the following
day. The disc was still populated with plages and smaller filaments
associated with the numerous sunspot groups visible on that day.
MDF (P): 8.29
I am very grateful to all our members who, despite often cloudy
weather, keep their solar images and drawings coming in to me on an
almost daily basis. That really helps me to know what is happening on
the Sun when I cannot observe because of the weather or other
commitments. The images are also highly valuable for these monthly

Bulletin compiled by Clive Down
(c) 2014 the Society for Popular Astronomy
The Society for Popular Astronomy has been helping beginners in
amateur astronomy -- and more experienced observers -- for more than
60 years. If you are not a member then you may be missing something.
Membership rates are extremely reasonable, starting at just £18 a year
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Astronomy, help and advice in pursuing your hobby, the chance to hear
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