Sky at Night
In
the footsteps of Harry Miller….
Dr
Anand Venkatakrishnan
Welcome Comet Wirtanen!
On December 16th, the
comet will pass within 12 million kilometers of Earth and should brighten to
about 3rd magnitude, though the diffuse coma and a nearly full Moon will make
observations difficult.
After a long time, yet again a promising
comet in our skies for naked eye and binocular observers. Amateur astronomers
and comet watchers will have to only fight light and atmospheric pollution and
moon light for hunting this favorably positioned but elusive comet. Comets are
small bodies of a few tens of kilometers, made of ices, dirt and other
primordial elements that vaporize to form a tail as it advances towards the sun
in its long elliptical orbits. As a comet nears the inner solar system crossing
the orbits, one by one, of planet Mars, Earth, Venus and Mercury,
circumambulating the Sun and moving outwards to where it came from, it exhibits
varying physical appearances depending on its distance from the sun and the
angle of view as seen from the Earth.
Comets were thought to represent omen
and much feared. We know from the Shakespeare’s play, Julius Ceaser, in which
Calpurnia says, “When beggars die there are no comets seen, the heavens
themselves blaze forth the death of princes!” Indeed, the kings feared the
comets as their apparition was to bring much misfortune and even death. During
the times of the king Harold II of England, in 1066, appeared the famous ‘Halley’s
Comet’ which was thought omen as it was assumed to have led to his downfall and
death. The comet is represented on the Bayeux
Tapestry [a 11th century embroidered cloth nearly 70 metres (230 ft) long and 50 centimetres
(20 in) tall, which depicts the
events leading up to the Norman conquest of England],
and described in the tituli as a star. Surviving accounts from the period describe
it as appearing to be four times the size of Venus
and shining with a light equal to a quarter of
that of the Moon. Halley came within 0.10 AU of Earth at that time.
Comets are maverick
objects which behave unpredictably somewhat like our weather forecasts! An
ordinary comet may put on a bright show or a totally unexpected comet might
create a sensation of spectacular visibility, whereas, a much expected one may
turn out to be dull and uninteresting. Due to the technological advancement
nowadays, astronomers have been able to predict their sky position, brightness
and angular measure to a great degree of certainity.
In this electronic
digital era of instrumentation, comets could be easily photographed by even amateur
astrophotographers who are lucky to have a reasonably good telescope, mount,
camera and a lap top computer. Astronomy is an all-encompassing natural science,
provoking deep fundamental questions, in the minds of interested youngsters. It
is a fascinating hobby for the younger generation. The sky is the natural
laboratory for the astronomer. Comet hunting, new and old, adds to the appetite
of the seasoned amateur astronomer, in his or her endeavor to fill his light
collecting basket (like collecting flowers in a garden) collecting clusters,
nebulae and galaxies! The experience of which is scientifically and spiritually
rewarding!
Comet Wirtanen, 46P,
was discovered on a photographic plate at +17th magnitude, in
January 15, 1948, by the American astronomer Carl Alvar Wirtanen, at the Lick
Observatory, California, USA. It is a short-period comet with an orbital period
of 5.4 Earth years. It belongs to the Jupiter family of comets, all of which
have aphelia between 5 and 6 astronomical units, abbreviated AU. Comets like
the Planets revolve around the Sun with elliptical orbits, only much longer and
usually highly inclined, to the normal orbital plane of the planets around the
sun.
There are long period
comets and short period comets. There are periodic comets and non-periodic
comets that visit the solar system only once and to go back never to come
again! As a comet approaches the inner solar system, i.e., falling inside the
orbit of planet Mars, towards the Sun, it warms up, and the ices and gases on
its surface vaporize and form a coma around the evaporating nucleus of the
comet. This coma may measure hundreds of thousands of kilometers and as the
comet accelerates towards the Sun, it gets pushed out into forming a tail of
gases and dust. The closer the comet gets to the Sun, the longer a tail it may
develop. Depending on the size and composition and albedo (surface brightness)
of the comet and its distance and angle of presentation, to the Earth, it may
exhibit differing features of brightness, apparent angular size and tail
length. Some comets put on spectacular show like comet West, with a huge bright
tail. Comet Hale Borp and comet Hyakutake were visible to the unaided eyes like
a spread out broom. In 1994, the great comet Shoemaker-Levy collided with
planet Jupiter after breaking into 21 fragments hitting headlines in all the
newspapers. They are potentially dangerous but by no means omen, as believed
earlier. Knowledge helps mankind to prevent disasters or at least to be
prepared to meet any natural eventuality.
Like before, even in
this digital era, comets are named after the observer who discovers them. Some
have single names whereas others have been named after the first three
astronomers who happen to have discovered them like comet ‘Bappu-Bok-Newkirk.’ *This
comet was accidentally discovered by the trio: exposure taken by the 22-year
old ‘Indian astronomer Vainu Bappu’ during the dawn of July 2, 1949, comet
noticed by ‘Gordon Newkirk’ through a binocular microscope and its presence
confirmed by ‘Bart Bok’ at Harvard University the next day. *(https://britastro.org/node/2595).
Comets were called dirty snow balls by
Whipple, and more appropriately now mentioned as snow balls with some dirt. The
only comet to have been named after its predictor (not discoverer) is ‘Halley’s
comet’ as Edmund Halley predicted the next apparition of this comet, to be in
1759, (which occurred after his death) by investigating the annals of its historical
astronomical record. And the comet was named in
Halley's honor by French astronomer Nicolas-Louis de Lacaille in
1759.
How to look for comet Wirtanen….
The best way is to look at the evening
dusk sky after sunset. First, is to get a feel of the sky and the general
direction in which the comet appears on you particular observing day. This is
done, just with the unaided eye. After locating the approximate region of the sky
in which the comet is, use any low power good binoculars and keep observing and
sweeping in the same area, for any diffuse object. Experience is always helpful
and the comet could be very elusive to detect. Detection requires averted
vision (looking at the edge of the binocular field of view, when you suspect
the object is in the field centre – this helps to detect it!).
On the 16th
of December 2018, the comet will pass 0.0781 AU (11,680,000 km) (~30 Lunar
Distance, LD) from the Earth, an astronomical
unit, AU, being the Earth-Sun distance of ~150,000,000 kms. This occurs a little less than four
days after the comet passes its minimum distance from the Sun of 187 million
kilometers. Its magnitude (brightness) could peak as bright as magnitude +3
around the 16th of December, 2018. In this regard, planet Venus is of magnitude
~ -4, Jupiter ~ -2.5, the 1st brightest star Sirius being ~ -1.58.
Human eyes visible limit on a very clear night is +6; each order of magnitude
corresponding to a difference of 100 times.
Knowledge of constellations would be of
great help in trying to locate the comet in respect to the familiar patterns of
stars. And this needs practice for a few nights. With the beginning of December, the comet will be near the
border between the constellations Cetus and Eridanus at a declination near
–20°. Along the way, on the evening of
December 15th it can be conveniently found passing almost midway between the
Hyades and Pleiades star clusters. On December 21st, it will from a triangle
with the lowest two stars that make up “The Kids” asterism in Auriga: 10 Eta
Aurigae and 8 Zeta Aurigae. And on the nights of the 22nd and 23rd, it will
pass within a few degrees to the south and east of the brilliant yellow-white
star Capella.
After
Full Moon, dark sky opportunities open up in the evening sky. There will be a
46-minute window of darkness between the end of evening twilight and moonrise.
A week later, the comet will be visible
most of the night without any lunar interference, and about a half hour after
the start of the New Year it will stand more than 70° above the northern
horizon, and probably hover at around 4th magnitude.
It
is expected, the comet would rapidly swell during early December, reaching an
apparent angular diameter somewhere between 1degree and 1.5 degrees — two to
three times the apparent diameter of the Moon — on the night it is closest to
Earth. And as you can smart guess, a diffuse object spreads its light in
proportion to its area of spread and therefore would be disappointingly dimmer.
Perhaps, after all, only its central nucleus area could be made out! In any
case, it will be worth the watch! All that matters is only interest and persistent
observation. And keep your expectations low as the comet would certainly not
appear as bright as it is seen in exposure photographs. The thrill lies in
hunting the faint wisps of light in the real night sky! Good luck.
