Sky at Night

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 11^th 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 +17^th 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 16^th 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 1^st 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.