Australian Sky & Telescope - 04.2019

48 AUSTRALIAN SKY & TELESCOPE April 2019

COMETS by David Seargent

Anti-tails and shadows

Remembering two of the 21st century’s most interesting comets.

T

hese days, the sky is continually

monitored by automated search

programmes seeking potentially

dangerous asteroids and other

astronomical transients. Although

comets are not the main object of

these searches, inevitably many are

found. And it is also inevitable that

most of the potentially brighter ones

are first recorded well before they reach

perihelion. This is good for advanced

preparation of observations, but it is not

too good for amateur comet hunters.

It therefore came as a welcome

surprise on November 7 last year

when three amateurs — Don

Machholz, Shigehisa Fujikawa and

Masayuki Iwamoto — discovered a

10th-magnitude comet in the morning

sky. (It was an even bigger surprise

that one of these, Machholz, found

it visually). Then, on December 18,

Iwamoto did it again, photographically

discovering another new comet of

magnitude 11–12; quite bright by the

standard of today’s comet discoveries.

Both C/ 2018 V1 (Machholz-

Fujikawa-Iwamoto) and C/2018 Y1

(Iwamoto) were found while emerging

from the morning twilight. The former

reached perihelion on December 3

and quickly became poorly placed for

observation, but the latter did not

arrive at perihelion (at 1.29 a.u.) until

February 6, passing just 0.31 a.u. from

Earth six days later. It may still be

around magnitude 11 at the start of

April, although being in the southern

reaches of Perseus it will have already

succumbed to evening twilight for mid-

southern latitudes. It will be somewhat

better placed for the Top End (nearly

23° altitude at the end of astronomical

twilight for Darwin on April 1), but

mostly it will be a Northern Hemisphere

object during April as it fades away.

Two of the best

Although April this year is not, therefore,

expected to bring anything too dramatic,

this month has, in the past, brought

forth some truly remarkable objects.

Two examples are especially noteworthy,

namely C/1956 R1 (Arend-Roland)

which passed its perihelion on April 8,

1957, and C/1962 C1 (Seki-Lines) which

passed just 0.03 a.u. from the Sun on

April 1 of its discovery year.

Arend-Roland was discovered some

six months in advance by Belgium

professional astronomers S. Arend and

G. Roland and evolved into the first

comet visible to the naked-eye from

European latitudes since the 1910 return

of Halley and the magnificent C/1910 A1

that deserved the title of ‘Great’.

A bright object throughout April, first

from the Southern and then from the

Northern Hemisphere, Arend-Roland

became remarkable for its unusual

anti-tail as Earth passed through the

plane of its orbit late in the month.

Such features are really perspective

effects. Coarse particles (of the type

that would cause meteors if encountered

by Earth’s atmosphere) spread along a

comet’s orbit in a broad but thin sheet.

Normally invisible, this sheet becomes

apparent as our planet crosses the orbital

plane of the comet and enables us to

observe it edge-on. On these occasions, a

comet appears to display a narrow spike

pointing directly toward the Sun. These

so-called anti-tails are not rare, but the

one sported by Arend-Roland was truly

remarkable and may rate as the best ever

recorded. On April 25, the feature was

estimated as some 15° long as seen by

the naked eye. As the main tail of the

comet was then around 30° in length,

the entire comet spanned one quarter of

the entire vault of the heavens!

Seki-Lines was the second visual

discovery be Japanese amateur T. Seki,

and it too became a beautiful naked-eye

object. By late March, it had become as

bright as Mercury, low in the evening

sky and sporting a fan-shaped tail

clearly visible in the twilight. It was so

bright that many expected it to become

a naked-eye sight in broad daylight as it

passed perihelion, within two degrees

of the Sun, on April 1. Nevertheless,

searches by eye and with small

instruments failed to re-locate it. Had it G. RHEMANN/G. BACHMAYER; UNIVERSITY OF CALIFORNIA

C/1956 R1 (Arend-

Roland) sported an

impressive anti-tail

during its April 1957

apparition.

C/2018 Y1 (Iwamoto),

discovered on December

18, 2018, was primarily

an object for Northern

Hemisphere observers.