Australian Sky & Telescope — July 2017

50 AUSTRALIAN SKY & TELESCOPE July 2017

COMETS by David Seargent

Kiwi comet makes its return

Comet 71P/Clark will zoom through the Milky Way in July.

A

s we move into July, the bright

periodic comets which graced

our skies earlier in the year have

now faded to mere shades of their

former selves. Nevertheless, another

relatively bright object of short period

(due to reach perihelion at 1.59 a.u.

on June 30) will be well placed for

southern observers in Scorpius during

July. Despite its position within a

rich region of the Milky Way band,

the comet should be a relatively easy

object for small- and medium-size

telescopes, as it is expected to reach its

maximum brightness this month, with

some forecasts placing it at around

magnitude 10.

The comet in question is71P/Clark.

This object is of special interest for

southerners as it was discovered by

Michael Clark of Mt John Observatory

in New Zealand on June 9, 1973 during

a photographic variable-star patrol

conductedbytheBambergObservatory

inGermany.Atthetimeofdiscovery,

the comet’s magnitude was estimated

as13.Havingaperiodof5.5years,the

comet experiences alternate favourable

and unfavourable apparitions, with this

year’s being one of the former.

The two comets of long period

that have been features of our skies

in recent months should, however, be

easier objects for small telescopes and

binoculars as they move away from

their respective perihelia.

Having passed perihelion (at

1.05 a.u.) on May 9, C/2015 ER61

(PANSTARRS) begins July moving

slowly through Aries, reaching the

border of Taurus by the end of the

month. The brightness of this comet

has not been entirely steady, with an

outburst having occurred last autumn,

so magnitude predictions are necessarily

uncertain, although relatively

conservative parameters suggest a slow

fade from about magnitude 9.5 to 10.5

during the month of July.

Also retreating from its perihelion

passage (on June 12 at 1.64 a.u. from

theSun)thedynamicallynewC/2015

V2 (Johnson)remains very well placed

forsouthernobserversasitslowlyfades.

Starting the month in Virgo, the comet

passes the meeting point of Virgom

Libra and Hydra around the middle

of July, before continuing southward

into Centaurus by the month’s end.

The scatter in brightness estimates of

this comet was quite large as it moved

toward perihelion, probably because

it displayed a large and diffuse outer

coma surrounding a relatively small and

strongly condensed inner one. Back in

April, magnitude estimates ranged from

as bright as the faint sevens to as dim

as the mid tens! Assuming a value near

the middle of the range and an ‘average’

response to solar distance, implies a

brightness close to magnitude 8 in early

July, fading by about one magnitude

before the end of the month.

■ DAVID SEARGENT is the discoverer

of comet 1978 XV. His latest book,

Visually Observing Comets (published

by Springer), is now available both as a

paperback and an ebook.

S Comet C/2015 V2 (Johnson) — shown here

as it was at the end of March — will be well

placed for southern observers during July,

but is beginning to fade.

S C/2015 ER61 (PANSTARRS) will move through

Aries during July, glowing at around magnitude

9.5 to 10.5. This is how it looked on April 6.

G. RHEMANN