Astronomy

1°

E PERSEUS

AURIGA

Capella

Path of

Comet PANSTARRS

May 1

26

21

16

11

6

April 1

c

¡

d

58

N

Comet PANSTARRS (C/2016 R2)

15"

Ganymede

Callisto

Jupiter

April 18, 11:30 P.M. EDT

S

W

Angled to the extreme

42 ASTRONOMY • APRIL 2018

COMETSEARCH

Bright comets typically put on

brief shows as they pass close

to the Sun. Comet PANSTARRS

(C/2016 R2) follows a more gen-

tle arc that brings it no closer

to the Sun than 2.6 astronomi-

cal units. (An AU is the average

Earth-Sun distance.) Its distant

orbit means a slower speed and

many months of visibility. Just

as important, its orbit inclines

steeply to the solar system’s

ecliptic plane.

This combination produces

a distance to both the Sun and

Earth that changes gradually,

allowing the comet to maintain

its brightness. The biggest

uncertainty rests on the comet’s

activity. If it continues to outgas

as it has been and hangs in at

10th magnitude, you’ll be able

to pick it up with a 4-inch scope

under a dark sky. But a magni-

tude or two fainter would require

an 8- to 10-inch instrument.

C/2016 R2 appears highest in

April’s early evening sky. It starts

the month in Perseus and crosses

into Auriga at midmonth. By

April’s close, you can find it 2.5°

south of magnitude 0.1 Capella.

Although this area lies only a few

degrees from the Milky Way’s

equator, we’re peering through

a fairly unpopulated segment of

our home spiral. No glowing gas

clouds lie along our line of sight,

just some dark dust that reduces

the number of field stars.

PANSTARRS bids a long farewell

EVENING SKY MIDNIGHT MORNING SKY
Venus (west) Jupiter (southeast) Mercury (east)
Mars (south)
Jupiter (southwest)
Saturn (south)
Neptune (east)

WHEN TO VIEW THE PLANETS

— Continued from page 37

If you view Jupiter the night
of April 7/8, you’ll notice the
black shadow of Ganymede
crossing the north polar region
from 1:17 to 3:01 a.m. EDT.
The big moon itself traverses
the same region starting at
4:25 a.m. and wrapping up at
5:27 a.m. You can see another
shadow transit of this satellite
between 5:14 and 6:59 a.m.
EDT on April 15.
But an even better example
of the orbital tilt arrives the
night of April 18/19. Both
Ganymede and Callisto are
then on the far side of Jupiter
and appear south of the plan-
et’s disk. Although Jupiter does
pass in front of Ganymede
early that evening, the moon
returns to view by 11 p.m. EDT.
Callisto lies well south of the

M22. But Mars, in its closer

and faster orbit around the

Sun, moves eastward at a com-

paratively rapid pace. By the

time a waning gibbous Moon

passes by the morning of the

7th, the planets appear nearly

3° apart. At the end of April,

14° separate the pair.

planet all night and never

enters the shadow.

As Jupiter climbs high

in the south, Saturn and

Mars appear in the southeast.

The two rise together around

2:30 a.m. local daylight time

April 1. Just 1.4° separate

them that morning, and the

gap closes to 1.3° at their con-

junction on the 2nd. The col-

orful pair appears against the

stunning backdrop of north-

ern Sagittarius, with the

5th-magnitude globular star

cluster M22 just 0.4° south-

west of Mars. The Red Planet

shines at magnitude 0.3, and

yellowish Saturn glows 0.2

magnitude fainter.

Saturn remains almost

stationary this month, never

straying more than 2° from

Although naked-eye and

binocular views of Saturn are

impressive, plan to spend

time observing the planet

through a telescope. In mid-

April, the planet’s disk mea-

sures 17" across the equator

while the ring system spans

39" and tips 25° to our line of

This visitor from the distant Oort Cloud moves from Perseus into Auriga

in April, ending the month near brilliant Capella.

Both Ganymede and Callisto appear south of Jupiter the night of April 18/19,

when the planet’s equator tilts near its maximum to Earth.