Astronomy

Mars slides past Neptune

36 ASTRONOMY • DECEMBER 2018

Visible to the naked eye

Visible with binoculars

Visible with a telescope

MARTIN RATCLIFFE and ALISTER LING describe the

solar system’s changing landscape as it appears in Earth’s sky.

December 2018: Venus shines brilliantly

SKYTHIS

MONTH

D

ecember’s night sky

features several treats.

Skywatchers can antici-

pate two fine planetary

conjunctions: Mars

and Neptune pass exception-

ally close to each other in the

evening sky, while Mercury

and Jupiter meet in morning

twilight. Meanwhile, Venus

shines at its brightest for the

year. But December’s most

promising event has to be the

prolific Geminid meteor show-

er, which peaks under a largely

Moon-free sky.

Let’s kick off our tour with

one of the month’s more chal-

lenging planets. Saturn lies

low in the southwest during

December, Mars appears con-

spicuous in the southern sky.

The Red Planet shines at mag-

nitude 0.0 and dominates the

background stars of its host

constellation, Aquarius. As

darkness falls, the ruddy object

lies halfway to the zenith —

nearly double its peak altitude

at opposition this past summer.

As you gaze at Mars with

your naked eye, you might

assume it’s the only object of

interest in Aquarius. But target

the planet through binoculars

and you can pick up the much

fainter glow of Neptune. The

distant ice giant lies 3.6° east-

northeast of Mars on the eve-

ning of the 1st. Although both

worlds move eastward relative

to the starry backdrop, Mars

moves much faster. The dis-

tance between the two drops

by about 0.6° every day.

A quick calculation shows

that the two should be nearly

on top of each other within a

week, and mathematics once

again reveals its power. On the

North American evening of

December 6, Neptune lies 23'

evening twilight in early

December. The magnitude 0.5

world appears as a lonely

point of light in the gathering

darkness. On the 1st, it stands

about 10° high 45 minutes

after sunset. A week later, on

the 8th, it’s only 7° high at the

same time. But a slender, two-

day-old Moon accompanies it

that evening. They lie 3° apart

and appear beautiful through

binoculars. The ringed planet

succumbs to bright twilight

within a week. It will pass on

the far side of the Sun from

our viewpoint at the begin-

ning of January.

While twilight partially

obscures Saturn in early

east-northeast of Mars. The

two switch positions the fol-

lowing evening, with Neptune

16' southwest of Mars. (The

magnitude 6.1 star 81 Aquarii

stands 12' north of Mars.)

Although you’ll need binocu-

lars or a telescope to spot

magnitude 7.9 Neptune, you’ll

rarely get a better guide than

you’ll have these two nights.

The actual conjunction

between the two planets

occurs at 9:08 a.m. EST on

the 7th, when Mars passes 2.2'

north of Neptune. The two

then lie below the horizon

from North America, but

observers in Eastern Europe

and the Middle East will have

front-row seats to the event.

This is the second close

conjunction between these

planets in a couple of years.

On December 31, 2016, the

two appeared slightly nearer

to each other, though this

month’s event occurs higher

in a dark sky. The future isn’t

as bright, however. You’ll have

to wait nearly two centuries,

until October 19, 2210, for

A Geminid meteor slices between Taurus (with brilliant Jupiter intruding)

and Orion at the height of the 2012 shower. Observers should get another

great show this year with the Moon out of the sky. AMIRREZA KAMKAR

Skygazers enjoyed the color contrast between ruddy Mars and bluish

Neptune in January 2015. The two worlds meet again December 7. ALAN DYER

Mars

Neptune