Astronomy - September 2015

Moon’s distance from Earth

Twice the Earth-Moon distance

½° ¼°

34 ASTRONOMY • SEPTEMBER 2015

A: Mars’ moons are easily vis-

ible at night from the surface

of the Red Planet. Phobos —

the nearer, larger, and brighter

of the two — would be obvi-

ously non-round. A sharp-eyed

observer would be able to make

out the moon’s largest craters

and see the jaggedness of its

terminator — the dividing line

between light and dark. Pho-

bos rises in the west and sets in

the east in the span of around

four hours a couple times each

martian day and is up at some

time each night. It also notice-

ably changes phase between

rising and setting. From the

Curiosity rover’s point of view,

on some evenings Phobos rises

out of the slowly fading blue-

gray twilight in crescent phase,

transits as a quarter moon, and

sets as a gibbous moon. Near

fall or spring equinox, it goes

into eclipse before becoming

full. Near the start or end of

totality, Phobos would remain

faintly visible. And on a few

days each year, it also transits

the Sun during the daytime

in an annular or even a partial

solar eclipse.

Deimos appears much

smaller but is easily visible and

brighter than any star in the

martian sky at night. It is a

little outside the areostationary

orbit — it goes around Mars

almost as fast as the planet

spins. So it is visible for a cou-

ple nights, then gone for a few

nights, and then back. During

the night, the stars move past

it, but it does rise in the east

and set in the west. Eclipses

involving Deimos are rare but

do occur near the solstices.

During dust storms, the

rising and setting of the moons

would be lost in the murk,

but they still would be visible

when they are high in the sky

at night. Out of dust storms,

Phobos is sometimes visible

in daylight, high in the early

morning or late afternoon sky.

Mark Lemmon

Texas A&M University

College Station

Astronomy’s experts from around the globe answer your cosmic questions.

MOON

MUSINGS

Q: WHAT EXACTLY IS A

DEGREE IN TERMS OF THE

DISTANCE BETWEEN STARS?

Stephanie Johnson

Maple Grove, Minnesota

A: A measurement in degrees

has nothing to do with the true

distances between celestial

objects. When we talk about

apparent distances or sizes, how-

ever, degrees do come into play.

Here’s an example: The

Moon’s true diameter is 2,159

miles (3,474 kilometers). But

the width that we see of the

Full Moon — its apparent

diameter — is ½°. That’s how

much of an angle it subtends at

its distance from us. To prove

there’s no connection between

true and apparent diameter,

let’s imagine that we can dou-

ble the Moon’s distance from

Earth. Its apparent size would

shrink to ¼° from our vantage

point, but its true diameter

would remain unchanged.

Michael E. Bakich

Senior Editor

Q: CAN A MOON HAVE ITS

OWN MOON?

Garvin McDaniels

Benton, Kansas

A: Yes, it is possible for a moon

to itself have moons and/or

rings, and there are a few

places where scientists have

suggested that this is (or was)

the case. However, moons tend

to rotate slowly, and this makes

it difficult for a moon’s moon

to find a stable orbit.

In 2008, some scientists

claimed that an electromag-

netic detector on board the

Cassini spacecraft had found

rings around Saturn’s moon

Rhea. This claim did not hold

up when the larger community

carefully checked it out, but

some astronomers still believe

that markings around Rhea’s

equator are the remnants of

an ancient ring that fell onto

the surface.

Iapetus, another moon of

Saturn, is best known for its

highly contrasting bright and

dark hemispheres. But it has

several other mysterious char-

acteristics, including a bulging

shape that indicates it once

spun much faster than it does

now as well as an equatorial

mountain range. Some scien-

tists have proposed a model

involving an ancient moon

around this moon, which might

have produced both features as

it spun inward toward Iapetus

and then broke up into a ring.

It is actually impossible in

most cases for a moon around

a moon to be stable in the

long term because nearly all

moons rotate once per orbit so

they always keep the same face

toward their planet (Earth’s

Moon is the best-known exam-

ple). Such a slow rotation rate

means that any moon-orbiting

object would orbit faster than

the moon rotates, which would

cause the moon-orbiting object

ASKASTR0

Q: ON THE MARTIAN SURFACE,

WOULD THE MOONS DEIMOS

AND PHOBOS BE VISIBLE TO

THE UNAIDED EYE?

David De Roo, Silver Spring, Maryland

NASA’s Curiosity rover watched Phobos, Mars’ largest

moon, occult the diminutive Deimos in 2013 using the

long lens of its Mastcam. NASA/JPL-CALTECH/MSSS/TEXAS A&M UNIV.

Astronomers suspect the odd ridge

on Saturn’s moon Iapetus that

makes it look like a walnut might

have been formed when a moon or

set of rings crashed to the surface.

Degrees of sky are irrelevant to true distance between stars. If the Moon’s

distance from Earth doubled, its apparent size would shrink by half, even

though its physical diameter would remain unchanged. ASTRONOMY: ROEN KELLY

Moving the Moon

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE