Astronomy

Mercury^ (Perihelion)

Merc

ury^ (Aphelion)

Venus

(Aphelion)

Venus

(Perihelion)

Earth (Perihelion)

Earth

(Aphelion)

70 ASTRONOMY • JUNE 2018

A: In 2012, Cassini revealed

that, based on data taken

between 2006 and 2011,

Saturn’s largest moon, Titan,

changes shape due to tides

raised on the satellite as it cir-

cles the planet. Over the course

of its nearly 16-day orbit,

Titan’s surface deforms by

more than 33 feet (10 meters).

This amount of tidal defor-

mation is associated with a

malleable, likely liquid ocean

layer inside the moon. Current

estimates place Titan’s ocean at

more than 62 miles (100 km)

thick. If Titan were solid all the

way through, the expected

deformation of the surface

throughout its orbit would

total only about 3 feet (1 m).

However, like most of the

solar system’s larger satellites,

Titan is tidally locked to

Saturn. A tidally locked satel-

lite simply rotates once per

every orbit around its parent

body, always showing the same

face to the planet. Such satel-

lites can still experience tides.

Because Titan’s orbit is ellipti-

cal, the gravitational inf luence

of Saturn from the near to far

side of the moon varies

throughout its orbit, which

causes the deformations

recorded by Cassini.

Alison Klesman

Associate Editor

Q: HOW LARGE DOES

THE SUN APPEAR FROM

MERCURY AND VENUS,

AS COMPARED TO HOW

WE SEE IT FROM EARTH?

Robert Harrison

Los Ranchos, New Mexico

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

THE TIDES

ON TITAN

A: The apparent size of the Sun

(with a physical diameter of

about 865,000 miles [1.4 million

kilometers]) varies with its

distance from the observer. On

Earth, where we average a

distance of 93 million miles

(150 million km) from our star,

the angular diameter of the

Sun is about half a degree (0.5°).

Mercury orbits the Sun at

an average distance of about

36 million miles (58 million km).

As a result, the angular diam-

eter of the Sun from Mercury

is much larger: about 1.4°.

Venus’ average distance from

the Sun is about 67 million

miles (108 million km), and the

Sun’s angular diameter from

this planet is about 0.7°.

It is worth noting that the

planets’ orbits are not quite

circular. Between perihelion

and aphelion, the angular

diameter of the Sun as seen

from Earth changes by about

3 percent. On Mercury, that

change is nearly 53 percent,

while on Venus, it’s a little

over 1 percent.

Alison Klesman

Associate Editor

Q: I’VE READ THAT THE

PLANNED CREWED MARS

MISSIONS WILL TAKE SIX

MONTHS OR TWO YEARS

TO ARRIVE. WHICH IS IT?

COULD A LONGER TRIP

BE DUE TO THE HEAVY

PAYLOAD? OUR ROVERS

TOOK ONLY EIGHT TO NINE

MONTHS TO ARRIVE.

Ronald Greene

Kingman, Arizona

A: When it comes to a trip to

the Red Planet, your mileage

may vary — literally. Earth and

Mars are constantly moving,

but they don’t stay a constant

distance apart. Furthermore,

spacecraft from Earth don’t

travel in a straight line to the

Red Planet. Instead, astronauts

leaving Earth would follow a

path known as the Hohmann

transfer orbit, an ellipse from

where Earth is now to where

ASKASTR0

Q: DOES TITAN EXPERIENCE ANY TIDES

IN ITS OCEANS, OR IS IT TIDALLY LOCKED

WITH NO TIDES? Richard Robinson, Clay, New York

The Sun’s apparent size varies with

distance, appearing larger from

Mercury and Venus than from

Earth. Because the planets’

orbits are not circular

(Mercury’s is more

elliptical), the Sun’s

apparent size can vary

between aphelion

and perihelion.

ASTRONOMY: ROEN KELLY

Titan’s orbit is slightly elliptical, bringing it closer to Saturn during some points and taking it farther during

others. The moon is most spherical at the farthest point from the planet, and most football-shaped when it passes

closest to Saturn; the amount of deformation Titan experiences requires a liquid ocean beneath its surface. NASA/JPL