Astronomy - USA (2020-03)

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LOCATING ASTEROIDS I

Tracking Vesta through Taurus

THE BRIGHT LUMINARY Aldebaran, the ruddy eye of the

celestial Bull, guides us toward the field of Vesta in March.

Beginning the night high in the west, as twilight deepens, the

main belt asteroid slowly approaches the Hyades star cluster,

which actually lies far in the background.

As the fourth asteroid to be discovered, Vesta was once thought

to be part of an exploded planet. Our current understanding,

bolstered by data from the orbiting Dawn spacecraft, is that it

slowly grew as small planetesimals steadily globbed together.

Unlike the darker space rocks out there, Vesta reflects 40 percent of

the sunlight that hits it, helping it reach magnitude 8.5 and making

it relatively easy to spot with a 4-inch scope from the suburbs.

You might want to skip the first week’s game of “asteroid or

star?” when Vesta drifts across the nondescript background of the

outer Milky Way. Instead, start about an hour after sunset, and note

the asteroid’s position. Then return about three hours later to

notice its displacement. On March 11 and 26, Vesta separates from

6th-magnitude field stars. And it forms a line with two brighter

background stars on the evening of March 19.

Observers with 8-inch scopes should make sure to check in on

Vesta on March 7, when it pairs with a star only slightly brighter

than itself. Much like Saturn doesn’t twinkle to the eye, Vesta

should have a slightly “flatter” character than stars, as long as our

atmosphere is not too turbulent.

March evenings find this 8th-magnitude asteroid lurking among the myriad

background stars of western Taurus.

TAURUS

43

h 5

Path of Vesta

March 1

6

11

16

21

26

31

N

E

1°

The Bull stares down Vesta (^)
but Saturn’s dramatic rings
span 37", while the planet itself
stretches 16" across. At a dis-
tance of 957 million miles,
Saturn stands twice as far
from us as Jupiter. Under a
dark sky at 5:30 a.m., the pair
of planets stand 15° high.
Let’s return to Jupiter for a
moment, as the world is now
high enough to witness some
of the exciting activity of its
Galilean moons. Ganymede’s
giant shadow falls on Jupiter’s
northern hemisphere March 10
at 6:53 a.m. EDT and is fully
on the disk by 7 a.m.
Although Callisto missed
the giant planet’s disk during
2019, its transits are back in

2020. It begins a trek across

the far northern latitudes of

the jovian disk at 7:12 a.m.

EDT March 14, when

Midwestern observers can

spot Jupiter riding 16° high.

Earlier that morning, you

can watch Io leave Jupiter’s

disk at 4:09 a.m. EDT fol-

lowed by Ganymede reappear-

ing on the planet’s eastern

limb at 4:51 a.m.

Finally, on March 31,

Jupiter rises as Callisto is near

the end of a transit, and the

moon exits the jovian disk just

after 6 a.m. EDT. In addition

to Io’s transit the morning of

March 20, there will be a few

more events involving the

innermost Galilean moon

this month and many more

throughout the year.

By mid-March, Mercury

makes an earnest leap into the

morning sky following last

month’s inferior conjunction.

Because of the low tilt of the

ecliptic to the predawn eastern

horizon, the pint-sized planet

stays at a relatively low altitude

throughout March. Rising

an hour before the Sun on

March 10, Mercury glows at

magnitude 1. By 7 a.m. local

daylight time, its climbed to 7°

high, though the exact height

differs depending on latitude.

Your best opportunity to

spot the innermost planet is

March 20 and 21, when a

wafer-thin crescent Moon

stands nearby. On both morn-

ings, the Moon and Mercury

rise an hour before the Sun.

Thirty minutes later, Mercury

climbs 5° high in the east-

southeast. The Moon and

the tiny planet sit 15° apart

March 20, and just 5° apart the

following day. Mercury glows

at a modest magnitude 0.2.

Mercury reaches greatest

western elongation March 23,

at 28° west of the Sun, but the

small planet doesn’t gain any

10°

March 26, 1 hour before sunrise

Looking southeast

SAGITTARIUS

AQUILA

CAPRICORNUS

Altair

Saturn

Mars

Jupiter

Three planets triple the predawn pleasure

When Mars stands midway between Jupiter and Saturn on March 26,
it highlights the closest meeting of these three worlds in 20 years.

altitude. By March 31, you’ll

find it has brightened to mag-

nitude 0.0, and stands 4° high

30 minutes before sunrise.

GET DAILY UPDATES ON YOUR NIGHT SKY AT

http://www.Astronomy.com/skythisweek.

Martin Ratcliffe provides

planetarium development for

Sky-Skan, Inc., from his home

in Wichita, Kansas. Alister

Ling, who lives in Edmonton,

Alberta, has watched the skies

since 1975.