Astronomy - USA (2020-08)

Path of Iris

M23

0.5°

SAGITTARIUS

N

E

August 1

5 10

15

(^302520)
WWW.ASTRONOMY.COM 43
LOCATING ASTEROIDS I
Steady as she stays
IN THE SUITE of characteristics that make for a good asteroid
hunt, 7 Iris hits a rare one this month: It is nearly motionless on
the sky. It spends the month in a single low-power field, making
it perfect for one sketch to mark its gradual shift against the back-
ground stars.
When Earth, on its inside orbital track, catches up to and passes
a more distant object, the asteroid or planet appears to slow down
and switch direction, called retrograde motion. Usually the loop is
so big our chart just shows a curved line, but Iris happens to be
close enough to the ecliptic this year that the path is a hairpin turn.
If your scope does not slew you directly to the asteroid, star-
hop to or dial in Messier 23, a lovely star cluster in northern
Sagittarius, then shift 1° to the southwest. Look for a trapezoid
made by four stars and scan west to pick out the fainter magni-
tude 9.8 pinprick of this main belt object.
The downside to this single-field advantage is that it will be
a true challenge to see it ooze position over the course of one
observing session.
Iris was discovered by Englishman John Hind in 1847.
Iris executes a hairpin turn (^)
August, Mars brightens to
magnitude –1.8, rising about
10 P. M. local time. Its eastern
track slows as it approaches
next month’s retrograde turn.
The apparent size of the
martian disk grows from 15" to
19" this month as the distance
between Earth and Mars con-
tinues to decline, leading up to
its October 13 opposition. The
planet reaches perihelion on
August 3. Its gibbous disk
changes from 86 to 92 percent
lit during August.
We’re entering peak Mars
observing season, which will
last through mid-December.
Right now, the best time to
observe it is in the predawn
hours when it is high above the
horizon. By 4 A.M. local time on
August 31, Mars stands nearly
60° high due south for midlati-
tudes in North America.
Mars rotates in 24 hours
and 37 minutes. The Red
Planet’s visible features depend
on the time of night you
observe, and landmarks appear
to shift backward when
observed at the same time of
night. Mars is approaching
southern summer, so its south
polar cap, tilted toward us, is
small. The bright Hellas basin
is often confused with the polar
cap due to its prominence.
Uranus rises just after mid-
night local time on August 1
and is well up in the eastern sky
on August 31 at the same time.
It can be spotted with binocu-
lars quite easily, shining at
magnitude 5.8. No other bright
stars are nearby. A scope
reveals a greenish-colored disk
spanning 3.6".
Uranus sits 9° northeast of
the Last Quarter Moon soon
after midnight on August 10.
On August 31, Uranus stands
14° northeast of Mars and lies
roughly midway between
Hamal, the brightest star in
Aries, and Menkar, the bright-
est star in Cetus.
Ve nu s is a brilliant object
in the eastern sky before dawn.
It’s already up by 3:30 A.M. local
time on August 1 and glows at
magnitude –4.5 just south of
Zeta (ζ) Tauri, the southern
horn of Taurus the Bull. Its
43-percent-lit disk spans 27".
As the month progresses,
Venus treks across northeast-
ern Orion and then across
Gemini the Twins. Between
August 12 and 13, the disk is
half lit. Observationally, the
exact moment it reaches 50
percent is hard to judge. When
do you think it occurs?
On August 15, Venus is 4°
due south of a waning crescent
Moon. It’s a beautiful morning
view in the east, along with
Taurus, the Pleiades, and Orion.
By August 31, the planet has
faded to –4.3 and is less than 9°
south of Gemini’s brightest star,
Pollux. Your scope will reveal a
59-percent-lit disk spanning 20".
Mercury is brief ly visible in
the morning sky in the first few
days of August. On August 1, it
stands 5° high 45 minutes
before sunrise, shining at mag-
nitude –0.9. Each following
morning it drops lower toward
the horizon, although it also
brightens. Try catching it
August 4 at magnitude –1.1,
Uranus floats in darkness (^)
when it’s 3° high 45 minutes
before sunrise. It quickly sinks
out of view.
Mercury reaches superior
conjunction on the far side of
the Sun on August 17 and reap-
pears in the evening sky in late
August. It remains difficult to
see and disappears again in
mid-September.

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.

August opens with asteroid Iris traveling west. Between August 16 and

20, the tiny world swings around a magnitude 9.5 star, reversing course.

The ice giant

Uranus makes an

excellent target

with binoculars

or a telescope

this month. This

false-color image

shows the planet

as it appeared in

2010. DAMIAN PEACH