–2.0–2.2–2.4–2.6–2.8June 25 July 1 July 9 July 15 July 31 Aug. 10 Aug. 19 Aug. 28 Sep. 4
DateMagnitude20"21"22"23"24"24"21"23"20"60 ASTRONOMY • MAY 2018
make quality observations long before and after that
date. At the start of May — a full three months before
closest approach — Mars will shine at magnitude –0.4
and measure 11" across, nearly 50 percent of its maxi-
mum size this year. At that size, detail will be visible.
Because Mars will really put on a show for more than
a month on either side of opposition, the darkness of
your site doesn’t matter much. Indeed, some ambient
light actually is welcome when you observe the Red
Planet. A white light off to your side (not directly in your
field of view) lighting up your surroundings will cause
your daytime vision — which is superior to night vision
in both resolution and color sensitivity — to kick in.
Want to observe from a parking lot in Chicago? If it’s
clear, you’ll see some detail, even through small tele-
scopes. But you shouldn’t observe from a parking lot
because the steadiness of the air above your site — what
astronomers call seeing — makes all the difference. The
better the seeing, the more detail your scope will reveal.
Unfortunately, any parking lot stores lots of daytime heat
in the summer and releases it at night, ruining the view
of even bright objects like Mars. So leave the city behind.What can you see?
Albedo features are the areas most subject to seasonal
changes such as brightening or darkening. Through a4-inch or larger telescope, you’ll see large albedo features
— regions distinguished by the amount of light they
ref lect. Mars’ best include Syrtis Major (an easily seen
dark, triangular feature), Chryse, Elysium, the Hellas
Basin, Libya, and Solis Lacus.
Mars’ day, which astronomers call a sol, is 37.4 min-
utes longer than an Earth day. So, if you observe Mars at
the same time each night, its markings will appear to
move 9.11° per day to the west. In a little more than five
weeks, the planet seems to slowly rotate backward one
full spin. All of Mars’ prominent features will, at some
time during this period, lie on its meridian, the line
splitting the planet’s visible disk from top to bottom, as
seen from Earth. All martian features look their best
when on the planet’s meridian.
Even including albedo highlights, the Red Planet’s
best surface features are its polar ice caps. At opposition,
Mars’ south polar cap will tilt 11° in our direction.
Due to the temperature range at the martian poles,
astronomers subdivide each ice cap into larger “sea-
sonal” and smaller “residual” caps. Residual caps last
through the martian year. The southern residual ice cap
measures about 200 miles (320 km) across. The northern
residual ice cap spans about 600 miles (960 km).
Winters are more severe in Mars’ northern hemi-
sphere, and the seasonal ice cap there has reached a“Mars will look as large as
the Full Moon to the naked
eye.” How exciting — if
only it were true. This
deliberately misleading
statement first appeared
online during mid-2003,
when Mars was as close to
Earth as it will be for thou-
sands of years. And if thatwasn’t bad enough, we
saw this statement again
before Mars’ next half-
dozen oppositions.
Nothing could be farther
from the truth, but expect
this lie to pop up again this
year. Just tell your friends
it ’s someone’s idea of an
internet joke. — M.E.B.WE WON’T GET FOOLED AGAIN
How bright will Mars be?
This sequence,
taken June 5,
2016, shows Mars’
rotation over
slightly more than
2 hours. The left
image was taken
at 1h17m36s UT,
the center one
at 1h48m36s UT,
and the right one
at 3h26m36s UT.
DAMIAN PEACH
How big
and bright Mars
appears depends
on when you
observe it. This
graph shows how
the Red Planet’s
apparent size
grows before
opposition and
shrinks afterward.
ASTRONOMY: ROEN KELLY