Sky and Telescope - July 2018 - 23

e've waited 15 long years and now it's time to
party. Mars reaches perihelic opposition - an
opposition when a planet is at its closest point to
the Sun - on July 27th. This is the ﬁrst perihelic opposition of Mars since August 2003, meaning that it's bigger
and bolder in the night sky than it's been in more than a
decade. Are you as eager as I am to roam its deserts and
poles with a telescope? Maybe even track a dust storm or
catch sight of clouds capping mighty Olympus Mons?
Mars, a planet that requires the patience of marble,
has a more eccentric orbit than most denizens of our
solar system. At approximately two-year intervals, Earth
lines up with Mars at opposition, but a majority of those
alignments occur at the same time Mars is relatively far
from the Sun. Not this year. Mars is almost at perihelion
at the same time as opposition, so it will be a snug .38
au (57.6 million kilometers) away from Earth, close
enough for telescopic observers to have a ﬁeld
day ferreting out dark surface markings and
changeable weather.
On the night of opposition, in the company of the waxing gibbous Moon, the
Red Planet will burn an intense magnitude -2.8, equaling Jupiter at peak
brightness. That's a lot of light to

muster for a tiny planet only twice the size of our Moon.
It just goes to show how distance trumps size when it
comes to things celestial.
At the same time, the planet's disk will balloon to 24.3″
(arcseconds), only 0.8″ smaller than during the 2003
opposition when Mars came its closest to Earth in 59,635
years. Because Mars won't arrive at perihelion until September, its closest approach to Earth is slightly delayed,
occurring on July 31st. After that date, the two planets
begin to part ways.
June opens with Mars already 15.5″ across and shining
at magnitude -1.2, nearly the equal of Sirius. Even users of
small telescopes should have no problem seeing the south
polar ice cap and numerous dark albedo markings. By
July 1st, it ﬁ lls out to 21.1″ and reaches its greatest size at
month's end as the polar cap continues to shrink.
Despite its glorious girth, northern observers will pay
a price during this juicy Mars apparition. At most perihelic oppositions, including this one, the planet retreats to
the belly of the ecliptic low in the southern sky. On July
27th, Mars gleams from southwestern Capricornus at
declination -25° 34′ and at culmination stands only 23°
high from Chicago and 13° from London. Low altitude
often means more air layers to peer through, resulting in
increased turbulence, poor seeing, and soft images.

Aldebaran

M35
Mars

β
α

p WINTER VISITOR Astrophotographer Alan Dyer captured Mars (center) as it looped through Gemini during its 2007-2008 apparition. Gemini rises
above the eastern horizon. To the right of the Twins, Orion raises his club and shield to challenge the red-eyed bull, Taurus.
s k y a n d t e l e s c o p e . c o m * J U LY 2 0 1 8

M A R S: N ASA / JPL- CA LTEC H; PH OTO: N ASA / ESA / A L A N DY E R

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Sky and Telescope - July 2018

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