Sky & Telescope - September 2024 - 72

BEGINNER'S SPACE by Sean Walker
o
When Is the Best Time
to Observe Mars?
MARS IS A UNIQUE PLANET in many
ways. It's the only planet in the solar
system whose surface can be viewed
directly in a telescope. And unlike the
other outer planets, which reach opposition
every year, Mars does so once
every two years.
Because of its reputation and brightness,
Mars is a tempting target for
new observers and their telescopes.
Unfortunately, the planet frequently
disappoints, because it only appears big
enough to impress in telescopes for a
few months every two years. So, when
is the best time to cast your gaze toward
the famed Red Planet?
As discussed in this column in the
March 2023 issue (page 72), the outer
planets look biggest and brightest when
appearing opposite to the Sun in our
sky, a period known as opposition. But
because Jupiter, Saturn, Uranus, and
Neptune are so distant, the difference is
relatively insignificant in the telescope.
Because Mars is comparatively nearby,
Mars in 2024−2025
Best time to observe
10
11
12
13
14
7
8
9
5 20 5 20 4 19 4 19 3 18 2 17 4 19 3 18 3
Sept Oct Nov Dec Jan Feb Mar April May
OBSERVING SEASON This graph shows the angular size and magnitude of the Red Planet
throughout this coming apparition. Mars should yield satisfying telescopic views on steady nights
between November 12, 2024, and March 10, 2025.
72 SEPTEMBER 2024 * SKY & TELESCOPE
the planet's disk dramatically grows in
size as opposition nears. The apparent
size of Mars varies between a minuscule
3.5 arcseconds at its farthest distance
to a maximum of 25.6″ at its nearest
apparitions. Also important is the date
of closest approach, which often occurs
a few days before or after opposition.
This time, closest approach is January
12, 2025, four days before its January
16th opposition.
During the lead-up to closest
approach, it's generally accepted that
good views of the planet typically come
when the planet's apparent diameter
reaches 10″ and greater. At that point,
most any telescope will resolve the largest
dark surface markings, such as Syrtis
Major, Sinus Meridiani, Mare Cimmerium,
and others. The bright white
polar caps will also be clearly visible, as
will ice clouds and, if you're lucky, the
occasional yellowish dust storms. That
luck might not be considered good if the
storm spreads across the entire globe.
Aphelion
of Mars
THE CYCLE OF MARS Due to the elliptical
nature of the planets' orbits, Mars comes
closer to Earth in some years than in others in a
period that repeats over 15 years. This diagram
shows the position of Mars and Earth at opposition
between 2018 and 2033.
Each apparition of Mars is different
due to the elliptical nature of its orbit.
That's why some approaches are closer
than others. For example, the closest
approach next January is a distant or
aphelic opposition, in which the planet
will only reach 14.6″. Compared to the
most recent closest or perihelic apparition,
which occurred on July 27, 2018,
when the planet achieved an angular
size of 24.3″, it will appear a bit more
than half that size this time around.
But just because Mars won't be as big
as it can get during the next apparition,
you shouldn't ignore it - especially if
you observe from northerly latitudes.
The planet will be at its best at the same
time it's favorably positioned in the
most northerly extent of the ecliptic,
placing the planet high in the sky when
it climbs to the meridian.
So, when exactly does Mars reach
that 10″ apparent diameter threshold?
That occurs this year on November
12th, a date that marks the start of a
four-month observing season that ends
on March 10, 2025, when its disk once
again shrinks to below 10″.
So, get ready for Mars season. The
more often you look, the more you'll
see. And keep an eye on upcoming
issues for more information about
Mars's 2024-25 apparition. ¢
Perihelion
of Mars
t
2018
Apparent diameter (arcseconds)
Closest approach
Jan 12
Opposition
Jan 12
APPARITION GRAPH: BEATRIZ INGLESSIS / S&T; ORBIT: ROY BISHOP /
RASC OBSERVER'S HANDBOOK
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Mar.
Dia
Jun.
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Sept.
Dec.
mete
2022
r
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O
Nov.
a
Apr.
r
M
2025
b
f
i
s
Oct.
May
O
r
2033
b
i
f
2029
E
a
r
t
2020
h
Jan.
Feb.
Aug.
Jul.
2027
2031

Sky & Telescope - September 2024

Table of Contents for the Digital Edition of Sky & Telescope - September 2024