Sky & Telescope - August 2020 - 52

JUNE 2020 OBSERVING
Exploring the Moon by Charles Wood

Mare Humboldtianum

Mare Marginis

August
Librations

Mare Smythii

This month presents an
excellent opportunity to view
several limb-hugging maria.
Mare Australe

AUGUST 2 02 0 * SK Y & TELESCOPE

of superposed impact craters, is the
third-youngest large impact basin on
the Moon. The best opportunity to view
the basin in August is on the evening of
the 31st due to the nearly 6° libration
in latitude that favorably tips the entire
northeastern limb our way. The Moon's
phase is nearly full on the 31st, making the dark Mare Humboldtianum the
easiest feature to spot. Look closely and
you should detect the bright mountainous peaks of the 320-km-diameter
inner basin ring that contains the mare.
About two-thirds of the way from the
inner ring to the dark-ﬂoored Endymion crater is the basin's 620-km-wide
main rim. This feature rises just above
the surrounding terrain but drops about
500 meters to the smoother terrain
between the basin rim and inner ring.
Nearly 40° south of Humboldtianum
lies Mare Marginis, an irregular dark
patch with no evidence for an underlying basin. It was assumed in the 1970s
that the Marginis lavas ﬁlled an ancient
impact basin whose rim subsequent
impacts had long since destroyed. But

p Four limb-hugging maria are well placed for
observers during the last week of August. This
simulation shows the Moon as it will appear on
the evening of the 28th.

laser topography from orbiting spacecraft
failed to detect the expected depression,
and high-resolution gravity data from
NASA's GRAIL (Gravity Recovery and
Interior Laboratory) spacecraft found
that Marginis has only a weak and diffuse positive gravity anomaly, meaning that there is no signiﬁcant excess
mass hidden below the surface such as
found beneath other circular mare. As
a result, Marginis is no longer considered a basin. Yet Marginis retains two
interesting distinctions - it is antipodal
to the Orientale Basin, and it contains a
faintly visible swirl just northeast of the
crater Goddard.
South of the distinctive, dark-ﬂoored
crater Neper is Mare Smythii and
its underlying basin. Mare Smythii is
two-tone, with lavas in the northern
half being distinctly darker and younger
than those in the southern portion.
Interestingly, the lavas in all the eastern

N ASA'S SCIEN TIFIC VISUA LIZ ATION ST UDIO
CREDIT

f you think of the Moon as a head, it
does an endless series of rolling tilts
with cycles of nodding up and down
"yes" and shaking back and forth "no".
These librations tip the polar regions
alternately towards and away from
the Earth (libration in latitude) and
similarly for the east and west limbs
(libration in longitude). Librations can
be as extreme as 8°, and anything over
5° gives a noticeably better perspective
on features near the limb than the typical view. The last week of August shows
most of the eastern limb tilted Earthward by 5° to 6°, providing worthwhile
opportunities to compare four different
lunar maria and basins of different ages
and character.
From north to south, the maria in
question are Humboldtianum, Marginis, Smythii, and Australe. Like other
lunar maria, most of these dark lavas
ﬁll large impact basins - most, but not
all of them, as we shall see.
Along the Moon's northeastern
limb is Mare Humboldtianum and
basin, which, based on the number

Sky & Telescope - August 2020

Table of Contents for the Digital Edition of Sky & Telescope - August 2020