Sky and Telescope - October 2015 - 53

Contributing Editor Charles A. Wood (www2.lpod.org) is coauthor
of the book 21st Century Atlas of the Moon.

Charles A. Wood

Southeast Imbrium
Stratigraphy
0

This timescale
illustrates the
sequence of events
described in the
text. You can create a similar stratigraphic timeline
for any area you
observe carefully
on the Moon.

Dark halo craters

50 km

2
Eratosthenes
3
Archimedes

CHARLES A. WOOD

Mare lava ﬂows

4
4½

Imbrium basin
Pre-Imbrium

With the Sun shining directly down on the lunar landscape, the rim of Eratosthenes is nearly invisible. Not far to its north
are a series of small pits called dark halo craters (arrowed) that punched through a bright crater ray from Copernicus.

Magma must have also risen up along fractures underlying Archimedes, explaining how mare lava partially
ﬁlled the crater's ﬂoor and buried its central peaks.
Shoemaker noticed that the 59-km-wide crater Eratosthenes, which interrupts the Montes Apenninus,
has small secondary craters and faint rays visible at full
Moon on the nearby plains of Mare Imbrium. So Eratosthenes must have formed after the last lava ﬂows in this
area. Moreover, because it maintains some rays (which
erode away rather quickly), it's probably younger than
other craters on Mare Imbrium that lack ray patterns.
Are any nearby craters likely to be younger than
Eratosthenes? There's one obvious candidate: Copernicus. Based on the brightness and extent of its rays and
secondary craters, 96-km-wide Copernicus is the youngest large landform in this area. Its ejecta clearly lie atop
those of Eratosthenes. Ages determined by counts of
small craters superposed on Copernicus' wide apron of
debris indicate that this impact occurred 800 million to
1 billion years ago. This is ancient by Earth standards -
but relatively recent for events on the Moon.
Finally, are any features in this southeast Imbrium
quadrant even younger than Copernicus? Many small
craters a few kilometers in diameter appear younger,
because they display crisp rims that would have been
smoothed and eroded over time.
But can we use the law of superposition to prove that
they're younger than Copernicus? Yes! During full
Moon, point your telescope to the tract of mare just
north of Eratosthenes. They're a challenge to see, but
under high magniﬁcation you might notice about a
half-dozen dark splotches. At the center of each is a 3- to

5-km-wide impact crater gouged into bright ray material from Copernicus. These small pits, known as dark
halo craters, excavated underlying mare lavas and spread
them as pulverized debris on top of the ray.
Within this eyepiece-wide view of southeast Imbrium,
a careful observer can identify landforms created during
4 billion years of lunar history, from battered pre-basin
ruins to very young dark halo craters cutting through
bright rays. Using the benchmarks explored here, now
have a look at every other feature in the ﬁeld of view and
try to decipher where it ﬁts in this lunar sequence. ✦

The Moon * October 2015
27
October 1

Phases
LAST QUARTER

October 4, 21:06 UT
NEW MOON

October 13, 0:06 UT
NASA / LRO

NASA / LRO

Eratosthenes

Age (billions of years)

Copernicus
1

FIRST QUARTER

October 20, 20:31 UT
FULL MOON

For key dates, yellow dots indicate which part
of the Moon's limb is tipped the most toward
Earth by libration under favorable illumination.

October 27, 12:05 UT

Distances

Librations

Apogee
252,518 miles

October 11, 13h UT
diam. 29′ 40″

Perigee
222,739 miles

October 26, 13h UT
diam. 33′ 3″

Byrd (crater)

October 1

Short (crater)

October 22

Mare Humboldtianum

October 27

Sk yandTelescope.com October 2015

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Sky and Telescope - October 2015

Table of Contents for the Digital Edition of Sky and Telescope - October 2015