54 August 2014 sky & telescope
OBSERVING
Exploring the Moon
Looking at the lunar surface from Earth’s perspective,
it’s often diffi cult to detect even signifi cant changes in
topography. Ordinarily, you’d slew your telescope to a spot
illuminated by the grazing light of a rising or setting Sun,
so that even small bumps cast long shadows on the lunar
landscape. This technique works well for hills, scarps,
and crater rims, but it’s less successful for detecting
gentle changes in elevation.
For example, I’ve observed the 77-mile-wide (124-km)
crater Fracastorius hundreds of times. When the seeing
is excellent, I can occasionally spy a thin rille that bisects
the crater fl oor from east to west. Fracastorius formed on
a rim of the Nectaris impact basin that bounds a region of
subsidence inside the basin. The Fracastorius rille marks
where this drop-off toward the deep central basin fl oor
begins. It turns out that this regional change in eleva-
tion is rather extreme: the terrain drops more than 0.5
mile from the rille to the north edge of the crater fl oor —
much more than I expected from mere visual inspection.The Lowdown on Lunar Features
It’s still fun to judge the lunar highs and lows by eye, but
now there’s a much more accurate method. Since 2009
the cameras on NASA’s Lunar Reconnaissance OrbiterLunar Highs and Lows
An online tool lets you measure a crater’s depth.
(LRO) spacecraft have provided an unending torrent of
extraordinary, ultra-high-resolution images of the Moon
— and, thanks to its stereo imagery, the elevations of 100
billion individual points on the surface.
These data are available through the QuickMap inter-
face (target.lroc.asu.edu/q3). I introduced S&T readers
to this impressive, fun-to-use utility in my August 2012
column (page 54), and the 300-terabyte database is so rich
with possibilities that it’s worth a second look.
You might start by using the Path Tool (found by click-
ing the “wrench” icon at the top-right of the screen) to cre-
ate a topographic cross-section across a big crater. Do that
for Copernicus and you’ll fi nd that the rim rises about 0.6
mile above the surrounding terrain, and that the crater
fl oor is about 2.4 miles below the rim crest. Considering
that Copernicus has a diameter of 58 miles, it’s actually
a quite shallow depression — its depth-to-diameter ratio
is only 0.04. (The original, just-formed crater would have
been deeper, but a combination of rim slumping and
pooling of debris fi lled in the fl oor.)
You can use the LRO QuickMap to examine the topog-
raphy of other interesting craters. For example, watch the
Sun rise over 110-mile-wide Petavius when the Moon is
a thin waxing crescent, and you’ll see that its high centralDECEPTIVELY FLAT The lunar crater Fracastorius seems to have a broad, fl at fl oor with a narrow, challenging-to-see east-west rille.
However, altimetry data from NASA’s Lunar Reconnaissance Orbiter shows that the crater fl oor slopes downward toward its northern
rim. In the display from the QuickMap interface at target.lroc.asu.edu/q3, the plot shows elevations along the green line at right.ACT-REACT QUICKMAP