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skyandtelescope.com • AUGUST 2019 53
usually linear segments. This gives rays
their characteristic feathery and dis-
continuous look, as visible in those seen
near Pytheas and the more splotchy
ones from Tycho visible on the fl oor of
Pitatus. Each bright segment includes
a patch of secondary craters as well as
rock fl our.
Looking straight down on Tycho, a
striking non-uniformity is seen: the
near-absence of rays to the west. This
“zone of avoidance” indicates that
the projectile that impacted the lunar
surface to form Tycho came from the
west at a low angle, perhaps only 20°
to 25° above the lunar surface. The
momentum of the projectile carried
bright ejecta downrange for hundreds
of kilometers. If you look closely, you
can see that the young craters Proclus
and Thales have bright rays as well as
distinct zones of avoidance.
One aspect of rays still not under-
stood is their pattern near their originat-
ing crater versus those seen farther out.
For example, Copernicus is surrounded
by a circle of ray brightness extending
about one crater diameter on all sides.
Emerging from that are around 15 to 20
rays, some short, others more exten-
sive. The ray to the east of Pytheas is
nowhere near radial to Copernicus, and,
in fact, if extended back toward Coper-
nicus, it completely misses the crater’s
east rim. Similarly, Tycho’s bright ray
that tracks southwest over Longomon-
tanus is tangent to Tycho’s rim. How
does this occur, particularly without an
appreciable lunar atmosphere?Like Copernicus, the 29-km-wide
crater Kepler is surrounded by a large
halo of bright material, with rays
extending some 200 to 300 km. Notice
that those rays radiating west of Kepler
have two narrow zones of avoidance.
One bright ray extends to the north-
west and another goes southwest. If you
observe when the Sun is about 25° above
Kepler so that both the rays and the
surface topography are visible, you’ll see
that between Kepler and the dark mare
areas with few rays lies a short linear
ridge, and to its northwest, two more
hills. The 500-meter-high ridge and hills
seem to have blocked Kepler’s rays, indi-
cating that the material must have had
been ejected nearly horizontally.pLeft: The rays produced by ejecta from the impact that created Kepler were impeded by hills and a long ridge to the crater’s west. Right: The crater
ray east of Pytheas contains many secondary craters originating from Copernicus.Also, notice the wispy ray in the
middle of the dark zone (seen in the
image at top left) that got through the
gap between the ridge and the hills.
Two reasons that rays are still not
fully understood is that there are very
few measurements of their lengths,
something that can now be easily
accomplished using LROC’s Quickmap
software (quickmap.lroc.asu.edu). The
other reason is there currently isn’t a
standardized classifi cation system. So
that begs the question: What are you
waiting for?¢Contributing Editor CHUCK WOOD
frequently explores the LROC Quickmap
to further his understanding of the Moon.qPatchy ray material originating from Tycho is visible on the fl oor of Pitatus.KeplerHillsRidgeUnblocked
raysPytheasCopernicus
secondary
craters