Sky and Telescope - July 2015 - 12

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News Notes

ECLIPSES I Not-Quite-Total Lunar Eclipse?
Sunlight refracts in atmosphere
Penumbra

SKY & TELESCOPE DIAGRAM

Red "sunset-sunrise"
light leaks into umbra

Umbra

Penumbra

A little sunlight refracts through our atmosphere and into Earth's umbra, coloring the lunar disk
during totality. The atmosphere also makes the umbra appear 1% to 2% larger than it should be.
(Not to scale.)

An intriguing eclipse of the Moon on
April 4th (S&T: Apr. 2015, p. 50) delighted
observers from western North America to
Australia and the Far East. What makes
it intriguing is whether this was a "barely
total" eclipse, as had been almost universally predicted - or "barely partial," with
a hair-thin rim of sunlit lunar surface
remaining just outside the umbra of

Earth's shadow. Many observing reports
received by Sky & Telescope suggest it was
the latter. But the most detailed mathematical analysis suggests that it was
probably just barely total.
The core issue is that our atmosphere
enlarges Earth's shadow by a not-quiteexact amount. The U.S. Naval Observatory, which routinely adds 2% to the

GALAXIES I New Dwarfs near Milky Way
Two teams of astronomers went panning for gold in the Southern Hemisphere sky - and both struck it rich.
Between the two, the teams discovered
nine candidate dwarf galaxies near the
Milky Way. Future observations will
determine whether these are really dwarf
galaxies or merely globular clusters.
The teams both used data from the
Dark Energy Survey, an optical and nearinfrared project at Cerro Tololo InterAmerican Observatory in Chile. Combing
through the ﬁrst year of images taken
by the Dark Energy Camera, a team led
12

July 2015 sky & telescope

by Keith Bechtol (University of Chicago)
found eight clusters of stars well outside
the boundaries of the Milky Way; the second team, led by Sergey Koposov (University of Cambridge, UK), found the same
eight dwarf candidates, plus one more
near a gap in the detector.
The dwarf candidates range in size
from 120 to 1,300 light-years across. The
nearest, Reticulum 2 at a distance of
almost 100,000 light-years, is the strongest candidate. Even though it's small
(200 light-years across), it's elongated,
so it's less likely to be a globular cluster

umbra's "geometric" radius to account
for this eﬀect, predicted 12.2 minutes of
totality for April's eclipse. The French
national almanac oﬃce and eclipse guru
Fred Espenak instead use the "Danjon
method," and both predicted just 4.7
minutes of totality. However, Chicago
amateur Curt Renz points out that neither method includes Earth's oblateness,
which is about 0.3%. This tiny shortfall
in our planet's polar radius, normally
insigniﬁcant, becomes a factor in such a
borderline case.
Eclipse specialist David Herald conducted a full analysis that includes both
oblateness and the best new information
about the size of the umbra from a massive study of lunar eclipse crater timings
he recently undertook with Roger Sinnott
(S&T: June 2015, p. 28). Herald concludes
that April's lunar eclipse had a magnitude
(a measure of its completeness) of 1.002.
In other words, by the numbers, it was
total by just one part in 500, putting the
theoretical edge of the umbra beyond the
limb of the Moon by a mere 4.6 arcseconds at maximum eclipse. But the result
remains uncertain, because Herald and
Sinnott ﬁnd that the atmosphere's eﬀective eclipsing layer can diﬀer in thickness
by a few kilometers for reasons unknown.
■ J. KELLY BEATTY & ALAN MACROBERT

posing as a dwarf galaxy. The second
most-likely dwarf galaxy, Eridanus 2, is
also stretched out and lies the farthest
away at a whopping distance of 1.2 million light-years.
Astronomers will need to measure the
total mass of each object to determine
whether they are all really dwarf galaxies.
Unlike in globular clusters, most of the
mass in galaxies resides in dark matter.
The teams will collect spectra and measure the stars' velocities and bulk motions
to determine their hosts' mass and
whether these clusters of stars are moving
as satellites of the Milky Way or not.
■ MONICA YOUNG

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Sky and Telescope - July 2015

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