Sky and Telescope - November 2016 - 52

OBSERVING

Exploring the Solar System

"Last Contact"
Percival Lowell made his ﬁnal observation 100 years ago this month.
"Last Contact" and "Certainly Now" would be the last
lines ever penned by Percival Lowell in his observing
logbook, on November 11, 1916. By the following evening, the iconic American astronomer was dead, at the
age of only 61 years 8 months.
Lowell seemed to be in good health, typically brimming with project ideas, as he entered what would be the
ﬁnal year of his life. Although 1915 ended with a heavy
snowstorm in Flagstaﬀ - the depth at the observatory
reached 50 inches - the New Year began with a run of
clear skies.
Mars, always a chief preoccupation on Mars Hill
whenever it was well placed, came to a February opposition that year. Lowell and staﬀ astronomer E. C. Slipher
divided their time between the Red Planet and Saturn,
whose satellites and subtle ring divisions they'd been
measuring for over a year.
Lowell believed he had made an important discovery.
Astronomers had found that the Cassini Division maintains a 1:2 orbital resonance with Saturn's innermost
known moon, Mimas. Lowell and Slipher had recorded
more subtle ring divisions that seemed to be in sync
with other Mimas resonances - except that they were

Below: The night before he died,
Lowell recorded this observation
of Io's reappearance from within
Jupiter's shadow.

LOWELL OBS. / PUTNAM COLLECTION CENTER

LOWELL OBS. / JAMES E. PURDY

Percival Lowell (1855-1916), a wealthy
Bostonian, built a major observatory
in Flagstaﬀ, Arizona, from which he
intensely studied the planets.

November 2016 sky & telescope

consistently located too far out. Invoking a nifty bit of
celestial mechanics, Lowell showed that the discrepancy
could be explained if Saturn consisted, beneath the outer
rind of its clouds, of a series of nested shells of diﬀerent
oblateness that spun at diﬀerent speeds. Saturn was, he
said, "rotating like an onion in partitive motion."
If Saturn had such an interior structure, why not
Jupiter as well? This was one of the burning questions
Lowell hoped to answer as the giant planet neared opposition in late 1916.
On October 19th, returning to his Flagstaﬀ observatory after an exhausting barnstorming lecture tour of
colleges in the Paciﬁc Northwest and the West Coast, he
and Slipher began a nightly vigil of the moons of Jupiter.
As the divisions of the rings of Saturn had seemingly
disclosed the internal structure of that planet, Lowell
hoped that the moons of Jupiter would do the same for
its gas-giant cousin.
Although many of their measurements were of the
already well-studied Galilean satellites, they also hoped
to lay the wires of their ﬁlar micrometer on little Amalthea, the ﬁfth satellite of Jupiter, which had been discovered visually by E. E. Barnard with the 36-inch refractor of Lick Observatory in September 1892 and whose
motion was not at the time completely understood.
Circling just 110,000 kilometers from Jupiter's cloudtops,
Amalthea seemed an ideal probe of the gravitational ﬁeld
around the giant oblate spheroid of the planet.
But would it even be visible in the 24-inch Clark?
It would certainly be challenging. Amalthea can only
be seen near its western and eastern elongations, at
which its maximum separation from the planet's disk
never exceeds one Jupiter diameter. Also, since it's four
million times brighter, Jupiter completely swamps Amalthea (magnitude 14.1 at best) in its glare.
To disclose the shy little moon, special techniques
are needed. Lowell and Slipher employed an eyepiece
occulting bar to block Jupiter. Their strategy succeeded;
indeed, they observed Amalthea at its eastern elongation
on the very night that Lowell returned from his demanding lecture tour.
For the next several weeks, during which the skies
stayed persistently clear, Lowell and Slipher scrutinized
the Galileans and the tiny inner moon at every opportunity. On Saturday evening, November 11th, after a long

Sky and Telescope - November 2016

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