Sky and Telescope - June 2018 - 37
largest in the world until 1962 - to the observatory's instruments.) He signed a 99-year lease for the observatory land in
June 1904 and commenced moving his Yerkes colleagues to
California. His conﬁdence was again rewarded in December
of that year when he ﬁnally received the Carnegie funding
message. Hale resigned from Yerkes Observatory two weeks later.
1908 was a big year on Mount Wilson.
Hale used a modiﬁed spectroheliograph to
show that sunspots were magnetic, the ﬁrst
time that magnetic ﬁelds were identiﬁed on
an extraterrestrial body. Later work would
yield the Hale-Nicholson Law, which states
that magnetic polarities in sunspots are
symmetric across the solar equator and that
polarities in each hemisphere switch from
one sunspot cycle to the next - phenomena that are still
being investigated today (S&T: Jan. 2018, p. 18).
The 60-inch mirror that might have earlier found a home
at Yerkes was installed in its telescope mount in December
1908 after four years of grinding and ﬁguring by Hale's chief
optical engineer, George Ritchey. It was the ﬁrst major telescope to use a coudé focus, which sent light to a point outside
the telescope, allowing astronomers to change instruments
without disturbing the telescope's tracking. The design also
featured a mercury-ﬁlled bearing system that supported 95%
of the structure's weight. Successful operation of the 60-inch
telescope effectively marked the end of refractors for research.
But Hale was planning for a bigger telescope even before
the 60-inch reﬂector was completed. John Hooker, a Los
Angeles business executive, amateur astronomer, and friend
to Hale, offered to pay for the casting and grinding of a
100-inch disk, with the understanding that it would be the
biggest telescope in the world
and would be named after him.
It was a risky commitment, as
the 60-inch telescope hadn't yet
been fully checked out, but Hale
pressed forward in September
1906 with a disk order to the
Saint-Gobain glassworks, the
French company that had cast
the 60-inch disk and the only
operation willing to attempt a
The ﬁrst disk looked like a
failure. It contained air bubbles,
and Ritchey doubted it would
withstand grinding and polishing. Saint-Gobain tried again
with new methods and equipment, but without success. In
desperation, Hale gambled that
the original disk was salvage-
able and decided to have it completed over Ritchey's strong
objections. The mirror was ﬁnished after seven years but,
given their tensions over the mirror, so was the relationship
between Hale and Ritchey (S&T: Oct. 2016, p. 66).
As before, Hale began the project boldly, but without sufﬁcient ﬁnancing: Hooker's money wouldn't
cover critical components like the mount
and housing, and other engineering difﬁculties raised the total price. Eventually,
Hale had to convince Andrew Carnegie to
supply another $10 million via the Carnegie Institution to complete the work.
First light for the 100-inch on November 1, 1917, came with some suspense.
Several hours were needed for the mirror
to adjust to ambient temperatures. Once
the mirror had stabilized, however, it showed its true potential, and attendees realized that Hale had produced another
"world's largest telescope."
Some of the most important work of 20th-century astronomy happened inside the Hooker dome on Mount Wilson.
Edwin Hubble determined that Andromeda was a galaxy outside our own by measuring distances to Cepheid variables and
established that the universe was expanding. A. A. Michelson
used a 20-foot interferometer to make the ﬁrst measurement
of a star's diameter. Fritz Zwicky determined that the gravi-
UNIV ERSIT Y OF CHICAGO / Y ERK ES OBSERVATORY (2)
Hale was the
specialist at a time
wasn't even a
p TOPPING IT OFF The great dome of
Yerkes Observatory was tinned and cemented in the autumn of 1896.
t MAKING PLANS Telescope makers Warner & Swasey Company included this drawing of the Yerkes Observatory's 40-inch
scope, 90-foot great dome, and 75-foot
elevating floor in a 1900 circular describing
their latest work.
sk yandtelescope.com * J U N E 2 018