THE STRUGGLES TO FIND THE NINTH PLANET
BY CLYDE W. TOMBAUGH
Clyde Tombaugh (at age 22) shown with his homebuilt
9-inch reflecting telescope that he built before being hired by the Lowell Observatory to
search for Percival Lowell's "Planet X."
have been written and published on the triumphal detection and discovery of Pluto. As the
sole survivor of the Lowell Observatory staff in that frustrating year of 1929, I thought
that it might be of some interest to relate the "down mood" and emotions
experienced by those persons involved. I shall give a little history to illustrate the
setting and circumstances.
In the period 1905 thru 1907, the first
photographic search was made with a 5 inch lens having a useable field of 5 degrees along
the Invariable Plane which is inclined 1.6 degrees to the Ecliptic. Lowell scanned the
pairs of plates by laying one plate over the other and using a hand magnifier. At that
time, Pluto was outside of the search strip, and also too faint to be recorded.
By carefully analyzing the residuals of
Uranus, Lowell hoped to predict the position of Planet X in the sky -- namely Libra. In
1911, a photographic search was made with the 40-inch reflector, using exposures of ten
minutes or less; but due to tangential coma, the field was limited to one degree. After
one year, this attempt was abandoned.
Next, Lowell borrowed a 9-inch camera from
Swarthmore College 1914 to 1916. Nearly 1,000 plates were taken over a considerable area
of the sky. Curiously, only two of those plates contained the image of Pluto near the
plate limit. The Lowell Observatory now owned a Carl Zeiss Blink-Comparator for scanning
the pairs of plates, but the images of Pluto were missed. I was shocked when I learned
that these two plates were taken on March 19 and April 7, 1915, respectively- about the
worst possible time for detection. The advantage of opposition time was ignored.
Considering Pluto's position in 1915, those search plates should have been taken in
December. The 'quick find' attitude seemed to dominate in the earlier searches, resulting
in improper procedures, and lack of thorough checking of planet suspects. They were
looking for a planet of the 13th magnitude (10 times brighter than Pluto).
In 1915, Lowell drastically revised the
preferred region of search to eastern Taurus. This region is in the Milky Way, and the
plates were extremely rich in stars, which slows down the rate of scanning.
On November 16, 1916, Lowell dies suddenly
of a stroke at the age of 61- a discouraged and exhausted man. No more planet searching
was done for the next 13 years.
Lowell's widow tried to break the will
regarding the estate. Unfortunately, this bitter litigation dragged on for 10 years
robbing the resources of the estate thru court costs and excessive attorney's fees. This
resulted in a severe curtailment of funds for publications and acquisition of a more
powerful planet search camera.
Guy Lowell became the observatory sole
trustee. In 1925, he purchased 13-inch unfinished disks of glass from Reverend Joel
Metcalf's widow, hoping that funds would be available later to complete the camera. Guy
Lowell dies in 1927, and the trusteeship passed to Roger Lowell Putnam, Percival's nephew.
He was determined that his Uncle Percy's Planet X should be found. After being turned down
by some possible sources for funds, Roger persuaded his uncle, Dr. A. Lawrence Lowell
(then President of Harvard University) to provide $10,000. By building most of the
mounting in the observatory shop, they could stretch the $10,000 to complete the
With funds assured, V. M. Slipher plunged in
to the work on the design. The very important task was to find a highly competent optician
to finish the objective lens, which was to be a 3-element Cooke type astrograph. One was
C.A.R. Lundin of the Alvin Clark firm. He made the 40-inch Lowell Reflector in 1909.
Completion was delayed because one of the lens components had been ground quite thin. Then
came the shocking bill of $6,000.
Putnam promptly sent Lundin a check for the
final payment. This upset Slipher because he felt that a final test should have been made
by actual photographic performance.
Here is where I came into the picture. I had
made several drawings of Jupiter and Mars at the eyepiece of my newly constructed 9-inch,
f9, reflector in the Fall of 1928, in Western Kansas. I decided to send them to the Lowell
Observatory. Perhaps, my drawings were compared with their current photographs. Anyway,
Slipher as impressed, and invited me to join the staff on a 3 month trial basis.
After spending 28 hours in a chair car on
the Santa Fe, I arrived at Flagstaff in the early afternoon of 15 January 1929. Dr.
Slipher met me at the depot and drove me up Mars Hill. The yellow pine forest was in
marked contrast to the treeless plains of Western Kansas. I was rather unnerved by it all,
everybody were strangers, 1,000 miles from home, and not enough money in my wallet for a
return ticket to home, Slipher assigned a bedroom to me on the second floor of the
Administration Building (now known as the Slipher Building).
The next day, I was taken out to the new
dome and shown the new 13-inch telescope. Stanley Sykes and his son were installing some
hand controls on the mounting. For the first time I was told that the new telescope was to
find Lowell's predicted Planet X. This sounded exciting.
The 13-inch objective lens arrived from the
Alvin Clark firm on 11 February, and was bolted onto the front end of the steel tube. The
next few weeks were spent in making tests.
Although, a few 11x14 inch plates were tried
at first, Slipher decided to use 14x17 inch plates. These large wooded plateholders were
constructed which would bend the plates to the exact Petzval curvature of the field. Every
plate was adjusted on a special measuring table in the dark room before being placed in
the telescope. The focus was critical, with a tolerance of only 1/5th of a millimeter in a
focal length of 1,700 millimeters (66 inches) at f5.3.
Some plates snapped in two during the one
hour exposure in the cold dome with a large BOOM-- badly startling me. Slipher seemed
resigned to accepting a few broken plates, but I could not. The observing schedule was
full enough without having to make repeat plates. I had to find a way to prevent
plate-breaking. By changing the order of locking in the screws on the back of the plate
holder, the plates were no longer under undue strain, and the breaking completely ceased.
One of the earliest problems was a
"pulsing" in the drive mechanism at certain hour angles. It required a lively
manipulation of the east-west buttons on the control box. This really worried Slipher,
because this kind of guiding can soon wear-out the observer. Finally, I tried throwing the
telescope a little out of balance by moving the counterweights on the Declination axis,
forcing the driving clock to exert a little more torque. This would keep the worm screw in
constant contact with the teeth on the 4-foot worm wheel. This stopped the pulsing-making
for much easier guiding during the long exposures.
Soon another problem presented itself-double
images! While guiding on one hour exposures, a sudden slight shift on the Declination axis
end-wise occurred-- displacing the guide star off of the crosshairs by about 15 or 20
arc-seconds. Quickly, I got the guide star back on the crosshairs. Upon development, all
the star images were double- most discouraging!
On another plate a few nights later, it
happened again. I tried tightening the large collars on the Declination axis, but I could
not eliminate the "chug" until they were too tight to move the telescope in
Declination. I noticed that the "chug" always occurred at 0h42m west of the
meridian. So when an exposure was due to go thru this critical angle, I initiated the
procedure of swinging the telescope westward to chug the axis before starting the
exposure. This stopped the double image curse.
Still other problems loomed. This larger
camera was sensitive to the "steadiness of seeing" which could be ignored with
smaller scale cameras. I found that the soft images with "2" seeing would not
match well with the pin-prick images taken under "4" seeing.
It is of the utmost importance to have the
plates of a pair well-matched in regard to images quality and magnitude limit- otherwise,
the pair was unblinkable. It was an advantage to take the plates as near to the meridian
as possible. If not, the duplicate plate had to be taken at nearly the same hour angle--
because of the differential refractive index of the atmosphere. I had to schedule taking
the duplicate plate on a night of about the same quality of seeing. Also, if there was a
slight haze, I would prolong the exposure by 10 or 15 minutes over the hour to maintain
the same magnitude limit-- otherwise it is most distractive in blinking.
The old Greek philosopher, Socrates, is
credited with saying: "Know thyself". I say: "Know your telescope".
In April, 1929, the planet search actually
got underway. Since the up-dating of Lowell's favored position was not (sic: should be
"now") in Gemini. Slipher wanted Gemini photographed immediately. It required 3
pairs of 14x17 inch plates to span the width of Gemini. The scale was 30 millimeters per
During latter April and early May, the two
Sliphers blinked the pairs of Gemini plates in 2 weeks-- much too fast to scan some
700,000 stars! They missed seeing the images of Pluto on the Delta Geminorum pair. The
progress in blinking is inversely proportional to the number of star images that one must
see in scanning. The star density in the Milky Way was shocking-- up to 400,000 star
images per plate in Western Gemini!
I had just come upstairs from the darkroom,
when I saw V. M. Slipher removing the last pair from the Blink-Comparator. "Did you
find Planet X?" I asked. In a resigned voice, he said, "No, we didn't find
anything." He appeared to be very sad-- as if all hope had fled. Naturally, he wanted
to be the one to find Planet X. He had met defeat, and he knew it. Perhaps, Planet X was
in some other region of the Zodiac. Because of their other work, no one of the senior
staff could devote much time to the laborious task of blinking. I have often wondered what
they expected to see on plates taken so far from 'opposition'. By this time, all the
asteroids in Gemini would be moving eastward again after a brief temporary stationary,
even Pluto just a little past its western stationary point.
I began to realize that Slipher was under
pressure. Now they had the super-camera, they had better find Planet X.
By the middle of June, one hundred big
plates had accumulated, but only a few pairs had been blinked. I was exhausted from a
heavy 2-week run at the 13-inch telescope, when V. M. Slipher came to my office and said
they wanted me to 'blink' the plates. I shuddered at the prospect of this grim assignment.
After blinking two pairs, I realized that I had a dilemma. How was I to distinguish an
asteroid near its stationary point from Planet X?
During the Flagstaff rainy season, I studied
the changing positions of Uranus and Neptune from the 1928 and 1929 American Ephemeris.
The advantage of taking the plates at opposition became obvious. Also, that for a given
region in the Zodiac, there was only a one month observing window.
In September 1929, I started taking plates
in Pisces, and blinked them in the following full moon period. I encountered a few false
planet suspects of the 16th and 17th magnitude on almost every pair of plates. These were
beyond the reach of the 5-inch companion camera. These had to be checked with a third
plate taken on a third date. Taking three plates had the additional advantage in that one
could select the best matched pair for blinking. Each succeeding month, I marched 30
degrees eastward to keep up with the opposition region. Several times during the Fall of
1929, V. M. Slipher would come to the Blinker-room to inquire if I was finding anything,
(namely Planet X). As the Milky Way was approached in Taurus, the great number of
star-images slowed down the blinking.
In January 1930, I re-photographed Gemini.
On the crystal clear night of 21 January, I took the first plate of the Delta Geminorum
region. Soon after I started the exposure, a strong Northeast wind came up, and the 3rd
magnitude guide star was darting about wildly, and swelled up to over 2 times the angular
diameter of Jupiter-completely fading away at times. I never saw worst seeing in my life
before or since. I continued the exposure until the end of the hour and quit for the
night. The images were badly swollen, but showed Pluto's image in the right place-
consistent with the shift on the 23 and 29 January plates, which I detected at about 4:00
PM Mountain Time on 18 February 1930.
The following years, 1931-1932, I
photographed two strips around the sky, one on each side of the Zodiac. After learning of
Pluto's 17 degree inclination, it seemed best to search thru a wider area. I always
calculated the necessary overlap when there was a gap in time between adjacent plate
regions. I continued the search at greater distances from the Zodiac until 1943 when World
War II engaged my teaching Navigation in a Navy V-12 school, set up on the Arizona State
College at Flagstaff.
1929 was a very hectic year compared to the
triumphant year of the discovery of Pluto in 1930.
shadow: After the discovery, Mrs. Lowell wanted to name the new planet "Lowell".
She soon changed her mind and wanted the planet names "Constance". This went
over like a lead balloon at the observatory, and V. M. Slipher chose to ignore this
delicate matter. Instead of Plutonium, you all might have known it as
with permission from the author, 4/5/95
points of view regarding the search for the ninth planet, the reader is invited to read
William Graves Hoyt's book Planets X and Pluto, published by The University of Arizona
W. Tombaugh passed away on January 17, 1997 in his home in New Mexico. He was 90 years