The Mule Driver Who Measured the Expanding Universe

In the late 1920s, a man sat in the cold, dark dome of the largest telescope on Earth and kept a faint smudge of light on a slit for hours at a time, night after night. The smudge was a galaxy in Pegasus called NGC 7619. It was so dim that its light had to be collected for 33 hours to make one usable spectrum, and 45 hours to make another. When the plates were finally developed and measured, they showed that NGC 7619 was rushing away from us at about 3,779 kilometres per second, twice as fast as any galaxy anyone had ever clocked.

That number, published in March 1929, helped turn a hunch into one of the most famous laws in science: the farther away a galaxy is, the faster it recedes. The universe is expanding. The man at the eyepiece was Milton Humason. He had quit school at 14. He had first come to Mount Wilson as a hotel worker and a mule driver, and he had joined the observatory staff as its janitor.

His partner, Edwin Hubble, got the law, the space telescope and most of the glory. But the redshifts that pushed that law out across the universe, and proved it, were very largely Humason’s. This is the story of how a kid who hauled freight up a mountain trail ended up measuring the speed of the cosmos.

A boy on the mountain

Milton La Salle Humason was born on August 19, 1891, in Dodge Center, Minnesota, according to the finding aid for his papers at the Huntington Library. His family later moved to California, and in 1905, as a teenager, he went to a summer camp on Mount Wilson, the peak above Pasadena where George Ellery Hale was building a new solar observatory. He fell for the place. That fall he dropped out of high school and went to work on the mountain.

Mount Wilson was not easy to reach. When the observatory got going, the summit sat at the end of a steep and dusty nine-mile footpath, with no way up except on foot, on horseback or on a mule. The Mount Wilson Observatory’s own history says the Snow solar telescope went up in 60 trips by mule train, and the next big instrument, the 60-inch telescope, was carried by mules and a primitive truck. Humason worked at the Mount Wilson Hotel and then as a mule driver. According to the Huntington finding aid, he led mule-powered wagons up the Mount Wilson Toll Road with timber, iron and other supplies for the telescopes and buildings.

The mountain gave him more than a job. In 1910 he met Helen Dowd, the daughter of Merritt C. “Jerry” Dowd, the observatory engineer whom the observatory’s history calls its chief electrician. They married in 1911. For a while the couple moved down to the valley, where Humason worked as foreman on a relative’s ranch in La Verne. But, as one profile put it, he missed the mountain.

The janitor’s job

In 1917 his father-in-law told him that a janitor’s job would soon open at the observatory. Humason took it, knowing that the post could lead to work as a relief night assistant on the telescopes. The Director’s annual report for 1918 announced the completion of the giant new 100-inch telescope. In the back of the same report, the name Milton Humason appeared for the first time, as janitor.

The 100-inch itself had nearly not happened. In 1906 the Los Angeles businessman John D. Hooker offered to pay for the mirror, and the glass was ordered from St. Gobain in France on September 19 that year. The disk that arrived in Pasadena in December 1908 weighed about 9,000 pounds. The observatory’s history calls it “wine bottle glass,” and it had sheets of bubbles between its layers. It was nearly rejected. After years of failed recastings, the staff tested the original disk again, ground it and figured it. By 1916 it was ready: almost 101 inches across and still, the observatory says, the largest solid glass mirror ever made.

On July 1, 1917, the mirror went up the toll road on a truck. Nearly 200 men went with it, many holding ropes on the switchbacks, on an eight-hour trip at about one mile per hour. Some were guards, because the day before someone had phoned in a threat to blow up the mirror on the way.

First light, on the night of November 1, 1917, was a scare. Pointed at Jupiter, the telescope showed what Walter Adams remembered as “six or seven partially overlapping images.” The dome had been left open in the day and the sun had warmed the mirror. Hale and Adams agreed to meet again at 3 a.m. When they did, a star came to a brilliant pinpoint in the eyepiece. The 100-inch would remain the largest telescope in the world until 1949, and the former mule driver was about to become one of its best users.

Humason learned fast. Hugo Benioff taught him to use the observatory’s 10-inch photographic telescope, and he became so good at it that Benioff recommended he take over the work. He began taking plates for Harlow Shapley’s observing program too. In 1919, he used the 10-inch to hunt for “Planet X,” the extra planet beyond Neptune predicted by W. H. Pickering.

He nearly found it. After Clyde Tombaugh discovered Pluto in 1930, Seth Nicholson and Nicholas Mayall went back through Humason’s old plates and found that Pluto was on them. The Huntington finding aid says it sat so near the edge of the plates, where the images were poor, that it is no surprise he missed it. A popular story says Pluto fell on a flaw in the emulsion. That seems unlikely, because it appears on four plates taken over three nights.

From janitor to astronomer

Humason’s patience and skill at the telescope caught Hale’s attention. In 1919, Hale appointed him to the scientific staff, a man with no degree and no high-school diploma. The Christian Science Monitor says the move came “in the face of stern protests”; the Linda Hall Library says it caused some disgruntlement. The Huntington finding aid dates his promotion to the Stellar and Nebular Division to 1920.

1919 was also the year Edwin Hubble arrived at Mount Wilson. The two could hardly have been more different. Hubble was a Rhodes scholar who had studied law at Oxford and, the observatory’s history notes, had been a talented boxer. Humason was the one-time “mule skinner.” The observatory calls them “perhaps the most unusual team in the history of astronomy, and one of the most important.”

By 1924 Humason was Hubble’s chief assistant, using the 100-inch to look for variable stars in spiral nebulae. That search paid off in a big way. Hubble found Cepheid variables in the Andromeda nebula and other spirals. Cepheids are stars whose pulsing period gives away their true brightness, a relationship worked out by Henrietta Leavitt at Harvard. They showed that the spirals lay far outside our own Milky Way. When the result was announced on New Year’s Day 1925, the old debate over whether the spirals were “island universes” was effectively over. They were galaxies.

The problem of the fleeing nebulae

Knowing how far away galaxies were was only half the puzzle. The other half had come from Arizona. In 1912, Vesto Slipher at Lowell Observatory became the first to measure the shift in the spectral lines of a galaxy. When light from a receding source is stretched to longer, redder wavelengths, the lines in its spectrum slide toward the red. This is the redshift, and its size tells you how fast the source is moving away. By 1917 Slipher had measured 25 spiral nebulae, and all but three were moving away from us, some at remarkable speeds.

Spectroscopy, the same tool that revealed helium in the Sun, was now telling astronomers that the galaxies were fleeing. Slipher’s list eventually reached 41 galaxies, 36 of them receding. Arthur Eddington printed it in his textbook on relativity. The fastest, at about 1,800 kilometres per second, was a galaxy called NGC 584.

But there was a ceiling. The spectra of faint galaxies are agonizingly dim, because a spectrograph spreads an already feeble light into a rainbow. To push further out, someone would have to sit at the 100-inch and keep a nearly invisible patch of light perfectly on the spectrograph slit for hours, through many nights, without a single slip. That someone was Humason.

Seventy-eight hours on one galaxy

In his short paper on NGC 7619, communicated to the National Academy of Sciences in January 1929, Humason explains that about a year earlier Hubble had suggested he try to measure fainter nebulae that sat in groups. Galaxies in a group should all be at roughly the same distance, so the brightest members could stand in for the faint ones.

NGC 7619, a galaxy in a group in the constellation Pegasus, was one of these. Humason took two spectrograms of it with a spectrograph on the 100-inch, one with an exposure of 33 hours and one of 45 hours. The plates were measured by Miss MacCormack and by Humason. The result was a velocity of +3,779 kilometres per second, “twice as large as any hitherto observed,” as he put it. It was more than double Slipher’s record for NGC 584.

Humason also noted that the new figure fell on the line extended from Hubble’s relation for nearer nebulae. Hubble’s own paper, “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae,” appeared in the same issue of the Proceedings.

The diagram that changed the universe

Hubble’s 1929 paper plotted velocities against distances and showed a rough straight line: the farther the galaxy, the faster it seemed to be moving away. The cosmologist Robert Kirshner, writing in PNAS in 2004, points out that Hubble cited no source for most of his velocities. Every galaxy in the table was one of Slipher’s, and the numbers were almost identical to Eddington’s list, “except for the four new ones from his Mount Wilson colleague, Milton Humason.” For galaxies such as NGC 7619, which had only Humason’s new redshift, Hubble ran the relation backward and used the velocity to estimate the distance.

Hubble’s slope, the number now called the Hubble constant, came out at about 500 kilometres per second per megaparsec. That is far too high, because his distance scale was off, but the straight line was real. Hubble also knew the evidence was still thin. In the paper he wrote that “Mr. Humason at Mount Wilson has initiated a program of determining velocities of the most distant nebulae that can be observed with confidence,” and that new data “if confirmatory, will lead to a solution having many times the weight.”

Pushing the line outward

The new data came fast. By 1931, according to the American Institute of Physics, Hubble and Humason had velocities for 40 more galaxies, 26 of them in eight groups. The greatest distance was now more than a hundred million light years, 50 times farther than the most distant spirals in the 1929 paper, and the line still held. In 1934 they showed that isolated galaxies followed it too. By 1936 they had a hundred more, and the relation was “still sensibly linear to a distance of 70 million parsecs,” more than 200 million light years.

In 1931 Albert Einstein visited Mount Wilson, and a well-known photograph shows Humason, Hubble and Einstein together in the Hale Library. The papers drew praise from the Dutch cosmologist Willem de Sitter, and Hubble’s reply, quoted by Kirshner, shows how careful the two observers were about what their numbers meant:

“Mr. Humason and I are both deeply sensible of your gracious appreciation of the papers on velocities and distances of nebulae. We use the term ‘apparent’ velocities to emphasize the empirical features of the correlation. The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.”

Humason kept going for a quarter of a century, refining the art of exposures that ran over several nights. The Huntington finding aid says he photographed the spectra of about 600 galaxies before he retired, first with the 100-inch and, from 1949, with the new 200-inch Hale Telescope at Palomar. The Christian Science Monitor puts his total at 620. His great summary paper, “Redshifts and Magnitudes of Extragalactic Nebulae,” written with Nicholas Mayall and Allan Sandage and published in the Astronomical Journal in 1956, ran to 66 pages and listed redshifts for 620 galaxies from the Mount Wilson and Palomar program, plus about 300 from Lick Observatory. Kirshner notes that today’s telescopes measure redshifts about 1,000 times faster than in Hubble’s time.

Secretary of the observatory

The janitor ended up in the front office. On October 1, 1948, Humason became Secretary of the Observatory, succeeding Alfred Joy. The post handled public relations, which meant answering letters and phone calls from students, photo seekers and people with their own theories of the universe.

His colleague Robert S. Richardson remembered that many of those writers spent their time “denouncing such impostors as Newton and Einstein.” Answer one, and another letter came back demanding more. Humason’s policy, Richardson wrote, “was to answer every such letter once, but a second time –no.” Otherwise, he said, you would soon find you were working not for the observatory in Pasadena, California, but for some correspondent “in Pasadena, Newfoundland.”

Humason never earned a degree. In 1950 the University of Lund in Sweden gave him an honorary doctorate. He retired on July 1, 1957, and moved to Mendocino in northern California, where the observatory staff still sometimes asked his advice about the telescopes, especially the 100-inch. In 1961, already retired, he discovered a comet, now designated C/1961 R1 (Humason). It was an unusually active “giant” comet whose closest approach to the Sun was well beyond the orbit of Mars. He died at his home on June 18, 1972, aged 80.

Whose law is it, anyway?

Credit for the expanding universe has always been tangled. In 1927, two years before Hubble’s paper, the Belgian priest and astronomer Georges Lemaître published a derivation of the velocity-distance relation, with an estimate of its slope, in a little-read French-language journal. When an English translation appeared in 1931, the key passage was missing. Some astronomers suspected deliberate censorship. In 2011 the astrophysicist Mario Livio reported in Nature that a letter in Lemaître’s archive showed Lemaître had cut it himself.

In October 2018, members of the International Astronomical Union voted to recommend calling it the Hubble-Lemaître law. According to Science, 4,060 votes were cast: 78% in favour, 20% against and 2% abstaining. It is only a recommendation, and some historians, including Helge Kragh, called the exercise bad history. Nobody proposed adding Humason’s name, though the 1929 diagram leans on Slipher’s velocities, the most distant point came from Humason’s plates, and Hubble himself had relied on Leavitt’s Cepheid relation for his distances.

Even the story of the janitor-astronomer has its own myth to watch out for. The historian William Ashworth, writing for the Linda Hall Library, warns that journalists tend to turn Humason into a bootstrap legend. He points to an oral history Humason gave in 1965, which shows a modest man who knew exactly what he was good at, tracking faint objects and measuring redshifts, and who left the theory to others. “Science needs the talents of both the Humasons and the Hubbles,” Ashworth writes. “But it is unwise to imagine that they are doing the same thing.”

The number is still argued over

Humason’s craft, coaxing a spectrum out of almost nothing, is still at the center of cosmology. Astronomers still measure the expansion rate with the same two ingredients, distances and redshifts, though now with space telescopes and instruments that collect the light of thousands of galaxies at once. And they still disagree. Measurements built on the cosmic distance ladder, which still starts with Cepheids, give about 73 kilometres per second per megaparsec. Values inferred from the cosmic microwave background, such as the Planck mission’s 2018 figure of 67.4, come out lower. The gap, called the Hubble tension, is now statistically significant and still unexplained.

Mount Wilson’s telescopes still stand above Los Angeles, the same peak where Albert Michelson once clocked the speed of light. Look at the line on any modern Hubble diagram, stretching out across billions of light years, and the first long step along it was taken by a man who once walked mules up that mountain.

If you enjoyed this, read how Cecilia Payne-Gaposchkin worked out what stars are made of, another outsider whose measurements changed astronomy.

Further reading