He was dying of congestive heart failure, the result of a heart attack that he'd suffered weeks earlier.
Although he was only 61, his years in the park had prematurely aged him. He wore a beard, and his skin was tanned by his exposure to the sun. He was thin.
He suffered from several ailments. A blood clot in his heart. Gangrene in one foot. Pneumonia in his lungs. For days he lingered, but doctors decided not to take the risk of operating on him.
At 6:03 p.m. on March 20, the man's heart stopped beating.
Days later his body was sent to a military cemetery for cremation after a check on his social security number revealed that he had served in the Air Force. A marble headstone bearing his name was placed on a wall among the names of other cremated veterans at Point Loma's Fort Rosecrans National Cemetery.
No one noticed that the name on the headstone was misspelled, the result of a clerical error on the man's death certificate.
No one at the hospital or at the cemetery knew the man, and no family members attended the placement of his cenotaph.
He was just a weather-beaten, penniless man who sold paintings of cats in Balboa Park who had grown old and died.
Years before he was a destitute painter, Robert Burnham Jr. had inscribed the universe. Writer, astronomer, finder of comets and asteroids and collector of ancient artifacts, Burnham was a singular Arizonan.
He was a scientist whose work at Lowell Observatory in Flagstaff helped advance the understanding of the sun's neighborhood in space.
He was an author whose name has become so familiar to some readers it has become a sort of shorthand, like Audubon to birders, Hoyle to card players, Webster to poor spellers, Robert to parliamentarians.
More than 30 years after its first publication, Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System remains a sort of real-life hitchhiker's guide to the galaxy, a compendium with something to say about nearly every cosmic destination worth visiting.
Part travel guide, part history text, part encyclopedia, it's like a handheld natural-history museum of the universe. And for decades it's held a grip on the imaginations of most people who ply the night skies with telescopes, people who yearn to travel in space and know that they can, any dark and clear night.
Reading Burnham's massive, three-volume work is like reading the notes of an adventurer who has spent a lifetime studying the treasures of a lost civilization: Its 2,138 pages are loaded with tables of data, technical passages and illustrations interspersed with historical arcana and ancient poetry. And all of it is meant as an incentive for the reader to recover those treasures by merely looking upward.
It is rarely compared to other books because there simply is none other like it. No other popular work approaches its utility and completeness; few other scientific texts contain its sense of wonder and even spirituality.
Despite Burnham's abiding fame among skywatchers, few people knew much about the man himself. Partly, that was because of confusion over another man with the same name. An editor at a science magazine, the other Robert Burnham published frequently during the same period that the Celestial Handbook gained popularity, causing readers to assume that the two were one and the same.
But Robert Burnham Jr. published almost nothing else besides his Handbook, and shunned publicity.
He led an extraordinary, but ultimately tragic, life. He also was a bundle of contradictions.
Burnham was a recluse, and yet he craved public recognition. He devoted years of labor to extraordinary, disciplined work, and yet he was incapable of staving off poverty. He was a brilliant writer who had an uncommon memory, yet words failed him in social situations.
He knew the night sky like few other people have, but was oblivious to earthly concerns.
He felt betrayed by his publisher and others who had benefited from his years of remarkable work, and he sank into depression and bitterness at the same time his reputation soared.
His books are revered by tens of thousands, yet he died alone and unnoticed.
And that's apparently just what he wanted.
After vanishing from his Phoenix home in 1986, he resisted attempts by his family to communicate with him. His sister, Phoenix resident Viola Courtney, only learned of her brother's death after he had been dead for two years, and it took her nearly a year longer to find out where he had died. She didn't communicate the news of her brother's death to the community of readers who know his name well.