Ever since it was launched from the temple-like headquarters of the National Geographic Society in Washington in 1888, National Geographic magazine has illuminated the world’s hidden places and revealed its natural wonders.

On Wednesday, the iconic ­yellow-bordered magazine, beset by financial issues, entered its own uncharted territory. In an effort to stave off further decline, the magazine was effectively sold by its nonprofit parent organization to a for-profit venture whose principal shareholder is one of Rupert Murdoch’s global media companies.

In exchange for $725 million, the National Geographic Society passed the troubled magazine and its book, map and other media assets to a partnership headed by 21st Century Fox, the Murdoch-controlled company that owns the 20th Century Fox movie studio, the Fox television network and Fox News Channel.

Under the terms announced Wednesday, Fox will control 73 percent of the operation, called National Geographic Partners, with the balance held by the National Geographic Society. The partnership, based in Washington, will include a portfolio of National Geographic-branded cable TV channels, digital properties and publishing operations, most notably the magazine that has advanced the society’s founding mission — “the increase and diffusion of geographic knowledge.”

The agreement provides a financial lifeline not just for the much-honored magazine, but also for the National Geographic Society itself, the organization’s chief executive acknowledged Wednesday. Like many print publications, National Geographic has been hurt by the onset of the digital era, which has put it on a slow trajectory toward extinction.

National Geographic magazine’s July 2014 issue. (National Geographic)

The magazine’s domestic circulation peaked at about 12 million copies in the late 1980s; today, the publication reaches about 3.5 million subscribers in the United States and an additional 3 million subscribers abroad through non-English-language editions. Advertising has been in steady decline.

“It has become apparent that ensuring the future of the society would require something bold,” the society’s chief executive, Gary Knell, said at an all-staff meeting Wednesday. Continuing as a media organization and potentially absorbing future losses, he said, “presented enormous and real existential risks. We . . . truly believe the path we’ve chosen presents the greatest potential upside.”

In his address to employees, Knell noted that keeping the magazine while cutting costs “would be a short-term fix for a long-term set of issues.”

The society first partnered with Fox in 1997 to launch the National Geographic cable channel, and later a fleet of smaller TV channels. The TV channels have grown into the organization’s most valuable assets; the venture had operating profits surpassing $400 million last year, according to one executive, although the society’s actual dividend from the partnership has not been disclosed.

Until Wednesday, the magazine and other publishing operations had remained fully under the National Geographic Society’s ownership and direction.

Although the partners spoke optimistically about the new marketing and promotional potential of their enlarged venture, the news sent shudders through the magazine’s downtown Washington offices. The one-word reaction from one of the magazine’s journalists: “Dread.”

Some of the anxiety stems from the fear of becoming an afterthought in an enterprise dominated by a shareholder, Fox, that operates a company with $29 billion in annual revenue. It also reflects uncertainty about what could happen to the magazine’s image and vaunted journalism under the Murdoch banner.

Magazine staffers point warily to Fox News Channel and to ­Murdoch-owned tabloids such as the New York Post and Britain’s Sun. They also looked closer to home — to the TV programs produced under the society’s long-running partnership with Fox.

Some of the programming on the National Geographic Channel has been a source of embarrassment to people at the society and its namesake magazine. Series such as “Doomsday Preppers” (about survivalists) and “Banged Up Abroad” (featuring “holiday horror stories”) have proved relatively popular but have caused discomfort within the organization because they are unlike the magazine’s high-minded articles and photography.

Even while she hailed the Fox partnership, Susan Goldberg, National Geographic’s editor in chief, was hesitant Wednesday in assessing the society’s TV collaboration with Fox. “Fox has acknowledged that they have not always represented the National Geographic brand in some of those programs in a way we loved or even they loved,” she said in an interview.

Nevertheless, Goldberg said the partnership will bring more resources and distribution muscle to National Geographic’s digital and print operations. “It’s great news. . . . It’s really a doubling down on our journalism and an investment in our journalism.”

She added: “There’s always anxiety over any change. . . . But holding still and doing the same thing is not a sustainable position.”

But some of the magazine’s subscribers sounded more concerned; many took to social media to register negative reactions. The outpouring “reflects the fear that the needs of shareholders will trump the expectations of [the readers] that National Geographic has nurtured for many years,” said Steve Beck, managing partner of CG42, a consulting firm.

But Beck said it’s not certain that Fox’s involvement will be a negative. Murdoch’s purchase of the Wall Street Journal and its parent company in 2007 hasn’t hurt the Journal’s quality, he said. “He acquired them because they have great content. That’s what he’s doing here. National Geographic has a history of developing really good, high-quality content and nurturing a really loyal audience. He and his team see a valuable asset” that can be improved.

In comments to National Geographic staffers Wednesday, Fox’s chief executive, James Murdoch — Rupert Murdoch’s youngest son — expressed his admiration for National Geographic, saying that he has read the magazine since childhood. In an interview, Murdoch said there were no immediate plans to change the publication.

Knell, the society’s chief executive, said in an interview that the sale will give “a longer runway” to the magazine. Neither Knell nor Murdoch would release data about the magazine’s financial health.

The society will remain a nonprofit education organization, separately governed from National Geographic Partners. The partnership will be governed by a board with an equal number of representatives from Fox and National Geographic. Declan Moore, chief media officer of the society, will become chief executive of National Geographic Partners.

Fox’s $725 million payment to the society will push the organization’s endowment to more than $1 billion and will allow the organization to double its spending on research, science and other projects, the society said.

The society said it would create an education center named for the Grosvenor family — whose members helped found the society and ran it for decades — and dedicated to improving high school students’ geography skills. It also said that it plans to establish a center for cartography, journalism and photography.

There was no word on whether the transaction, which has been approved by the society’s board and is expected to close at the end of the year, will lead to layoffs. Representatives from both sides are to meet next week to discuss how all of the organization’s assets will be integrated into the partnership.