(CNN) The Smithsonian museums have been closed since January 2 because of the government shutdown.

The loss of access to the 19 museums of the Smithsonian Institution -- the world's largest museum, education, and research complex -- has been tough for the thousands of people who visit them everyday. But it's been even harder on the Smithsonian's bottom line and its employees.

The 173-year-old institution is losing about $1 million a week because of the partial government shutdown, Smithsonian secretary David J. Skorton said in an op-ed for USA Today on Tuesday.

"The closure of restaurants, shops, IMAX theaters and other operations is costing the Smithsonian approximately $1 million in revenue each week," Skorton wrote. "These losses are not recoverable. They will have a lasting effect on our budget for this fiscal year. And that effect grows each day."

When the shutdown first began on December 22, the Smithsonian managed to keep its museums and the National Zoo, which it also runs, open for 11 days, thanks to carry-over funds from last fiscal year. But those funds ran out on the second day of this month.

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