Photos circulating online claim to compare the crowd size at former president Barack Obama's 2009 inauguration and Donald Trump's inauguration Friday.
An estimated 1.8 million people attended President Barack Obama's first inauguration.
The swearing-in ceremony began around 11 a.m.
"The best way is by aerial photography ... Take pictures and count heads, but even that is fraught," Seife said, according to Vox. "Pictures are taken at particular times, and (crowd) density may change over time. You're not going to see everybody. People will be popping in and out of buildings. There are people under trees. You can get a pretty sizable estimate, but it's not perfectly accurate."
"Almost everyone who has tried to make a crowd estimate has a vested interest in what the outcome of the estimate is," Seife said in a in 2011 interview. "Whenever you see a crowd estimate, you have to wonder where it's coming from."
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