It ticks along at a remarkably steady pace of about one new person every 10 seconds.
With only slight fluctuations, that rate has held remarkably steady for the last 100 years and is projected to continue almost unchanged for 50 more.
And in its methodical way, the U.S. Population Clock is now closing in on a milestone.
Sometime around the middle of this month, the U.S. Census Bureau clock will ring in the 300 millionth American.
Like the return of a comet or the close of a century, this blip in history will have far more emotional resonance than practical effect.
In fact, we will never know the name of the person who turns the counter to eight zeroes.
Gerber Products Co., the baby food manufacturer, asked the U.S. Census Bureau for guidance in anointing a newborn the 300 millionth American. But it turned out there wasn't much the bureau could do, said spokesman Robert Bernstein, because no one actually counts each new American.
The clock is an estimate based on an algorithm that takes into account births, deaths and immigration. The bureau collects monthly birth and death numbers from the National Center for Health Statistics, Bernstein said. Net immigration is derived from the American Community Survey, an annual polling of several million U.S. residents.
Right now the formula is one birth per seven seconds, one death per 13 seconds and one net increase in immigrants per 30 seconds. With the number of deaths subtracted from the number of births, immigration accounts for about 40% of population growth.
The Census Bureau hasn't forecast the day the milestone will occur, but the algorithm points to Oct. 16.
And while the who and where are purely speculative, that hasn't stopped academics from theorizing.
Demographer William Frey of the Brookings Institution predicts that it will be a Latino boy born in Los Angeles County.
He notes that Latinos are the fastest-growing segment of the U.S. population, that L.A. is the most heavily Latino area and that more boys are born than girls.
Mark Mather, a demographer for the nonprofit Population Reference Bureau in Washington, says the child could just as well be born to a white woman in a Midwest suburb.
He points out that more babies are still being born in America to non-Latino white women, even though the birthrate is lower than that for Latino women.
There's also a two in five chance that it won't be a baby at all but an immigrant, either arriving legally at an airport or illegally crossing the Arizona desert.
The algorithm used to calculate population growth has taken some adjusting over the years as birthrates have contracted or expanded.
In 1918, when World War I and a flu epidemic took a toll, the U.S. population registered its only one-year decline of the 20th century. After the stock market crash of 1929, population growth slowed to one new American every 25 seconds, and at the height of the Great Depression, the rate fell even lower, to just one new person every 43 seconds.
The year 1947 brought the postwar baby boom, nearly doubling the rate from one new American every 22 seconds to one every 12.
From then on, the clock settled into a pace that has hardly varied on an annual basis, never more than one new American every 12 seconds or less than one every 10.
The Census Bureau forecasts growth to continue at that rate through 2050, though forecasts can't take into account future wars, economic upheaval or more subtle social changes. In the late 1980s, the bureau underestimated the tide of coming immigration and forecast U.S. population to top out at about 302 million in 2038.
That was later revised upward to account for the higher than expected immigration and the higher birthrate among Latino women.
Current projections show the population hitting 400 million in 2043.
Continued growth will increasingly derive from immigration as the birthrate of ever more prosperous natives declines. Though it wasn't foreseen at the time, the liberalization of immigration law in the 1960s has been the decisive factor in keeping the population slope moving upward.
If the current national debate on immigration were to end in the sealing of the nation's borders, the United States could find itself, like most other developed countries, facing a declining population.
Though new immigrants still account for less than half of growth, their effect on the population is multiplied by higher birthrates that continue in successive generations. If the flow were to stop, the birthrate of their future generations would eventually decline.
In the nearly 30 years since the last milestone in 1967, when a beaming President Johnson heralded the 200 millionth American, there has been a sharp reversal in how Americans feel about growth.