Apple Inc. (Nasdaq: AAPL) is now officially the largest public company in the United States and the largest single component (by market-cap) in the S&P 500 index, replacing Exxon-Mobil (NYSE: XOM), which had held that crown for the past five years.

As of Tuesday’s close Apple had a market value of $336.3-billion, just above Exxon Mobil’s $335.1-billion figure.

On a down day for the market, Apple shares dropped 2.76 percent on Tuesday, but Exxon shares tumbled 4.41 percent.

These two iconic corporations are going in decidedly different directions.

Since July 1, Apple has climbed 6.0 percent in value, while Exxon has plunged 17 percent.
Apple is only the eleventh company to top the S&P 500 index (which was first formed in 1926).

Howard Silverblatt, senior index analyst at Standard & Poor’s,said Apple is now also the biggest public company on the planet.

He also explained that Exxon’s habit of buying back its shares so frequently has likely reduced its overall market cap.

“Exxon.. is the poster child for share buybacks (and share count reduction) [this] may account for some of the change,” he said.
“Apple has not reported buybacks since the second quarter of 2006. While we do not know where the Exxon price would be if they did not do all the buybacks, if we added back the shares they purchased over the past five years for share count reduction, at the current price they would be 17 percent larger than Apple -- however that’s pro forma.”

Apple’s rise to the top has been nothing short of breathtaking.
In December 1980 , the company (then called Apple Computer) conducted its initial public offering at $22 per share. It entered the S&P 500 index in November 1982 (at that time, AT&T was the largest company in the index; and Apple was at 153).

By the end of 1987 (just after the market crash), Apple was the 82nd largest company in the index. However, ten years later, with its fortunes going in reverse, the company plunged down to the 456 position.

After a dramatic turnaround and the development of an array of wildly innovative and popular products, Apple has surged in value. By April 2010, it supplanted Microsoft as the second largest US public company.

Now, it’s on top all alone.