Apple is the world's biggest company -- again

Apple (AAPL) on Monday once again became the world's most valuable publicly traded company ahead of energy giant ExxonMobil (XOM).

Shares in Apple added more than 1 percent in an otherwise down market, extending their remarkable rally after posting blowout fiscal first-quarter earnings last week. ExxonMobil, meanwhile, slipped fractionally amid a broader selloff related to the European debt crisis.

Apple's market capitalization, or the total value of all its shares outstanding, stood at $422 billion at the end of Monday trading, vs. $410 billion for Dow component ExxonMobil. Apple has added about $30 billion in market value since Jan. 23. For perspective, that's essentially equivalent to the entire market cap of computer maker Dell (DELL), drugstore chain Walgreen (WAG) or global shipping giant FedEx (FDX).

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Apple first surpassed Exxon as the world's largest listed company back in August, and the two firms have traded places a number of times since then.

Apple was briefly the world's largest company in intraday trading last week, as well as for short-lived periods in September and October. However, Apple finished the year far behind ExxonMobil, following the death of co-founder Steve Jobs and initial disappointment over the introduction of the iPhone 4S.

To compare the market caps of Apple and ExxonMobil going back to August 2011, see the chart below (courtesy of data from S&P Capital IQ):

(Credit: CBS MoneyWatch, Capital IQ)