The relative trends in share prices of the two companies suggest that the oil company may be knocked off its top spot - and possibly as soon as next week

If past trends are any guide, then some time in the next few months, Apple will become the largest company in the world by market capitalisation - passing the mighty Exxon corporation, which is presently valued at $331.2bn, compared to Apple's $274.2bn. (Microsoft, since you ask, is valued at $219.3bn.)

Remember that market capitalisation isn't about profit; it's about the share price multiplied by the number of outstanding shares (which is how one reaches a valuation for Facebook, or any small company when venture capitalists buy a chunk; or for any publicly-listed company). It can be described as the market's guess at the net present value of the total future profits of the company.

In the case of Exxon, it's been a tough couple of years; its share price has drifted down. Meanwhile Apple, with the iPod, iPhone and most recently iPad, has been going from strength to strength. (Apple has even passed PetroChina, formerly the world's biggest by market cap, presently worth about $262bn.)

How will we know when it's happened? Well, Apple has 913.6m shares outstanding (issued); Exxon, 5.092bn. A quick bit of maths shows that when Apple's share price reaches or exceeds 5.573 times that of Exxon, its valuation is also greater.

At present, it's sitting just below the 5.0 mark - so the differential needs to grow by another 10%, though WolframAlpha suggests that that could happen by January, based on current trends. (The picture shows a "random walk" forecast based on the share prices' previous movements.)

Exxon has of course come a long way down from its peak - in 2009 it touched about $500bn in market cap. But for Apple, it's been an enormously long way back from 13 years ago, when it was valued at just $2bn in May 1997, "reflecting Apple's loss of market share in an increasingly Windows-dominated world."

How things change. Though of course for Apple if it does take on the mantle of the world's most valuable company, there's only one way to go subsequently.

And meanwhile we might wonder: if Exxon is falling, where is the new energy company - based on wind, solar, nuclear or something else - to replace it? When there's a suitably big company that doesn't rely on extracting fossil fuels for its revenue, we'll know we're in a new energy paradigm.

But for now, keep watch on that Apple/Exxon ratio. No doubt there will be some glasses raised in Cupertino if it hits 5.5. And with Apple due to announce quarterly results on Monday evening - during which it will announce how many more million iPads and iPhone 4s have been sold - it might be worth watching that ratio when the US markets open on Tuesday lunchtime.