The Doppler effect (or Doppler shift), named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague, is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source.

It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

In this paragraph, what is `relative`?

Six years ago, lawyer-banker-scholar Charles Morris wrote a prophetic book - `Two-Trillion-Dollar Meltdown: Easy Money, High-Rollers and the Great Credit Crunch` - that foresaw the 2008 Great Recession before it clobbered America and the world.

Now Morris has reversed course and sees good times ahead. His forthcoming book, `Comeback,` predicts that surging U.S. energy independence will bring a buoyant rise in American manufacturing and jobs.

The implication of this passage is that you should pay attention to Charles Morris because:

An oil tanker, also known as a petroleum tanker, is a merchant ship designed for the bulk transport of oil. There are two basic types of oil tankers: the crude tanker and the product tanker. Crude tankers move large quantities of unrefined crude oil from its point of extraction to refineries. Product tankers, generally much smaller, are designed to move petrochemicals from refineries to points near consuming markets.

What is the primary difference between crude tankers and product tankers?