The singer's historic sales week becomes even more breathtaking when put into historical context.

The act of purchasing of an album -- whether from your computer or your local store -- has become an increasingly rare thing for folks to do. Other than a small 1.4 percent increase in album sales in 2011, every year for the past 15 years the album has been finding it harder and harder to breathe.

With that in mind, the fact that Adele's 25 is already breaking 24-year-old sales records, just over halfway through its debut week, is a staggering feat. Sending her already-impressive numbers back in time, however, shows just how dramatic the singer's success really is.

25 is projected to sell 2.9 million units in its debut week, slightly above the current record of 2.4 million achieved by *NSYNC in 2000 with the release of No Strings Attached. Album sales reached their historical peak in 2000, falling 60.6 percent through last year. Although digital track sales have partially filled that void, total sales have fallen sharply. No Strings Attached was released when total music sales (albums plus digital tracks) were about 2.5 times higher than today.

When comparing their different eras and sales climates, 25 handily beats No Strings Attached in terms of their respective slices of first-week music sales. Meaning: Adele's share of total music sales during 25's debut week is far greater than *NSYNC's was for No Strings Attached. Adele is on track to capture an estimated 42 percent of recorded music sales this week, considering both her album and track sales (a combined 3.15 million units when every 10 tracks are converted into one "album"). No Strings Attached grabbed a 16.7 percent share of sales in its first week of release.

Another way to compare today's apples to yesterday's oranges is by adjusting for "inflation." Everything from food prices to movie revenues are regularly adjusted for shifting currency values -- for example, $100 in 1980 would go as far as $290 would today. The difference, of course, is that music sales have fallen, not risen, since 2000. Adjusting for that contraction, Adele's projected first-week sales would be equivalent to her selling 7.59 million units in 2000.

It's important to note that some differences between the eras cannot be removed when comparing first-week sales. Because iTunes didn't exist in 2000, No Strings Attached didn't have the 900,000 pre-orders 25 is said to have had. Back then, when online retailers like Amazon and CD Now represented a small share of total sales, nearly all sales came from physical retailers, which would give big albums sale pricing and prominent shelf space for at least a month.

Nevertheless, the music industry has not lost its enthusiasm for following first-week sales as if it were a sport. Labels and artists (ahem) desperately want to reach number one on the charts. Four-week sales may be a better way to measure albums from different eras -- but there are no bragging rights for best four-week sales period.