Fox’s ”American Idol” was again down sharply on its second night of the season, with CBS comedy ”The Big Bang Theory” edging the show by a whisker in the adults 18-49 demo. While ”Idol” has been beaten in the past by special programming like the Winter Olympics, this is believed to be the first time a regular series has bested the primetime powerhouse in the key sales demo.

According to Nielsen’s final nationals, Thursday’s one-hour edition of ”American Idol” averaged a 5.7 rating/15 share in adults 18-49 and 18 million viewers overall , down 28% in the demo vs. the comparable night a year ago. This came one night after its premiere demo score (7.4) represented a 24% fall-off from last year.

Though ”Idol” was the night’s No. 1 show, it was CBS comedy ”The Big Bang Theory,” celebrating its 100th episode , that edged ahead from 8-8:30 p.m. in adults 18-49 (5.3/14, 15.8 million viewers overall), one year after ”Idol” crushed ”Big Bang,” 7.0 to 4.2. ”Idol” still led last night’s battle in total viewers, 16.2 million to 15.7 million.

”Idol” easily dominated the 8:30 p.m. half-hour as CBS’ newbie “Rob” 3.5/9 and 11.4 million for runner-up ”Rob” on CBS, which declined by 12% from its strong premiere of last week. ”Big Bang” itself came within a tick of its week-ago average.

CBS also was looking good at 9 p.m. with rookie drama ”Person of Interest” (3.2/8 in 18-49, 14.3 million viewers overall), which was up a tick from last week for a series high; the show seems to be benefiting from a stronger lead-in as well as reduced crime drama competition at Fox, where ”The Finder” (2.2/5 in 18-49, 6.8 million viewers overall), while up from last week thanks to its ”Idol” lead-in, is not nearly as strong as the show that had been airing at 9, ”Bones.”

”Person of Interest” placed second in 18-49 to ABC’s ”Grey’s Anatomy” (3.5/8 in 18-49, 9.3 million viewers overall) while moving ahead in adults 25-54 (4.5/10 to 4.2/9). The Eye also stayed on top, at least among the broadcast networks, in all key categories with ”The Mentalist” (2.9/8 in 18-49, 14.0 million viewers overall), which was up week to week while ABC’s ”Private Practice” was down (2.1/5 in 18-49, 6.1 million viewers overall).

NBC opened meekly with ”30 Rock” (1.6/4 in 18-49, 4.1 million viewers overall) and then edged up at 8:30 p.m. with ”Parks and Recreation” (1.9/5 in 18-49, 4.3 million viewers overall), which logged its best 18-49 score since Nov. 17 and tied with ABC’s ”Wipeout” for third in 18-49 at 8:30. ”Parks” also closed the gap a bit with CBS’ ”Rob” in adults 18-49, trailing by 4 shares after lagging by 6 shares a week ago; and in adults 18-34, the NBC comedy moved ahead (2.3/7 to 2.0/6).

Also for NBC, ”The Office” (3.0/7 in 18-49, 6.0 million viewers overall) was on par with last week, placing third in 18-49 and first in 18-34 (3.4/10), but ”Up All Night” (1.9/5 in 18-49, 4.0 million viewers overall) fell a tick from its modest Thursday premiere of last week. And drama ”The Firm” doesn’t figure to stay in its 10 p.m. slot much longer after dipping to a 0.9/2 in 18-49 and 3.4 million viewers overall.

At CW, ”The Vampire Diaries” (1.1/3 in 18-49, 2.7 million viewers overall) held up well opposite ”Idol,” dipping a tick in most categories and just 5% in total viewers. Also down a bit was 9 p.m. rookie drama ”The Secret Circle” (0.7/2 in 18-49, 1.7 million viewers overall).

The 18-49 averages for the night: Fox, 3.9/10; CBS, 3.5/9; ABC, 2.5/6; NBC, 1.7/4; Univision, 1.5/4; CW, 0.9/2.

In total viewers: CBS, 14.1 million; Fox, 12.3 million; ABC, 7.1 million; NBC, 4.2 million; Univision, 3.7 million; CW, 2.2 million.