Because the AP says so blog.

Page-one headline of the Tribune today, exactly as written: UN blesses U.S. plan to shift power to Iraq. I’ve seen this style usage before in this newspaper, and it makes me wonder -- why no periods for UN but periods for U.S.? Maybe the issue of periods for the United Nations was the subject of months of heated debate, culminating in a resolution in the General Assembly banning the use of periods in the abbreviation, which it called a “neocolonial practice.” Then the Tribune, in the spirit of international cooperation and harmony, decided to go along with the UN usage.

More likely, it reflects a mandate of the AP Stylebook. My copy is in my office, so I can’t look it up just now, but I suspect that’s the reason. AP says periods for U.S., none for UN. If so, it’s an example of a foolish consistency on the part of the Tribune. It looks silly in that head. Devotion to the AP way of doing things can reach silly levels among some editors.

I can also report that (and this will be the last time I mention it) according to today’s paper, that about 1000 people showed up at the Adler Planetarium yesterday morning to see the Transit of Venus. I figure that represents the hard-core astro-buffs. Lazy duffers like myself, who knew about the event, but didn’t go, probably represented 100 times as many people in the metro area. If that assumption is true -- and it’s only speculation -- that would mean 100,000 people out of 9.1 million or so in the metro area understood what was going on, celestially speaking. Nice to be part of a knowledgeable elite, especially if you don’t have to get out of bed early to be part of it.