Reverence for the sun deity Huitzilopochtli thrived among the Aztec peoples of Meso america as late as the 16th century. But students of history know that the true zenith of sun worship occurred in the 1970s, when engineers at Caltech constructed the first fully solar-powered George Hamilton and millions knelt at the feet of their fierce, tawny goddess Farrah Fawcett, whose gleaming tusks and shining blonde nimbus were foretold by Montezuma.

Widespread opinion held in those days that nothing could be more natural and invigorating than to start one's summer by getting a nice light sunburn—just to establish a good tanning base for the season, mind you. If you saw beachgoers slathering their skin with something, chances are strong that it was baby oil. For those of us so irredeemably untannable as to need protection against the sun, a bit of searching at the pharmacy might turn up a sunscreen with an SPF as high as 15.

Then those killjoy dermatologists began insistently connecting solar exposure to melanomas and other skin cancers, and America's love affair with the sun slowly set. Fashion certainly has not changed so far as to uphold pallor as the ideal for beauty, but a more healthy moderation has taken hold. Sunbathers know not to bake until blistered and always to wear protection. Almost everyone has heard and internalized some form of the advice, “You can't be too careful with the sun.”

Then again, maybe you can. Beginning on page 62, researchers John H. White and Luz E. Tavera-Mendoza describe the many ways in which vitamin D contributes to human health, from building strong bones to fighting inflammation. Astonishing numbers of people who live in temperate climates may suffer subtly from deficiencies of it, in part because our modern lifestyles prevent us from getting enough sunlight for our bodies to make the vitamin abundantly. Epidemiological evidence hints that too little vitamin D might contribute to the incidence of certain cancers, immune disorders and even infectious diseases. Consequently, while quite wisely shunning sun exposure for the sake of our skins, some of us may have crossed the line and raised our risks for other problems. A little moderation may offer superior defense.

But on the subject of defense, no threat is potentially more grave than that posed by nuclear weapons. Our special report, beginning on page 74, looks at the state of the world's nuclear arsenals, which remain terrifying to contemplate. The graphics speak volumes about who truly poses a danger to whom and what even a single nuclear explosion can do.

Nevertheless, thanks to nonproliferation and disarmament treaties, most of the world's nuclear warheads are aging. Now the U.S. military is looking to replace old W-76 warheads, a pillar of the nation's defenses, with newer, “more reliable” ones. Aside from whether other nations might see that move as a treaty violation, some critics worry how trustworthy such weapons will be when they cannot be tested. David Biello has the details in the second half of our report, starting on page 80; see also our comments on the larger problem of U.S. nuclear strategy (or lack of one) in scientificamerican Perspectives, on page 37. Then ask yourself: How much nuclear defense is too much?