Cape Town - In a moment of cosmic serendipity, the moon will rise in full this evening for the second time during a single calendar month, paying appropriate homage to the first human to set foot on its dusty surface.

This is the phenomenon of a rare “blue moon” that occurs on average only seven times every 19 years.

This blue moon is occurring on the same day as a private memorial service is held in the US city of Cincinnati to honour former Nasa astronaut Neil Armstrong, who died last Saturday aged 82. Armstrong stunned the world when he stepped on to the moon’s Sea of Tranquillity on July 20, 1969, and spoke the immortal, if slightly fluffed, line “One small step for man, one giant leap for mankind”.

The moon was also full on August 2.

“Once in a blue moon” commonly means something very rare or occasional. However, as the production editor of the online Nasa Science website Dr Tony Phillips points out, most blue moons look pale gray and white, indistinguishable from any other moon – but on rare occasions it really can appear blue.

“A truly blue moon usually requires a volcanic eruption,” he explains. “Back in 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded with the force of a 100-megaton nuclear bomb. Plumes of ash rose to the very top of Earth’s atmosphere, and the moon… turned blue!

“Certain forest fires can do the same trick. A famous example is the giant muskeg fire of September 1953 in Alberta, Canada, when clouds of smoke containing micron-sized oil droplets produced lavender suns and blue moons all the way from North America to England.

“There are plenty of wildfires burning in the hot, dry USA this month.”

Sadly, those particles are unlikely to reach Cape Town to influence our view of it. - Cape Argus

* To read Phillips’s full article, visit http://science.nasa.gov/