Humans have long had a fascination with lunar events – and with the rise of the Internet, all kinds of moon moments have become media sensations. Consider super moons, harvest moons, and the rare super blood moon total eclipse.
On 31 October, sky-watchers in the Eastern Hemisphere – including Australia and New Zealand – may be tempted to add another lunar moniker to the list: a “black moon,” popularly defined as the second new moon in a month.
However, it’s not exactly going to make for good sky-watching.
New moons occur when the moon’s orbit takes it between Earth and the sun, leaving the lunar orb’s unilluminated side facing Earth.
At night, this phase of the moon is impossible to see: Since new moons are in the same part of the sky as the sun, they rise and set with the sun and are overwhelmed by its glare.
A tiny sliver of the moon during the new moon phase in Northern California [Image: Tim Laman]
New moons can be readily seen only when they pass directly in front of the sun, causing solar eclipses. Otherwise, sky-watchers must look to the days before or after a new moon, when just a sliver of the moon’s sunlit side is visible from Earth.
Usually, new moons occur only once a month, but because there’s a slight disjunct between the moon’s phases—a 29.5-day cycle, on average—and the Gregorian calendar, some months can have two new moons: one at the beginning and one at the end. Joe Rao of Space.com notes that this double-dipping occurs once every 32 months or so.
In this sense, a black moon is like the evil twin of a blue moon, conventionally understood as the second full moon in a month.
But let’s be clear: This new moon—like any astronomical event—doesn’t bring ill tidings or herald the end of days, despite the astrological fear-mongering that has been seeping through the web.