'Horns' to the heavens

To measure the distance between the stars, clench a fist in a well-known heavy-metal sign.
To measure the distance between the stars, clench a fist in a well-known heavy-metal sign.

TRAWLING the music collections of readers of this column, I think it would be safe to assume I'd find more Debussy than Dio. Perhaps I'm wrong and there's many a heavy-metal fan reading this.

Even so, suggesting there's a connection between astronomy and music loud enough to make your ears bleed sees most readers in a pose resembling Rodin's The Thinker. Stay with me …

Listening to non-astronomers describing positions of sky objects can be confusing and brings a smile to my face. Typical directions sound like this: ''It's just two inches above the bright one'' or ''that fuzzy thing left of the Southern Cross?'' Unfortunately, linear measurements such as these don't cut it when measuring distances between stars.

In order to appreciate the apparent separation of stellar objects in the sky, we need to use an idea of the Babylonians: imagining the sky as an inverted bowl, a line from the horizon - any horizon - through the zenith to the opposite horizon represents half a circle, or 180 degrees. Astronomers refer to the distances between stars in degrees of arc. For example, the distance from the brightest ''pointer'', Alpha Centauri, to Acrux, the brightest star of the Southern Cross, is 15 degrees.

This is where Ronnie James Dio comes in. Every time he ''threw the horns'' (held out his index and little fingers at arm's length in the well-known heavy-metal salute), he was showing us that against the sky, they subtended an angle of 15 degrees.

It doesn't stop there: the width of the little finger's tip at arm's length is 1 degree - easily, and surprisingly, able to cover the full moon. Your thumb's width at arm's length is equal to a sky distance of 2 degrees. Index, middle and ring fingers held together subtend an angle of 5 degrees. To find 10 degrees of sky, employ the width of a clenched fist; and for 20 degrees, it's the distance from tip of thumb to tip of little finger using an open palm. This works no matter the size of the user.

Who would have thought a sign Dio got from his grandmother, and erroneously thought to represent devil's horns by metal fans, had an astronomical application?

This story 'Horns' to the heavens first appeared on The Sydney Morning Herald.