Australis, Austrinus, Australe - in constellations

Question

I was having a look at the official constellations, and I noticed three with similar names:

• Corona Australis
• Piscis Austrinus
• Triangulum Australe

Now the "Austral" definitely means "southern", as these constellations are otherwise known as the Southern Crown, Southern Fish, and Southern Triangle, respectively. (Also the origin of the name "Australia".) Predictably, they're also all in the southern celestial hemisphere as well. My question is - if they're all the same word, presumably of Latin origin, then why are they spelled in three different ways? I'd assume some sort of different noun-forms, which Latin is notorious for, but since they're all used in the same context, you'd think they'd be in the same declension. Anyone know why these names (and specifically these forms of the word) were chosen for these constellations?

Answer 1

Australis and austrinus are both Latin adjectives, and they have similar meanings. The first means ‘southern’ and the second means ‘from the south’. Those who named the constellations presumably had some reason for making the distinction.

Australis is a third declension adjective which, in the nominative, has the ending –is in both the masculine and feminine singular. Corona is a feminine noun, so the adjective has the –is inflection. Triangulum is a neuter noun, and the neuter inflection of third declension adjectives like australis is, in the nominative singular, -e.

Austrinus is a second declension adjective which has the inflection -us in the nominative masculine singular. Piscis is a masculine noun, so austrinus correctly agrees with it.