Question 1

Why is the Southern Cross useful for navigation?

Accepted Answer

Because there is no south pole star. Extending the long axis of the cross about four and a half times its own length lands close to the south celestial pole, which is the southern hemisphere's substitute for looking at Polaris.

Question 2

Where in the sky is Crux?

Accepted Answer

Its boundary is centred near right ascension 12h 27m, declination −61.9°. It stands at its highest around midnight in late March, which puts it in the evening sky for the two months after that and behind the Sun half a year later.

Question 3

Can I see it from where I live?

Accepted Answer

Every part of Crux clears the horizon from latitudes between the south pole and 21° N. Outside that band some or all of the region never rises at all, whatever the time of year — a constellation is fixed on the sky, and it is your latitude that decides how much of the sky you get.

Question 4

What is the brightest thing in it?

Accepted Answer

The brightest catalogued object in Crux is Acrux, at apparent magnitude 0.77 (V). That is apparent brightness — how it looks from Earth — so it mixes how much light a thing gives off with how far away it is, and the brightest object in a region is rarely the most interesting one.

Question 5

How are the stars in it named?

Accepted Answer

The brightest carry Greek letters, assigned roughly in order of brightness — by Johann Bayer in 1603, and for the deep southern constellations by Nicolas-Louis de Lacaille in the 1750s — followed by the Latin genitive of the constellation's name. Fainter naked-eye stars north of about −30° take a number from John Flamsteed's catalogue, which never covered the far south, and fainter still a designation from whichever survey recorded them.

Question 6

What exactly are the boundaries?

Accepted Answer

Lines of constant right ascension and declination, drawn by Eugène Delporte for the IAU in 1930 and referred to the sky as it stood in 1875. Precession has tilted the sky against them since, which is why the boundaries on a modern chart are very slightly askew.

Question 7

Are the stars in Crux actually near each other?

Accepted Answer

Almost never. The stars in a constellation lie at wildly different distances and are not related to one another in any way — the pattern is a line of sight from one particular planet. Two stars that look adjacent can be a hundred times further apart than either is from the Sun.

Question 8

What is the difference between a constellation and an asterism?

Accepted Answer

A constellation is one of the 88 regions the IAU recognises, with a fixed boundary and an official name. An asterism is any memorable pattern that is not one — the Plough inside Ursa Major, the Summer Triangle drawn across three constellations, the Great Square shared between two.

Question 9

Why are the names in Latin?

Accepted Answer

Because the list was assembled in Latin. Fifty of the 88 descend from Ptolemy's second-century catalogue of forty-eight — his ship Argo was later cut into three — and the rest were coined between the sixteenth and eighteenth centuries to fill the southern sky and the gaps. The genitive form is what star names use: Alpha Orionis means the alpha star of Orion.

Crux

BRIGHTEST OBJECTS

COMMON QUESTIONS