The sky — astronomy

The Southern Cross: Guide of the Southern Sky

The Southern Cross (Latin: Crux) is the smallest of the 88 IAU constellations and is best seen from the southern hemisphere. Extend its long axis about four and a half times its length and you reach the region of the south celestial pole, which points you toward true south.

The smallest of the 88 constellations

The Southern Cross, known in Latin as Crux, is small but unmistakable. The International Astronomical Union (IAU) divides the sky into 88 constellations, and Crux is the smallest of them by area. Its four bright stars form a compact kite or cross shape that is easy to recognise.

The main stars are named with Greek letters:

• Acrux (Alpha Crucis) is the bottom star and points toward the south. It is a multiple star, and even a small telescope splits it into at least two components.
• Mimosa (Beta Crucis) is a very bright star on the left-hand end of the crossbar for southern observers.
• Gacrux (Gamma Crucis) is the top star. It is a red giant, and its orange-red tint is visible to the naked eye.
• Delta Crucis is the fainter star at the other end of the crossbar.

There is also a fainter fifth star (Epsilon), but you do not need it for identification.

In ancient times this part of the sky could be seen from more northern latitudes. Because of the slow wobble of Earth’s axis (precession), it has gradually shifted south in the sky of the northern hemisphere. Today Crux lies below the horizon for most of Europe and North America.

Pointing to the south celestial pole

The northern hemisphere has a fairly bright Pole Star close to the north celestial pole. The south has no equivalent. The point near the south celestial pole lies in the constellation Octans and is marked only by faint stars. That is why observers in the south use the Southern Cross as a guide.

The method is simple:

1. Find the Southern Cross and identify its long axis, running from Gacrux to Acrux.
2. Extend that line beyond Acrux by about four and a half times the length of the cross.
3. The point you reach is close to the south celestial pole.
4. Drop an imaginary vertical line from that point straight down to the horizon. The spot where it touches is roughly due south.

You can double-check with two bright stars nearby in the constellation Centaurus, often called the Pointers: Alpha and Beta Centauri. Imagine the midpoint of the line between them and draw a perpendicular from it. That line also leads toward the cross and the pole region. Alpha Centauri is part of the nearest star system to the Sun, about 4.4 light-years away.

This technique is only approximate. For serious navigation, people use a compass and proper instruments. The sky is a lovely complement and a good skill to practise.

Beware the False Cross

Beginners often confuse the Southern Cross with an asterism called the False Cross, formed of stars in the constellations Carina and Vela. It is slightly larger but fainter, and it has no bright pointer stars beside it. The real cross also sits next to a dark patch, described below.

The Southern Cross on flags

Because it is so recognisable, the Southern Cross appears on the flags of several countries and territories in the southern hemisphere, including Australia, New Zealand, Brazil, Papua New Guinea and Samoa. On the Brazilian flag, the stars represent the sky over Rio de Janeiro on a particular date, and the Southern Cross is among them. In this way an astronomical constellation has become a symbol of national identity.

What else is in the southern sky

The area around Crux is full of attractions:

• The Coalsack is a dark nebula, a cloud of gas and dust that blocks the light of stars behind it. It is visible to the naked eye as a dark patch against the Milky Way.
• The Jewel Box (NGC 4755) is a bright open star cluster near Beta Crucis. In binoculars or a small telescope its stars sparkle in different colours.
• Omega Centauri is the largest globular star cluster visible to the naked eye from the southern hemisphere.
• The Magellanic Clouds are two irregular companion galaxies of the Milky Way, visible as faint, fuzzy patches.
• The Carina Nebula is a vast cloud of glowing gas where new stars are being born.
• The centre of our Galaxy lies toward Sagittarius and Scorpius, which ride high overhead on southern winter evenings.

To see them, look for a dark site with little light pollution and give your eyes about twenty minutes to adapt. More advice is in how to watch the sky.

When and where to look

The cross is easiest to observe from the southern hemisphere. Across most of Australia, New Zealand, southern Africa and South America it climbs high at certain times of year, and from the far south it almost never sets. Closer to the equator it sits lower, and from northern mid-latitudes it never rises at all. For exact times, check a planetarium program or sky app set to your location and date.

Positions in Astronativa are astronomically exact. The interpretation is for inspiration. · Glossary · Terms · Privacy