The Circinus Constellation

Celestial map of the constellation Circinus, the Pair of Compasses. Credit: Torsten Bronger

Welcome back to Constellation Friday! Today, in honor of the late and great Tammy Plotner, we will be dealing with the compass – the Circinus constellation!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

Over time, the number of recognized constellations has grown as astronomers and explorers became aware of other stars visible from other location around the world. By the 20th century, the IAU adopted a modern catalog of 88 Constellations. One of these is the Circinus constellation, a small, faint constellation located in the southern skies. It is bordered by the constellations Apus, Centaurus, Lupus, Musca, Norma, Triangulum Australe.

Name and Meaning:

Because Circinus was unknown to the ancient Greeks and Romans, it has no mythology associated with it. The three brightest stars form a narrow triangle. The shape is reminiscent of a drawing (or drafting) compass of the sort used to plot sea and sky charts. Nicolas Louis de Lacaille had a fascination with secular science and the thought of naming a constellation after a science tool fascinated him.

Lacaille’s table, showing his representations of the constellations. Credit:

In this case, Circinus represents a drafting tool used in navigation, mathematics, technical drawing, engineering drawing, in cartography (drawing maps) – and which many elementary school age children use to learn to draw circles and in geometry to bi-sect lines, draw arcs and so forth. In this case, the device should not be confused with Pyxis, a constellation associated with a ship’s compass… despite the similarity in names with the Latin language!

History of Observation:

The small, faint southern constellation Circinus was created by Nicholas de Lacaille during his stay at the Cape of Good Hope in the mid-18th century. Circinus was given its current name in 1763, when Lacaille published an updated sky map with Latin names for the constellations he introduced.

On the map he created, Lacaille portrayed the constellations of Norma, Circinus, and Triangulum Australe as a set of draughtsman’s instruments – as a ruler, compass, and a surveyor’s level, respectively. This constellation has endured and became one of the 88 modern constellation recognized by the IAU in 1920.

Notable Features:

Circinus has no bright stars and consists of only 3 main stars and 9 Bayer/Flamsteed designated stars. However, the constellation does have several Deep Sky Objects associated with it. For instance, there’s the Circinus Galaxy, a spiral galaxy located approximately 13 million light years distant that was discovered in 1975. The galaxy is notable for the gas rings inside it, one of which is a massive star-forming region, and its black hole-powered core.

Composite image of the central regions of the nearby Circinus galaxy, located about 12 million light years away. Credit: NASA/Chandra/HST

Then there’s the X-ray double star known as Circinus X-1, which is located approximately 30,700 light years away and was discovered in 1969. This system is composed of a neutron star orbiting a main sequence star. Circinus is also home to the bright planetary nebula known as NGC 5315, which was created when a star went supernova and cast off its outer layers into space.

Then there’s NGC 5823 (aka. Caldwell 88), an open cluster located on the border between Circinus and Lupus. Located about 3,500 light years away, this cluster is about 800 million years old and spans about 12 light years.

Finding Circinus:

Circinus is visible at latitudes between +10° and -90° and is best seen at culmination during the month of June. Start by taking out your binoculars for a look at Alpha Circini – a great visual double star. Located about 53.5 light years from Earth, this stellar pair isn’t physically related but does make a unique target. The brighter of the two, Alpha, is a F1 Bright Yellow Dwarf that is a slight variable star. This contrasts very nicely with the fainter, red companion.

For the telescope, take a look at Gamma Circini – a faint star a little over five hundred light years from the Solar System. In the sky, it lies in the Milky Way, between bright Alpha Centauri and the Southern Triangle. Gamma Circini is a binary system, containing a blue giant star with a yellow, F-type, companion. Gamma is unique because it possess a stellar magnetic buoyancy!

Location of the Circinus constellation. Credit: IAU

For larger binoculars and telescopes, have a look at galactic star cluster NGC 5823 (RA 15 : 05.7 Dec -55 : 36). This dim cluster will appear to have several brighter members which are actually foreground stars, but does include Mira-type variable Y Circini. While it will be hard to distinguish from the rich, Milky Way star fields, you will notice an elliptical shaped compression of stars with an asterism which resembles and open umbrella.

For large telescopes, check out ESO 97-G13 – the “Circinus Galaxy”. Located only 4 degrees below the Galactic plane, and 13 million light-years away (RA 14h 13m 9.9s Dec 65° 20? 21?), this Seyfert Galaxy is undergoing tumultuous changes, as rings of gas are being ejected from the galactic core. While it can be spotted in a small telescope, science didn’t notice it until 25 years ago!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.


The Constellation Caelum

the southern constellation Caelum. Credit:

Welcome back to Constellation Friday! Today, in honor of our dear friend and contributor, Tammy Plotner, we examine the Caelum constellation. Enjoy!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. Until the development of modern astronomy, his treatise (known as the Almagest) would serve as the authoritative source on astronomy. This list has since come to be expanded to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.

One of these constellations is Caelum, which was discovered in in the 1750s by French astronomer Nicolas Louis de Lacaille, and is now counted among the 88 IAU-recognized constellations. It is the eight-smallest constellation, with an area just less than that of Corona Australis (another southern constellation), and is bordered by the Dorado, Pictor, Horologium, Eridanus, Lepus and Columba constellations.

Name and Meaning:

The name Caelum, in Latin, literally means “chisel”, though the word can also mean ‘the heavens’. According to an antiquated school of thought, the sky (caelum, ‘sky, heaven, the heavens’) is rounded, spinning, and burning; and the sky is called by its name because it has the figures of the constellations impressed into it – just like an engraved (caelare) vessel. In Lacaille’s imagination, he saw this constellation as therefore representing “les Burins”, or the tools of a sculptor.

IAU map showing the location of the southern Caelum Constellation. Credit: IAU and Sky&Telescope magazine
IAU map showing the location of the southern Caelum Constellation. Credit: IAU and Sky&Telescope magazine

Notable Features:

The constellation of Caelum has very little to offer observers using either binoculars or telescopes, with only four primary stars visible to the unaided eye and only eight stars with Bayer/Flamsteed designations. However, Gamma Caeli is a widely separated binary star system with a distance of 0.22°. It is composed of a magnitude 4.5 red giant and a magnitude 6.34 white giant.

For an extreme challenge, try locating Alpha Caeli. At an approximate distance of 65.7 light years from Earth, this yellow-white F-type main sequence dwarf with an apparent magnitude of +4.44 has an an extremely faint companion. It is magnitude 13, with a position angle of 121º and a separation 6.6″.

If you like long-term variable stars, you could always look for R Caeli, a long-term Mira-type that ranges from from 6.7 to 13.7 every 391 days. Or how about X Caeli, a Delta-Scuti type star? It’s changes are much faster – but far less noticeably. It changes by one tenth of a magnitude (6.3 to 6.4) every three hours and fourteen minutes.

For those looking for Deep Sky Objects, a big telescope is necessary. This is because NGC 1679 is about all there is to see, and it doesn’t appear lightly. Located about two degree south of Zeta Caeli, there’s not even a magnitude guess at this small spiral galaxy – but it does measure about 3.2 arc minutes, and appears to be an irregularly-shaped galaxy. There are indications that it may be a dwarf starburst galaxy.

Seen as “Cela Sculptoris” in the lower right of this 1825 star chart from Urania's Mirror. Credit: Sidney Hall/Library of Congress
The Caleum constellation, depicted as “Cela Sculptoris” in the lower right of this 1825 star chart from Urania’s Mirror. Credit: Sidney Hall/Library of Congress

History of Observation:

Caelum was introduced by Nicolas Louis de Lacaille in the 1750s to help chart the southern hemisphere skies. Lacaille gave the constellation the French name Burin, which was originally Latinized to Caelum Scalptorium (“The Engravers’ Chisel”). English astronomer Francis Baily would alter shorten this name to Caelem, as suggested by fellow astronomer John Herschel.

In Lacaille’s original chart, the constellation was shown both as two types of chisels – a burin (a steel-engraving chisl) and an échoppe (an etching chisel) – although it has come to be recognized simply as a chisel.

Finding Caelum:

Though it is quite small and faint, locating Caelum is not difficult if you know where to look. Using stellar coordinates, you can find it by looking to the first quadrant of the southern hemisphere (SQ1), and then tracing it to between latitudes +40° and -90°. Or, start by picking out Canopus (the brightest of Carina‘s stars), pan due east, and then spot the small chisel between its neighbors.

Caelum is bordered by Dorado and Pictor to the south, Horologium and Eridanus to the east, Lepus to the north, and Columba to the west. The Caelum constellation occupies an area of 125 square degrees, and can be seen during the month of January at around 9 pm.

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations.


Messier 6 – The Butterfly Cluster

M6 open cluster (NGC 6405). Credit: Ole Nielsen

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at Messier 6, otherwise known as NGC 6405 and the Butterfly Cluster. Enjoy!

In the late 18th century, Charles Messier was busy hunting for comets in the night sky, and noticed several “nebulous” objects. After initially mistaking them for the comets he was seeking, he began to compile a list of these objects so other astronomers would not make the same mistake. Known as the Messier Catalog, this list consists of 100 objects, consisting of distant galaxies, nebulae, and star clusters.

This Catalog would go on to become a major milestone in the history of astronomy, as well as the study of Deep Sky Objects. Among the many famous objects in this catalog is M6 (aka. NGC 6405), an open cluster of stars in the constellation of Scorpius. Because of its vague resemblance to a butterfly, it is known as the Butterfly Cluster.

Continue reading “Messier 6 – The Butterfly Cluster”

Messier 4 (M4) – The NGC 6121 Globular Cluster

This M4 globular cluster, as imaged by the Wide Field Imager at ESO’s La Silla Observatory. Credit: ESO

During the late 18th century, Charles Messier began to notice that a series of “nebulous” objects in the night sky that he originally mistook for comets. In time, he would notice that they were in fact something significantly different. With the hope of preventing other astronomers from making the same mistake, he began compiling a list of these in what would come to be known as the Messier Catalog.

Consisting of 100 objects, the catalog became an important milestone in both astronomy and the research of Deep Sky objects. Among the many famous objects in this catalog is the M4 loose globular cluster (aka. NGC 6121). Located in the Scorpius (Scorpio) Constellation, this great cluster of ancient stars is one of the closest Messier Objects of its kind to Earth.

Continue reading “Messier 4 (M4) – The NGC 6121 Globular Cluster”