Constellations Old and New

The celestial sphere is a jigsaw puzzle with 88 pieces. The oldest piece is arguably the constellation Ursa Major, The Great Bear. Based on historical writings, prehistoric art, and the knowledge that this group of stars represented a bear in many cultures scattered throughout the world leads scholars to believe that this constellation was first described around 11,000 B.C., perhaps earlier.

The newest constellations are the 17 listed in the table below. Thirteen of these were invented by French astronomer Nicolas-Louis de Lacaille (1713-1762) during his stay at the Cape of Good Hope in 1751 and 1752, and the other four (Puppis, Pyxis, Vela, and Carina) are portions of the ancient enormous constellation Argo Navis, described by Ptolemy (c. 100 – c. 170). Though all of these constellations reside completely in the southern hemisphere of the sky (and thus can be best observed in the southern hemisphere), all but two of them (Mensa and Octans) have a portion that rises above the southern horizon as seen from Tucson, however scant and brief.

Newest Constellations

Constellation Description Declination
Puppis The Stern (of Argo Navis) -51˚ to -11˚
Pyxis The Compass (of Argo Navis) -37˚ to -17˚
Fornax The Laboratory Furnace -40˚ to -24˚
Antlia The Air Pump -40˚ to -25˚
Sculptor The Sculptor's Workshop -39˚ to -25˚
Caelum The Sculptor's Chisel -49˚ to -27˚
Microscopium The Microscope -45˚ to -27˚
Vela The Sail (of Argo Navis) -57˚ to -37˚
Horologium The Pendulum Clock -67˚ to -40˚
Norma The Carpenter's Square -60˚ to -42˚
Pictor The Painter's Easel -64˚ to -43˚
Telescopium The Telescope -57˚ to -45˚
Carina The Keel (of Argo Navis) -76˚ to -51˚
Reticulum The Net -67˚ to -53˚
Circinus The Compasses -71˚ to -55˚
Mensa The Table Mountain -85˚ to -70˚
Octans The Octant -90˚ to -74˚

Which (mostly) northern constellations were added last? Around 70 years prior to Lacaille, Johannes Hevelius (1611-1687) described the seven constellations in the table below. These constellations were first published posthumously in 1690.

Newest More Northerly Constellations

Constellation Description Declination
Lynx The Lynx +33˚ to +62˚
Lacerta The Lizard +35˚ to +57˚
Canes Venatici The Hunting Dogs +28˚ to +52˚
Leo Minor The Lion Cub +23˚ to +41˚
Vulpecula The Fox +19˚ to +29˚
Sextans The Sextant -12˚ to +6˚
Scutum The Shield -16˚ to -4˚

Let us now return to the oldest constellation, Ursa Major. The earliest extant literary work describing the constellations, including Ursa Major, is Phainómena by the Greek didactic poet Aratus (c. 315 BC – 240 BC). Phainómena is based on an earlier work by the Greek astronomer and mathematician Eudoxus of Cnidus (c. 408 BC – c. 355 BC), now lost. Earlier, the Greek poets Homer and Hesiod (~700 BC) mentioned the constellations, and we know that the Babylonians had a well-developed system of constellations (~2000 BC), as did the Sumerians even earlier (~4000 BC), later assimilated by the Greeks.

Here is what Aratus says in Phainómena about Ursa Major, in context.

The numerous stars, scattered in different directions, sweep all alike across the sky every day continuously for ever. The axis, however, does not move even slightly from its place, but just stays for ever fixed, holds the earth in the centre evenly balanced, and rotates the sky itself. Two poles terminate it at the two ends; but one is not visible, while the opposite one in the north is high above the horizon. On either side of it two Bears wheel in unison, and so they are called the Wagons. They keep their heads for ever pointing to each other's loins, and for ever they move with shoulders leading, aligned towards the shoulders, but in opposite directions. If the tale is true, these Bears ascended to the sky from Crete by the will of great Zeus, because when he was a child then in fragrant Lyctus near Mount Ida, they deposited him in a cave and tended him for the year, while the Curetes of Dicte kept Cronus deceived. Now one of the Bears men call Cynosura by name, the other Helice. Helice is the one by which Greek men at sea judge the course to steer their ships, while Phoenicians cross the sea relying on the other. Now the one is clear and easy to identify, Helice, being visible in all its grandeur as soon as night begins; the other is slight, yet a better guide to sailors, for it revolves entirely in a smaller circle: so by it the Sidonians sail the straightest course.

Between the two Bears, in the likeness of a river, winds a great wonder, the Dragon, writhing around and about at enormous length; on either side of its coil the Bears move, keeping clear of the dark-blue ocean. It reaches over one of them with the tip of its tail, and intercepts the other with its coil. The tip of its tail ends level with the head of the Bear Helice, and Cynosura keeps her head within its coil. The coil winds past her very head, goes as far as her foot, then turns back again and runs upward. In the Dragon's head there is not just a single star shining by itself, but two on the temples and two on the eyes, while one below them occupies the jaw-point of the awesome monster. Its head is slanted and looks altogether as if it is inclined towards the tip of Helice's tail: the mouth and the right temple are in a very straight line with the tip of the tail. The head of the Dragon passes through the point where the end of settings and the start of risings blend with each other.

Thank the Sumerians

Over five thousand years ago, the Sumerians in the area now known as southern Iraq appear to have been the first to develop a penchant for the numbers 12, 24, 60 and 360.

It is easy to see why. 12 is the first number that is evenly divisible by six smaller numbers:

12 = 1×12, 2×6, 3×4 .

24 is the first number that is evenly divisible by eight smaller numbers:

24 = 1×24, 2×12, 3×8, 4×6 .

60 is the first number than is evenly divisible by twelve smaller numbers:

60 = 1×60, 2×30, 3×20, 4×15, 5×12, 6×10 .

And 360 is the first number that is evenly divisible by twenty-four smaller numbers:

360 = 1×360, 2×180, 3×120, 4×90, 5×72, 6×60, 8×45, 9×40, 10×36, 12×30, 15×24, 18×20 .

And 360 in a happy coincidence is just 1.4% short of the number of days in a year.

We have 12 hours in the morning, 12 hours in the evening.

We have 24 hours in a day.

We have 60 seconds in a minute, and 60 minutes in an hour.

We have 60 arcseconds in an arcminute, 60 arcminutes in a degree, and 360 degrees in a circle.

The current equatorial coordinates for the star Vega are

α2019.1 = 18h 37m 33s
δ2019.1 = +38° 47′ 58″

Due to precession, the right ascension (α) of Vega is currently increasing by 1s (one second of time) every 37 days, and its declination (δ) is currently decreasing by 1″ (one arcsecond) every 5 days.

With right ascension, the 360° in a circle is divided into 24 hours, therefore 1h is equal to (360°/24h) = 15°. Since there are 60 minutes in an hour and 60 seconds in a minute, and 60 arcminutes in a degree and 60 arcseconds in an arcminute, it follows that 1m = 15′ and 1s = 15″.

Increasingly, you will see right ascension and declination given in decimal, rather than sexagesimal, units. For Vega, currently, this would be

α2019.1 = 18.62583h
δ2019.1 = +38.7994°

Or, both in degrees

α2019.1 = 279.3875°
δ2019.1 = +38.7994°

Or even radians

α2019.1 = 4.876232 rad
δ2019.1 = 0.677178 rad

Even though the latter three forms lend themselves well to computation, I still prefer the old sexagesimal form for “display” purposes, and when entering coordinates for “go to” at the telescope.

There is something aesthetically appealing about three sets of two-digit numbers, and, I think, this form is more easily remembered from one moment to the next.

For the same reason, we still use the sexagesimal form for timekeeping. For example, as I write this the current time is 12:25:14 a.m. which is a more attractive (and memorable) way to write the time than saying it is 12.4206 a.m. (unless you are doing computations).

That’s quite an achievement, developing something that is still in common use 5,000 years later.

Thank the Sumerians!