## 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.

 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.

 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.

## Exoplanets with Deep Transits

The list above shows the 35 stars presently known to dip in brightness by 0.02 magnitudes or more due to a transiting exoplanet.

The change in the star’s magnitude during transit is given by

$\Delta m = 2.5\log_{10}\left ( 1+\delta \right )$

where Δm is the drop in magnitude, and δ is the transit depth

The time between transits for these exoplanets ranges between 0.79 and 5.72 days, with a median period of 2.24 days.  You can generate your own ephemeris for any of these transiting exoplanets at:

https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TransitView/nph-visibletbls?dataset=transits

The transit duration for these exoplanets ranges between 1.08 and 3.11 hours, with a median duration of 2.11 hours.

The exoplanets with the deepest transits, HATS-6 b at 0.035 magnitudes and Kepler-45 b at 0.034 magnitudes, cross stars that are 15.2 and 16.9 magnitude, respectively, so these events might be out of reach for most amateur photometrists.  The only other star hosting a transiting exoplanet with a Δm ≥ 0.03m is Tycho 5165-481-1 in Aquila (WASP-80 b) which at visual magnitude 11.9 is a better candidate for smaller instruments.  The brightest star on our list (by far) is HD 189733 in Vulpecula, magnitude 7.7, with a drop in brightness that is almost as good at 0.026 magnitudes.

References
Fakhouri, O. (2018). Exoplanet Orbit Database | Exoplanet Data Explorer. [online] Exoplanets.org. Available at: http://exoplanets.org/ [Accessed 11 Dec. 2018].

## Faintest Constellations

There are a dozen constellations with no star brighter than +4.0 magnitude.  Many of them are deep in the southern sky.  They are:

ANTLIA, the Air Pump
Brightest Star: Alpha Antliae, apparent visual magnitude +4.25

CAELUM, the Engraving Tool
Brightest Star: Alpha Caeli, apparent visual magnitude +4.45

CAMELOPARDALIS, the Giraffe
Brightest Star: Beta Camelopardalis, apparent visual magnitude +4.02

CHAMAELEON, the Chameleon
Brightest Star: Alpha Chamaeleontis, apparent visual magnitude +4.047

COMA BERENICES, Berenice’s Hair
Brightest Star: Beta Comae Berenices, apparent visual magnitude +4.25

CORONA AUSTRALIS, the Southern Crown
Brightest Star: Meridiana, apparent visual magnitude +4.087

MENSA, the Table Mountain
Brightest Star: Alpha Mensae, apparent visual magnitude +5.09

MICROSCOPIUM, the Microscope
Brightest Star: Gamma Microscopii, apparent visual magnitude +4.654

NORMA, the Carpenter’s Square
Brightest Star: Gamma2 Normae, apparent visual magnitude +4.02

SCULPTOR, the Sculptor
Brightest Star: Alpha Sculptoris, apparent visual magnitude +4.27

SEXTANS, the Sextant
Brightest Star: Alpha Sextantis, apparent visual magnitude +4.49

VULPECULA, the Fox
Brightest Star: Anser, apparent visual magnitude +4.45