Eridanus

Authors: Susanne M Hoffmann, Youla Azkarrula, Doris Vickers

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Eri star chart (IAU and Sky & Telescope magazine, Roger Sinnott & Rick Fienberg).

File:Hugo-de-Groot-Syntagma-Arateorum MG 0631.tif

Hugo de Groot Syntagma Arateorum MG 0631

One of the 88 IAU constellations. In ancient Greece, the constellation simply had been named ‘The River’ with various interpretations. In Mediaeval times, e.g. in the Leiden Aratea, it is even depicted as a river god instead of a stream of water. The modern IAU-adoption renders the term as "Eridanus".

Etymology and History

In Greek, the constellation simply had been named ‘The River’ and is interpreted by authors as representing various real or mythological bodies of water. Aratus says it is the river Eridanus, but Eratosthenes also quotes others who believe it must be the Nile. Even in ancient times, some authors considered ‘Eridanus’ to be the name of a mythological river, while others considered it to be the name of the River Po in northern Italy.

Eridanus is a chain of stars next to Orion's foot. Like most stars in this chain, the star next to Orion's foot is not particularly bright (λ Eridani has a magnitude of 4.25). Since the southernmost star in the chain is brighter, many astronomers considered it to be the source of the celestial river. Eratosthenes notes that next to or below the river is the star ‘Kanobos’, Latinised as Canopus, which is the southernmost of all stars and is therefore also called the Earth-near (Perigeios). Canopus does not belong to the river, but it marks its southern end. At that time, it was probably located near ϑ Eridani. At least, that is the southernmost star listed by Ptolemy in his star catalogue.

For Ptolemy, the star next to Orion's foot is the beginning of the river and the southernmost star is the last in the chain. It may be that this list only indicates that Ptolemy systematically proceeded from north to south in his ‘inventory’ of the stars. In any case, he does not name any of the stars as the source or mouth of the river. It therefore remains unclear whether the river flows from north to south or from south to north, and the river therefore remains nameless. Eratosthenes, on the other hand, claims that the river flows from south to north, thus supporting the assertion that it must be the Nile, because this does not apply to any other river. He therefore does not need a mythological explanation, but sees the celestial river as a counterpart to geography.

In the 16th century, Dutch navigators extended this chain of stars, which originally only reached as far as ϑ Eridani, southwards for navigation purposes: on the star charts of Plancius (1598) and Bayer (1603), a line winds its way from the southernmost point of Eridanus to the south celestial pole. This asterism of a winding line was divided into two constellations: the extended Eridanus to the bright star that today bears the Arabic name ‘Achernar’ (end of the river) and the small water snake Hydrus, which was redesigned again in the 18th century.  

Origin of Constellation

Greco-Roman

Hyginus, Astronomica

Some call this the Nile, though many call it Ocean. Those who advocate the Nile point out that it is correctly so called on account of the great length and usefulness of that River, and especially because below the sign is a certain star, shining more brightly than the rest, called Canopus. Canopus is an island washed by the river Nile. (Mary Ward 1960)

Almagest Ποταμός

id	Greek (Heiberg 1898)	English (Toomer 1984)	ident.
	Ποταμοῦ ἀστερισμός
1	ὁ μετὰ τὸν ἐν τῷ ἀκρόποδι τοῦ δρίωνος ἐπὶ τῆς ἀρχῆς τοῦ ποταμοῦ	The star after the one in the foot of Orion, at the beginning of the river	lam Eri
2	ὁ τούτου βορειότερος ἐν ἐπικαμπίῳ πρὸς τῷ ἀντικυημίῳ τοῦ Ὡρίωνος	The one north of tbis, in thc curve near the shin of Orion	ß Eri
3	τῶν μετὰ τοῦτον ἐφεξῆς β’ ὁ ἐπόμενος	Thc rearmost of thc 2 stars ncxt in order after this	psi Eri
4	ὁ προηγούμενος αὐτῶν	The more advanced of t)lem	ome Eri
5	πάλιη τῶν ἐφεξῆς Β’ ὁ ἐπόμευος	The reannost of the next 2 in order again	mu Eri
6	ὁ προηγούμενος αὐτῶν	The more advanced of them	nu Eri
7	τῶν μετὰ τοῦτον γ’ ὁ ἐπόμευος	The rearmost of the 3 stars after this	xi Eri
8	ὁ μέσος αὐτῶν	The middle one of these	omi2 Eri
9	ὁ προηγούμενος τῶν τριῶν	The most advanced of the three	omi1 Eri
10	τῶν ἐν τῇ ἐξῆς διαστάσει δ ὁ ἑπόμενος	The reannost of the four stars in the-next interval	gam Eri
11	ὁ τούτου προηγούμενος	The one in advance of this	pi Eri
12	ὁ ἔτι τούτου προηγούμενος	The one in advance again of this	del Eri
13	ὁ τῶν ὁ προηγούμενος	The most advanced of the 4	eps Eri
14	ὁμοίως τῶν ἐν τῇ ἐφεξῆς διαστάσει δ ὁ ἑπόμενος	The reannost of the 4 stars in the next interval again	zet Eri
15	ὁ τούτου προηγούμενος	The one in advance of this	rho3 + rho2 Eri
16	ὁ ἔτι τούτου προηγούμενος	The one in advance again of this	eta Eri
17	ὁ τῶν ὁ προηγούμενος	The most advanced of the 4	HR 859
18	ὁ ἐν τῇ ἐπιστροφῇ τοῦ ποταμοῦ α’ ἀπτόμενος τοῦ στήθους τοῦ Κήτους	The first star _in the bend ofthe river, which [star] touches the ehest of Cetus	tau1 Eri
19	ὁ τούτῳ ἐπόμενος	The one to the rear of this	tau2 Eri
20	τῶν ἐφεξῆς τριῶν ὁ προηγούμενος	The most advanced of the next [group oll three	tau3 Eri
21	ὁ μέσος αὐτῶν	The midd1e one of these	tau4 Eri
22	ὁ ἐπόμευος τῶν τριῶν	The rearmost of the three	tau5 Eri
23	τῶν ἐξῆς ὡς ἐν τραπεξίῳ ὁ τῆς προηγουμένης πλευρᾶς ὁ βόρειος.	The next four stars, nearly fonning a trapezium: the northern one on the advance side	tau6 Eri
24	ὁ νοτιώτερος τῆς προηγουμένης πλευρᾶς	The next four stars, nearly fonning a trapezium: the southernmost on the advance side	tau7 Eri
25	τῆς ἐπομένης πλευρᾶς ὁ προηγούμενος	The next four stars, nearly fonning a trapezium: the more advanced one on the rear side	tau8 Eri
26	ὁ ἑπόμενος αὐτῆς καὶ λοιπὸς τῶν δ	The next four stars, nearly fonning a trapezium: the last of the 4, the rear one on that side	tau9 Eri
27	ὁ τῶν διεστώτων πρὸς ἀνατολὴν β συνεχῶν ὁ βόρειος	The northernmost of the 2 Stars close tagether at some distance to the east	ups1 Eri
28	ὁ ποτιώτερος αὐτῶν	The southernmost of them	ups2 Eri
29	τῶν ἐφεξῆς μετὰ τὴν καμπὴν β ὁ ἑπόμενος	The rearmost of the next 2 stars after the bend	ups3 Eri
30	ὁ προηγούμενος αὐτῶν	The more advanced of them	ups4 Eri
31	τῶν ἐν τῇ ἐξῆς διαστάσει γ’ ὁ ἐπόμευος	The rearmost of thc 3 stars in thc next interval	g Eri
32	ὁ μέσος αὐτῶν	The middle one	f Eri
33	ὁ προηγούμενος τῶν τριῶν	The most advanced of the three	h Eri
34	ὁ ἔσχατος τοῦ Ποταμοῦ λαμπρός	The last star of the river, the bright one	tet Eri
	ἀστέρες λδ, ὥν α μεγέθους ἂ, γ’ ἔ, δ’ ἧς, εἱ β.	{34 stars, 1 of the lirst magnitude, 5 of the third, 26 of the fourth, 2 of the fifth}

Convex Hull for the stars inside Potamos (CC BY Youla Azkarrula).

Stars within the Constellation Area

id	Label	IAU design.	description	Vmag
1	Cursa	HIP 23875	Constellation lines (Vertex)	2.79
2	Zaurak	HIP 18543	Constellation lines	2.94
3	Acamar	HIP 13847	Constellation lines (Vertex)	3.18
4	Rana	HIP 17378	Constellation lines	3.54
5	υ 4 Eridani	HIP 20042	Constellation lines	3.56
6	τ 4 Eridani	HIP 15474	Constellation lines	3.7
7	Ran	HIP 16537	Constellation lines	3.73
8	Theemin	HIP 21393	Constellation lines (Vertex)	3.82
9	Azha	HIP 13701	Constellation lines (Vertex)	3.87
10	Sceptrum	HIP 21594	Inside the hull	3.87
11	ν Eridani	HIP 21444	Constellation lines (Vertex)	3.928
12	Beemim	HIP 20535	Constellation lines (Vertex)	3.96
13	Dalim	HIP 14879	Inside the hull	3.98
14	μ Eridani	HIP 22109	Constellation lines (Vertex)	4
15	Beid	HIP 19587	Constellation lines	4.026
16	τ 3 Eridani	HIP 14146	Constellation lines	4.09
17	-	HIP 17874	Constellation lines	4.17
18	τ 6 Eridani	HIP 17651	Constellation lines	4.2
19	τ 5 Eridani	HIP 16611	Constellation lines	4.25
20	λ Eridani	HIP 23972	Constellation lines (Vertex)	4.27
21	f Eri	HIP 17797	Constellation lines (Vertex)	4.27
22	54 Eri	HIP 21763	Inside the hull	4.3
23	ω Eridani	HIP 22701	Constellation lines	4.4
24	π Eridani	HIP 17593	Constellation lines	4.42
25	Keid	HIP 19849	Constellation lines	4.43
26	τ 1 Eridani	HIP 12843	Constellation lines (Vertex)	4.46
27	υ 1 Eridani	HIP 21248	Constellation lines	4.51
28	-	HIP 17351	Constellation lines	4.583
29	τ 8 Eridani	HIP 18216	Constellation lines	4.623
30	τ 9 Eridani	HIP 18673	Constellation lines	4.66
31	Angetenar	HIP 13288	Constellation lines	4.77
32	64 Eri	HIP 23231	Inside the hull	4.77
33	Zibal	HIP 15197	Constellation lines	4.8
34	ψ Eridani	HIP 23364	Constellation lines	4.81
35	15 Eri	HIP 15382	Inside the hull	4.875
36	39 Eri	HIP 19777	Inside the hull	4.9
37	δ Fornacis	HIP 17304	Inside the hull	4.973
38	-	HIP 21644	Inside the hull	5.002
39	60 Eri	HIP 22479	Inside the hull	5.028
40	-	HIP 18213	Inside the hull	5.092
41	ξ Eridani	HIP 20507	Constellation lines (Vertex)	5.165
42	47 Eri	HIP 21296	Inside the hull	5.188
43	242 G. Eri	HIP 21297	Inside the hull	5.229
44	20 Eri	HIP 16803	Inside the hull	5.23
45	τ 7 Eridani	HIP 17717	Constellation lines	5.235
46	ρ 3 Eridani	HIP 14293	Constellation lines (Vertex)	5.3
47	ρ 2 Eridani	HIP 14168	Constellation lines (Vertex)	5.336
48	212 G. Eri	HIP 20264	Inside the hull	5.38
49	63 Eri	HIP 23221	Inside the hull	5.39
50	DU Eri	HIP 20922	Inside the hull	5.41
51	GU Eri	HIP 19398	Inside the hull	5.435
52	37 Eri	HIP 19483	Inside the hull	5.442
53	4 Eri	HIP 13782	Inside the hull	5.45
54	257 G. Eri	HIP 21685	Inside the hull	5.463
55	30 Eri	HIP 18141	Inside the hull	5.475
56	62 Eri	HIP 22958	Inside the hull	5.482
57	83 G. Eri	HIP 15643	Inside the hull	5.496
58	58 Eri	HIP 22263	Inside the hull	5.5
59	264 G. Eri	HIP 22028	Inside the hull	5.522
60	ρ Fornacis	HIP 17738	Inside the hull	5.54
61	258 G. Eri	HIP 21743	Inside the hull	5.563
62	94 G. Eri	HIP 16263	Inside the hull	5.576
63	106 G. Eri	HIP 16780	Inside the hull	5.58
64	160 G. Eri	HIP 18647	Inside the hull	5.589
65	174 G. Eri	HIP 19095	Inside the hull	5.59
66	122 G. Eri	HIP 17395	Inside the hull	5.61
67	169 G. Eri	HIP 19011	Inside the hull	5.611
68	AI For	HIP 15479	Inside the hull	5.622
69	ζ Fornacis	HIP 13942	Inside the hull	5.686
70	278 G. Eri	HIP 22860	Inside the hull	5.688
71	184 G. Eri	HIP 19511	Inside the hull	5.689
72	267 G. Eri	HIP 22086	Inside the hull	5.692
73	χ 2 Fornacis	HIP 16112	Inside the hull	5.702
74	R Eri	HIP 22881	Inside the hull	5.715
75	46 Eri	HIP 21278	Inside the hull	5.72
76	91 G. Eri	HIP 16142	Inside the hull	5.73
77	59 Eri	HIP 22325	Inside the hull	5.755
78	-	HIP 22336	Inside the hull	5.77
79	291 G. Eri	HIP 23916	Inside the hull	5.78
80	216 G. Eri	HIP 20465	Inside the hull	5.796
81	GW Eri	HIP 19571	Inside the hull	5.8
82	132 G. Eri	HIP 17798	Inside the hull	5.801
83	6 Eri	HIP 13835	Inside the hull	5.809
84	56 Eri	HIP 22024	Inside the hull	5.81
85	159 G. Eri	HIP 18606	Inside the hull	5.833
86	53 G. Eri	HIP 14110	Inside the hull	5.837
87	EM Eri	HIP 20271	Inside the hull	5.838
88	-	HR 997	Inside the hull	5.84
89	109 G. Eri	HIP 16989	Inside the hull	5.849
90	ϵ Fornacis	HIP 14086	Inside the hull	5.85
91	σ Fornacis	HIP 17618	Inside the hull	5.904
92	163 G. Eri	HIP 18723	Inside the hull	5.919
93	-	HIP 15357	Inside the hull	5.93
94	35 G. Eri	HIP 13479	Inside the hull	5.931
95	202 G. Eri	HIP 19996	Inside the hull	5.932
96	-	HIP 16029	Inside the hull	5.933
97	227 G. Eri	HIP 20892	Inside the hull	5.933
98	93 G. Eri	HIP 16266	Inside the hull	5.988
99	DO Eri	HIP 18339	Inside the hull	6
100	τ Fornacis	HIP 17007	Inside the hull	6.01
101	-	HIP 13402	Inside the hull	6.05
102	GZ Eri	HIP 20075	Inside the hull	6.052
103	236 G. Eri	HIP 21239	Inside the hull	6.052
104	205 G. Eri	HIP 20106	Inside the hull	6.06
105	240 G. Eri	HIP 21298	Inside the hull	6.063
106	171 G. Eri	HIP 19037	Inside the hull	6.111
107	247 G. Eri	HIP 21363	Inside the hull	6.111
108	-	HIP 13907	Inside the hull	6.131
109	281 G. Eri	HIP 23060	Inside the hull	6.139
110	14 Eri	HIP 15244	Inside the hull	6.143
111	γ 1 Fornacis	HIP 13197	Inside the hull	6.146
112	45 G. Eri	HIP 13883	Inside the hull	6.15
113	EK Eri	HIP 20263	Inside the hull	6.154
114	-	HIP 15024	Inside the hull	6.162
115	-	HIP 14551	Inside the hull	6.17
116	DL Eri	HIP 18455	Inside the hull	6.177
117	113 G. Eri	HIP 17057	Inside the hull	6.182
118	133 G. Eri	HIP 17734	Inside the hull	6.19
119	-	HIP 16628	Inside the hull	6.2
120	-	HIP 23311	Inside the hull	6.21
121	147 G. Eri	HIP 18173	Inside the hull	6.216
122	229 G. Eri	HIP 20997	Inside the hull	6.219
123	86 G. Eri	HIP 15776	Inside the hull	6.23
124	104 G. Eri	HIP 16677	Inside the hull	6.239
125	-	HIP 15125	Inside the hull	6.24
126	67 G. Eri	HIP 14814	Inside the hull	6.243
127	232 G. Eri	HIP 21110	Inside the hull	6.25
128	272 G. Eri	HIP 22439	Inside the hull	6.26
129	172 G. Eri	HIP 19121	Inside the hull	6.261
130	-	HIP 13768	Inside the hull	6.281
131	-	HIP 14972	Inside the hull	6.283
132	-	HIP 13947	Inside the hull	6.302
133	59 G. Eri	HIP 14355	Inside the hull	6.31
134	173 G. Eri	HIP 19111	Inside the hull	6.334
135	248 G. Eri	HIP 21428	Inside the hull	6.34
136	114 G. Eri	HIP 17136	Inside the hull	6.343
137	-	HIP 15700	Inside the hull	6.353
138	66 G. Eri	HIP 14797	Inside the hull	6.36
139	168 G. Eri	HIP 18990	Inside the hull	6.364
140	-	HIP 21377	Inside the hull	6.37
141	-	HIP 16449	Inside the hull	6.372
142	65 G. Eri	HIP 14757	Inside the hull	6.377
143	-	HIP 18428	Inside the hull	6.38
144	246 G. Eri	HIP 21324	Inside the hull	6.38
145	χ 1 Fornacis	HIP 15987	Inside the hull	6.385
146	189 G. Eri	HIP 19601	Inside the hull	6.4
147	-	HIP 15585	Inside the hull	6.404
148	-	HIP 13789	Inside the hull	6.411
149	213 G. Eri	HIP 20360	Inside the hull	6.429
150	-	HIP 16672	Inside the hull	6.43
151	188 G. Eri	HIP 19509	Inside the hull	6.44
152	186 G. Eri	HIP 19590	Inside the hull	6.44
153	166 G. Eri	HIP 18926	Inside the hull	6.45
154	-	HIP 16697	Inside the hull	6.48
155	117 G. Eri	HIP 17214	Inside the hull	6.485
156	-	HIP 15816	Inside the hull	6.499
157	-	HIP 13843	Inside the hull	6.5
158	-	HR 1359	Inside the hull	6.5

Mythology

Ancient Greek mythology tells the story of Phaeton and the river Eridanus is often associated with this constellation. Phaeton, the radiant one, is the mortal son of the sun god Helios, who begged to be allowed to drive his sun chariot. Despite all his father's warnings, he insisted and had an accident on the way because the heavenly horses were too strong and stormy for him. When he fell from the sky, he plunged into the river Eridanus.

According to Herodotus and the geographer Strabo, Eridanus is a mythological river with no real counterpart. It is located ‘at the end of the world’. At least a century after Herodotus, it was discovered that the Earth is a sphere and therefore cannot have an edge. By Strabo's time, this had long been the accepted doctrine. Therefore, the idea of the ‘edge’ or ‘end’ of the world is probably an association with death and the eternity that follows – a popular motif for the starry sky.

References

• References (general)
• References (Babylonian)
• References (ancient Greco-Roman)
• References (medieval)