Delphinus Constellation Guide – A Dolphin of Stars

In this article, we’ll be sharing with you everything you need to know about the constellation of Delphinus, including how to find it, deep space objects contained within it, and how to use it to find other night sky objects more easily.

The Constellation of Delphinus

Delphinus, which is Latin for ‘dolphin’ is one of the constellations Greek astronomer Ptolemy cataloged over 2000 years ago.

There are two Greek myths associated with Delphinus.

The first is about Poseidon, who was god of the sea, and Amphitrite, who was one of the 50 daughters of Nereus and Doris. Poseidon wanted to marry Amphitrite (who was a sea nymph) and when she refused his offer of marriage, he sent messengers to woo her.

The successful messenger was successful a dolphin.

Poseidon and Amphitrite married and had many children. Poseidon placed the dolphin in the night sky to honor it.

The second myth is about Arion, a Greek poet, court musician, and ruler of Corinth.

Arion was traveling from the Mediterranean to Greece by ship and the crew of the ship conspired to steal his valuables. Arion begged for his life, jumped into the sea, and was rescued by a dolphin.

Apollo, god of poetry and music, placed the dolphin in the night sky for saving Arion’s life.

To help you spot Delphinus, here’s what SkySafari 6 shows.

The small constellation of Delphinus, shown as a dolphin
The small constellation of Delphinus. Click for full-screen.

Delphinus has an area of 189 square degrees making it the 69th largest of the 88 recognized constellations.

Delphinus is often depicted as a kite with a long tail instead of a dolphin. It’s known for the diamond-shaped asterism, Job’s Coffin, which represents the celestial dolphin’s body.

Job’s Coffin is formed by the stars Alpha, Beta, Gamma, and Delta Delphini.

Job's coffin asterism in Delphinus
Job’s Coffin asterism in Delphinus constellation. Click for full-screen.

The image below shows the extent of the constellation, shaded gray. See it is near Cygnus and Aquila, but also the small constellations Vulpecula, Sagitta, and Equuleus.

The area of Delphinus, shaded gray.
Delphinus constellation shaded gray. Click for full-screen.

In the next section, we’ll learn how to find the dolphin constellation.

How To Find Delphinus In The Night Sky

Delphinus is part of the Heavenly Waters family of constellations and is visible to observers at latitudes between +90° and -70°.

Northern Hemisphere observers can see it from June to December in the late evening, with it being highest during August and September. Southern Hemisphere observers can see it from August to December.

The constellation of Delphinus is bordered by the constellations Aquarius, Aquila, Equuleus, Pegasus, Sagitta, and Vulpecula.

Delphinus bordering constellations
The constellations which border Delphinus. Click for full-screen.

Finding this small, faint constellation is not an easy task.

The best way to do so is to look for Job’s Coffin between the Great Square of Pegasus and the Summer Triangle, formed by the bright stars Vega, Deneb, and Altair.

Look for it towards the Altair end of the triangle, as shown in the image below.

Finding Delphinus between the Great Square of Pegasus and the Summer Triangle. Click for full-screen.

To find the exact position of Delphinus for your location on any night, use astronomy software such as Stellarium (free) or SkySafari 6.

The Brightest Stars of Delphinus

Delphinus is a small constellation with no particularly bright stars. It only contains seven stars brighter than magnitude 5.0 and only two of these are brighter than magnitude 4.0.

See the SkySafari 6 star chart below which shows them all for reference. Additional information about each is provided below the chart.

Alpha Delphini (Sualocin) – This double star is the second brightest star in Delphinus and marks the northwestern corner of Job’s Coffin.

The magnitude 3.77 magnitude blue-white subgiant primary and the magnitude 13.40 secondary components are 35.5 arcseconds apart. The system is 97 light-years away.

Sualocin is an eruptive variable star ranging in magnitude from 3.77 to 3.80.

Sualocin is the name ‘Nicolaus’ spelled backward and is a reference to Niccolo Cacciatore’s (a 19th-century Italian astronomer) Latinized first name.

Beta Delphini (Rotanev) – is the brightest star in Delphinus and marks the southwestern corner of Job’s Coffin.

Rotanev is another double star. The primary is a magnitude 3.63 yellow-white subgiant and the secondary is a main-sequence star shining at magnitude 5.02. The two components are 0.3 arcseconds apart.

Rotanev is 97 light-years away and its name comes from ‘Venator’ spelled backwards and is a reference to Niccolo Cacciatore’s Latinized surname.

Gamma Delphini – is another double star system and the third brightest in this constellation. It marks the northeastern corner of Job’s Coffin and the Dolphin’s nose.

The primary is an orange subgiant of magnitude 4.26 (Gamma 2 Delphini), while the secondary shines at magnitude 5.03 and is a yellow-white, main-sequence star (Gamma1 Delphini).

The two stars are 8.8 arcseconds apart and 126 light-years away.

Delta Delphini – This variable double star marks the southeastern corner of Job’s Coffin and is 223 light-years away.

The magnitude 4.42 orange giant primary and magnitude 5.00 secondary components are too close to be optically separated.

This pulsating variable ranges in magnitude from 4.38 to 4.48.

Epsilon Delphini (Adulfin) – is a magnitude 4.03 blue-white giant star and the fourth brightest star in Delphinus and marks its ‘tail’.

Epsilon Del. is 330 light-years away and 6 times more massive than the sun. Its diameter is 4.0 Solar diameters and it is 2.4 times hotter than our star.

Zeta Delphini – This variable double star is 220 light-years away. The 4.65 magnitude white main-sequence primary and secondary components are 13.5 arcseconds apart.

This eruptive variable ranges in magnitude from 4.54 to 4.69 with an irregular period.

Rho Aquilae – This magnitude 4.94 magnitude, white main-sequence star became a part of Delphinus in 1992. It’s 150 light-years away, has the mass of 2.5 suns, a diameter 1.9 times that of our sun, and it’s 1.6 times hotter than the Sun.

Star Hopping From Delphinus

Delphinus makes a poor starting point for star hopping due to its small size and dim stars.

Objects To See Within Delphinus

Delphinus contains no Messier objects and few other deep sky objects suitable for small telescope users. However, it does provide a home to two Caldwell objects.

NGC 6934 (Caldwell 47) – This globular cluster has a magnitude of 8.82 and an apparent size of 7.1 arcminutes. It’s 51,200 light-years away and is at right ascension 20h 34m 11s and declination 07° 24’ 16” (J2000.0).

NGC 7006 (Caldwell 42) – This globular cluster has a magnitude of 10.56 and an apparent size of 3.6 arcminutes. It’s 135,400 light-years away and is at right ascension 21h 01m 29s and declination 16° 11’ 14”.

Summary

Delphinus is a small, dim constellation known for the Job’s Coffin asterism.

It contains a small number of brighter stars and deep sky objects for the small telescope user. Hop to it from the Summer Triangle this summer.


Written by Tanya C. Forde