As soon as night falls, look high up in the eastern sky to see the three stars of the Summer Triangle: Vega, Deneb and Altair. The lowest of them, Altair, is the brightest star in Aquila (AK-will-uh) the Eagle. Now, reach your fist horizontally to the sky and measure off a little more than one fist to the left of Altair. Provided the sky isn't too light-polluted you'll see five stars in the shape of a small kite with a tail.

Delphinus lies to the left (east) of Altair below the Summer Triangle. The group is quite compact, measuring just 5.8 degrees across. (Stellarium)
Delphinus lies to the left (east) of Altair below the Summer Triangle. The group is quite compact, measuring just 5.8 degrees across. (Stellarium)

Or if you chose, you can imagine a mythological dolphin, the same as Greek sailors did in ancient times. They called it Delphin or Delphis, but we know it as Delphinus (del-FYE-nus). Its stars glimmer at fourth magnitude, not bright by any stretch, and yet the group is so compact and distinctive it's visible to the eye in muddled skies. If you have any difficulty spotting it, just use binoculars.

The dolphin made its way into the night sky after helping the sea god Poseidon reunite with his heart's desire. (Johannes Hevelius / Firmamentum Sobiescianum sive Uranographia 1687)
The dolphin made its way into the night sky after helping the sea god Poseidon reunite with his heart's desire. (Johannes Hevelius / Firmamentum Sobiescianum sive Uranographia 1687)

Delphinus arrived in the sky through its sweet-talking ways. Poseidon, Greek god of the sea, was looking for a wife to live with him in his underwater palace and met and courted the sea nymph Amphitrite. She resisted his advances and fled, so Poseidon sent messengers after her including a convivial dolphin, who convinced her to return. After she and Poseidon married he placed the dolphin's image in the stars in gratitude.

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Nicolaus Cacciatore (1770-1841), assistant and director of Palermo Observatory in Sicily, may have been involved in a clever scheme to have two stars in Delphinus named for himself. (Public domain)
Nicolaus Cacciatore (1770-1841), assistant and director of Palermo Observatory in Sicily, may have been involved in a clever scheme to have two stars in Delphinus named for himself. (Public domain)

My favorite Delphinus story involves its two brightest stars, Sualocin (Alpha Delphini) and Rotanev (Beta Delphini). We often come across odd-sounding names for stars, so on the face of it, these don't look out of place. The labels first appeared in the second edition (1814) of the Palermo Star Catalog compiled by Giuseppe Piazzi, the discover of the first asteroid Ceres. Nicolaus Cacciatore, Piazzi's assistant at the Palermo observatory, helped with the publication and later succeeded Piazzi as director.

Yet the origin of the stars' names resisted explanation until decades later when their meaning dawned on British astronomer Reverend Thomas Webb. Webb reversed the letters, discovering that Sualocin and Rotanev were Nicholaus Venator spelled backward. Venator is the Latinized form of Cacciatore, which means "hunter" in Italian. Aha! So the stars were named for the observatory's assistant. Very clever.

What's not known is whether it was a practical joke by Cacciatore or whether Piazzi did it to recognize the work of his assistant and successor. The topper is that the International Astronomical Union, the body officially in charge of star designations, formally approved the names Sualocin and Rotanev in September 2016. I suspect both astronomers are chuckling in their graves.

Gamma Delphini, an attractive double star for small and moderate-sized telescopes, lies 115 light years from the Earth. (Gianluca Masi)
Gamma Delphini, an attractive double star for small and moderate-sized telescopes, lies 115 light years from the Earth. (Gianluca Masi)

Delphinus is located astride the Milky Way, the reason it's thick with stars when viewed in a telescope or pair of binoculars. In size it ranks 69th among the 88 constellations (Crux, the Southern Cross, is smallest) but possesses its share of deep-sky treasures. My favorite is the double star Gamma Delphini, one of the prettiest in the sky for a small telescope. The two yellowish-orange stars are of similar brightness — magnitudes of 4.4 and 5.0 — and appear close together, only nine arc-seconds apart.

An arc-second is a small fraction of a degree. For example, the full moon is half-degrees in diameter. Degrees are subdivided into arc-minutes, 60 arc-minutes per degree. Since half-degrees equals 30 arc-minutes, we can alternately describe the moon's apparent diameter as 30 arc-minutes. Each arc-minute is further subdivided into arc-seconds — 60 per arc-minute — making the moon 1,800 arc-seconds across. In this context, you can better picture the nine arc-seconds that separate the stars in the Gamma pair.

The moon is currently waxing and will be full July 23, so a bright sky will make Delphinus appear a little fainter than usual this week. Haze from wildfires may also hinder the view depending where you live. Hang in there. The dolphin is patient. It remains breached all summer and fall waiting for you to turn, admire and applaud.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.