Thursday, October 16, 2008

The Little Dolphin

Just east of Altair, the southernmost star in the Summer Triangle, is Delphinus the Dolphin. Although a rather small, unassuming constellation, its distinctive central asterism (recognizable star pattern) actually resembles a leaping dolphin, so it’s fairly easy to spot once you’ve learned it.

Image from Flamsteed’s 18th century star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Delphinus the Dolphin (dell-FINE-uss) is an ancient constellation. In Greek myth, Delphinus was a messenger of the sea god Poseidon. When Poseidon pressed one of the sea nymphs at his court to marry him, the reluctant goddess fled. The persistent Poseidon sent a number of messengers to persuade her to return and become his wife, but only the beguiling dolphin succeeded. A grateful Poseidon elevated Delphinus to immortality in the night sky.

Star maps created with Your Sky

When I see the jeweled spray of stars that forms the Dolphin asterism high overhead in the early evening, I know that autumn has arrived. Let’s take a closer look at those five stars.

1) If you don’t know how to locate the Summer Triangle, read my earlier post. Look for the sprightly Dolphin a little east of Altair, the brightest star in Aquila the Eagle.

Dolphin asterism in the constellation Delphinus

2) Yellow-white Rotanev (ROH-tuh-nev) is the brightest star in Delphinus. Although slightly dimmer than Rotanev, blue-white Sualocin (SWAH-loh-sinn) is nevertheless the hotter of the two.

When these star names first appeared in an Italian star catalog in 1814, astronomers were puzzled as to their origin. Clever sleuthing by the English astronomer Thomas Webb revealed them to be the reverse spelling of Nicolaus Venator, the Latinized version of the name Niccolo Cacciatore. Cacciatore was the assistant to (and successor to) Giuseppe Piazzi, the famed director of the Palermo Observatory in Italy. It is unclear whether Cacciatore or Piazzi was responsible for the stellar wordplay.

Giuseppe Piazzi is best known as the discoverer of the largest asteroid found in our solar system to date: Ceres (SEER-eez).

3) The other two stars that, along with Rotanev and Sualocin, complete the “head” of the Dolphin asterism have no traditional names, so we call them Gamma and Delta, their star catalog designations. Gamma is a yellow-orange star, and Delta is a white giant.

4) The blue-white giant star that marks the dolphin’s tail is Deneb al Dulfim (DENN-ebb), from the Arabic for, blimey, dolphin’s tail! Now can you see the dolphin frolicking in the foam of the Summer Milky Way?

5) Delphinus contains an asterism within an asterism. The head of the Dolphin asterism— the quadrilateral formed by Rotanev, Sualocin, Gamma, and Delta— is an asterism known as Job’s Coffin. The origin of this odd name is unknown. It’s just another mystery lost to history. Perhaps someone should have put Thomas Webb on the case.