Poniatowski’s Bull

Hunt the skies for summer’s Taurus.
By | Published: August 1, 2022 | Last updated on January 15, 2024
This annotated version of chart IX within Johann Bode’s Uranographia shows the position of IC 4665. Unlike Messier or the Herschels, Bode plotted this object — although he failed to label it.
Image courtesy: History of Science Collections, University of Oklahoma Libraries

As most readers know, there are 88 constellations adorning the sky. These were codified in 1930 by the International Astronomical Union through international agreement. Before this, astronomers could create and publicize their own constellations, hoping they would catch on with the masses.

Although many of these obsolete constellations have faded away as footnotes in astronomical history books, there is one lost pattern in the summer sky that is fun to spot through binoculars. You’re probably familiar with winter’s Taurus the Bull. But what about Taurus Poniatovii?

Taurus Poniatovii, or Poniatowski’s Bull, was a small constellation created in 1777 by Marcin Poczobut, a Polish-Lithuanian astronomer and the director of Vilnius Observatory. Poczobut devised the constellation to honor Stanislaus Poniatowski, the king of Poland and Lithuania at the time.

Like its namesake constellation in the winter, Taurus Poniatovii is drawn from a V-shaped pattern of stars that fills the field of my 10×50 binoculars. The stars range from 4th to 6th magnitude and can be found just east of Cebalrai (Beta [β] Ophiuchi), the eastern shoulder of Ophiuchus. If you compare the formation to winter’s Taurus, then the role of bright-eyed Aldebaran is played by 6th-magnitude 73 Ophiuchi. The tip of the bull’s nose is marked by 68 Ophiuchi, while 66 Ophiuchi is the western eye. Fourth-magnitude 72 Ophiuchi marks the end of the eastern horn, while the western horn extends toward 6th-magnitude HD 163641 and beyond. Poczobut drew the bull’s body among the faint stars northeast of the V, but he lost me there.

Three of the stars that form the triangular head of Taurus Poniatovii — 67, 68, and 70 Ophiuchi — along with a scattering of about a dozen others belong to a sparse open cluster cataloged as Melotte 186 (Collinder 359). The group is centered on 67 Ophiuchi and spans 4°. But ironically, its size also makes it difficult to confirm. Which stars in this densely populated area belong to the cluster and which do not is a chore best left to the professionals. A study published in Astronomy and Astrophysics in 2006 suggested this may not be a cluster at all, but rather a “moving group” of stars following a similar path through the Milky Way.

That same study mentioned that Melotte 186 might be linked to our next target, IC 4665, as both troupes exhibit similar proper motion through the galaxy.
Located some 1,100 light-years away, there is no mistaking the identity of open cluster IC 4665 through binoculars. It can be found just 1° north-northeast of Cebalrai. On dark, clear nights, a faint hint of this cluster is visible to the eye alone. Swing your binoculars its way and IC 4665 resolves into a fine collection of about ten 7th- to 9th-magnitude stars, along with many fainter points. Although its stars are not tightly packed, IC 4665 stands out in binoculars. Depending on how faint you can see, you might see the brighter stars as forming a triangle, a diamond, a short-tailed kite, or even a distorted H.

Our last stop this month is the binary star 53 Ophiuchi. You’ll find it 3° south of Rasalhague (Alpha [α] Ophiuchi) at the top of the constellation’s pentagonal form. The binary’s 6th-magnitude primary star is accompanied by an 8th-magnitude companion, separately cataloged as SAO 122525, about 42″ to its south. Both are spectral type A white stars located around 370 light-years away. They are just on the edge of resolution through my 10x binoculars, but should be relatively easy at 12x and up. To resolve the companion, you need to reduce shaking, however. If you don’t have image-stabilized binoculars, brace them against something. Better yet, mount them on a tripod.

I welcome observations, suggestions, and comments. Contact me through my website, philharrington.net. Until next month, remember that two eyes are better than one.