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Posted by on Jul 5, 2017 in Tell Me Why |

Where Is Ursa Minor?

Where Is Ursa Minor?

Ursa Minor is the name of group of stars in the northern hemisphere. The word used in astronomy for a group of stars is “constellation”. The stars and constellations have Latin names. Ursa Minor (Latin: “Lesser Bear”, contrasting with Ursa Major), also known as the Little Bear, is a constellation in the Northern Sky. Its brightest star is called Polaris, and is centered over the North Pole. It is of great importance in helping sailors to find their bearings when navigating at night. Star maps of the sky will help you locate the constellations.

There are several mythological stories behind famous constellations. In Greek myth, Zeus was having an affair with the lovely Callisto. When his wife, Hera, found out she changed Callisto into a bear. Zeus put the bear in the sky along with the Little Bear, which is Callisto’s son, Arcas. In other myths, the constellation is not a bear at all, but is in fact a dog.

little dipper along with its partner the big dipper

Like the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, hence the North American name, Little Dipper: seven stars with four in its bowl like its partner the Big Dipper. It was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and remains one of the 88 modern constellations. Ursa Minor has traditionally been important for navigation, particularly by mariners, because of Polaris being the North Star.

Polaris, the brightest star in the constellation, is a yellow-white supergiant and the brightest Cepheid variable star in the night sky, ranging from an apparent magnitude of 1.97 to 2.00. Beta Ursae Minoris, also known as Kochab, is an aging star that has swollen and cooled to become an orange giant with an apparent magnitude of 2.08, only slightly fainter than Polaris. Kochab and magnitude 3 Gamma Ursae Minoris have been called the “guardians of the pole star”. Planets have been detected orbiting four of the stars, including Kochab. The constellation also contains an isolated neutron star—Calvera—and H1504+65, the hottest white dwarf yet discovered, with a surface temperature of 200,000 K.

Ursa Minor is bordered by Camelopardalis to the west, Draco to the west, and Cepheus to the east. Covering 256 square degrees, it ranks 56th of the 88 constellations in size. Ursa Minor is colloquially known in the US as the Little Dipper because its seven brightest stars seem to form the shape of a dipper (ladle or scoop). The star at the end of the dipper handle is Polaris. Polaris can also be found by following a line through the two stars—Alpha and Beta Ursae Majoris—that form the end of the ‘bowl’ of the Big Dipper, for 30 degrees (three upright fists at arms’ length) across the night sky. The four stars constituting the bowl of the Little Dipper are of second, third, fourth, and fifth magnitudes, and provide an easy guide to determining what magnitude stars are visible, useful for city dwellers or testing one’s eyesight.

The three-letter abbreviation for the constellation, as adopted by the IAU (International Astronomical Union) in 1922, is “UMi”. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 22 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 08h 41.4m and 22h 54.0m, while the declination coordinates range from the north celestial pole south to 65.40°. Its position in the far northern celestial hemisphere means that the whole constellation is only visible to observers in the northern hemisphere.

In the Babylonian star catalogues, Ursa Minor was known as the “Wagon of Heaven”, also associated with the goddess Damkina. It is listed in the MUL.APIN catalogue, compiled around 1000 BC among the “Stars of Enlil”—that is, the northern sky.

According to Diogenes Laertius, citing Callimachus, Thales of Miletus “measured the stars of the Wagon by which the Phoenicians sail”. Diogenes identifies these as the constellation of Ursa Minor, which for its reported use by the Phoenicians for navigation at sea were also named Phoinikē. The tradition of naming the northern constellations “bears” appears to be genuinely Greek, although Homer refers to just a single “bear”. The original “bear” is thus Ursa Major, and Ursa Minor was admitted as second, or “Phoenician Bear” (Ursa Phoenicia, hence Φοινίκη, Phoenice) only later, according to Strabo (I.1.6, C3) due to a suggestion by Thales, who suggested it as a navigation aid to the Greeks, who had been navigating by Ursa Major.

In classical antiquity, the celestial pole was somewhat closer to Beta Ursae Minoris than to Alpha Ursae Minoris, and the entire constellation was taken to indicate the northern direction. Since the medieval period, it has become convenient to use Alpha Ursae Minoris (or “Polaris”) as the north star, even though it was still several degrees away from the celestial pole. Its New Latin name of stella polaris was coined only in the early modern period.

The ancient name of the constellation is Cynosura (Greek Κυνοσούρα “dog’s tail”). The origin of this name is unclear (Ursa Minor being a “dog’s tail” would imply that another constellation nearby is “the dog”, but no such constellation is known). Instead, the mythographic tradition of Catasterismi makes Cynosura the name of an Oread nymph described as a nurse of Zeus, honoured by the god with a place in the sky. There are various proposed explanations for the name Cynosura. One suggestion connects it to the myth of Callisto, with her son Arcas replaced by her dog being placed in the sky by Zeus.

Others have suggested that an archaic interpretation of Ursa Major was that of a Cow, forming a group with Bootes as herdsman, and Ursa Minor as a dog. George William Cox explained it as a variant of Λυκόσουρα, understood as “wolf’s tail” but by him etymologized as “trail, or train, of light” (i.e. λύκος “wolf” vs. λύκ- “light”). Allen points to the Old Irish name of the constellation, drag-blod “fire trail”, for comparison. Brown (1899) suggested a non-Greek origin of the name (a loan from an Assyrian An‑nas-sur‑ra “high-rising”).

An alternative myth tells of two bears that saved Zeus from his murderous father Cronus by hiding him on Mount Ida. Later Zeus set them in the sky, but their tails grew long from being swung by the god. Because Ursa Minor consists of seven stars, the Latin word for “north” (i.e., where Polaris points) is septentrio, from septem (seven) and triones (oxen), from seven oxen driving a plough, which the seven stars also resemble. This name has also been attached to the main stars of Ursa Major.

In Inuit astronomy, the three brightest stars—Polaris, Kochab and Pherkad—were known as Nuutuittut “never moving”, though the term is more frequently used in the singular to refer to Polaris alone. The Pole Star is too high in the sky at far northern latitudes to be of use in navigation. In Chinese astronomy, the main stars of Ursa Minor are divided between two asterisms: 勾陳 Gòuchén (Curved Array) (including α UMi, δ UMi, ε UMi, ζ UMi, η UMi, θ UMi, λ UMi) and 北極 Běijí (Northern Pole) (including β UMi and γ UMi).

Content for this question contributed by Matthew Pearce, resident of Santa Cruz, Santa Cruz County, California, USA