Capricornus

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: Capricornus is a constellation in a roughly triangular shape that some people believe resembles a goat or a goat horn. Often portrayed as a goat with a fish tail, Capricornus can be found in the Southern Hemisphere and is most visible around September. Capricornus has a long history, as reflected in myths and tales of the ancient Greeks, Babylonians, and Sumerians. The Babylonians included Capricornus in the group of constellations known as the zodiac. Today, astronomers study Capricornus because it includes several interesting varieties of star as well as galaxies and clusters of stars.

The Goat Horn Constellation

Capricornus is a constellation, an area of space defined by a grouping of stars that appear to some observers to form patterns or pictures in the night sky. People have studied constellations for thousands of years. The types and descriptions of these star patterns vary widely, but the International Astronomical Union acknowledges eighty-eight major constellations. One of the best-known and most historically important of these is Capricornus, a Latin name that means "goat horn." Capricornus, which is located in the Southern Hemisphere, contains about thirteen primary stars. These stars are arranged in a roughly triangular shape thought by some to resemble a goat or a goat horn.

Capricornus owes much of its worldwide recognition to its inclusion in the zodiac. The zodiac is a group of twelve or thirteen constellations through which the sun appears to move along a path called the ecliptic. The ecliptic is in fact an illusion resulting from Earth’s orbit of the sun. However, the location of Capricornus and other constellations is measured relative to the celestial equator, not the ecliptic. Capricornus, for example, has a declination of about 20 degrees south of the celestial equator and a right ascension of twenty-one hours.

Attributes of Capricornus

Capricornus can be found in the Southern Hemisphere between the latitudes 60 degrees north and 90 degrees south. The constellation has a roughly triangular or arrowhead shape with an extra "spike" on the right top side. Although astronomers disagree on the exact number of stars in Capricornus, its main shape is composed of about thirteen primary stars. In addition, it includes many other stars and background objects. This constellation is just above average in size, being the fortieth largest of the officially recognized eighty-eight constellations. It is, however, unusually faint and can be challenging to locate.

This constellation is an important member of the zodiac and the star patterns of the night sky. It is located near many other constellations, including Aquila (the eagle), Microscopium (the microscope), Piscis Austrinus (the southern fish), and the zodiac constellations Aquarius (the water bearer) and Sagittarius (the archer).

Capricornus also contains many interesting stars. Greek letters such as alpha, beta, and so on were used to designate their relative brightness. Alpha Capricorni, also known as Algedi (the goat), is on the top right corner of the constellation and forms the tip of the horn. This star appears to be a binary star. However, that is actually an illusion based on two stars that coincidentally line up in the perspective of the Earth-based observer. The two stars involved are alpha 1, or Prima Giedi, and alpha 2, or Secunda Giedi. Just below and to the left of the Alpha stars is beta Capricorni (better known as Dabih), and farther left is Nu Capricorni (known as Alshat). These names, derived from Arabic, relate to the theme of goats and similar animals. Alshat means "the sheep," and Dabih refers to "the butcher" who will prepare the sheep for feasts or sacrificial rituals.

Other names relate to parts of the goat’s body, such as Deneb Algedi (goat’s tail), an alternate name of delta Capricorni. Meanwhile, the giant red star omega Capricorni is referred to as Baten Algiedi (goat’s belly) because it appears at the lower side of the constellation. Other important stars in the formation are Pi Capricorni, Rho Capricorni, and Omicron Capricorni, which form a triangle, and gamma Capricorni, better known as Nashira. The brightest of the stars in this constellation is delta Capricorni, which can often be seen without optical aids.

The Goat and the Sea

Capricornus was one of the first constellations to be identified in recorded history. The ancient astronomer Ptolemy (ca. 100–170 CE) listed it among the forty-eight constellations he and his contemporaries recognized. It also became entwined in the study of the zodiac. Ancient Babylonian astronomers thought the constellations of the zodiac had special powers because they appeared to be touched by the path of the sun. Over the centuries, many cultures applied their own interpretations to the meaning of the constellation. Most of these interpretations related in some way to goats. Other themes, such as the sea, also worked their way into the complex and growing mythology.

Early Greek astronomers identified the shape of the constellation as representing a goat or a goat horn. The symbol of the goat was important to ancient Greeks. In Greek mythology, the chief god Zeus was raised by a goat. This goat, known as Amalthea, fed and protected Zeus through his vulnerable childhood. Later, the horn of Amalthea broke off and became a cornucopia, or a horn of plenty. Another Greek myth relating to goats is about Pan, the god of the wild, who had a half goat, half human form. In one tale, Pan leapt into a lake to avoid a monster, and his unexpected swim caused him to partially morph into a fish.

Other mythological combinations of humans, goats, and fish appear in Babylonian records as well. As early as 3000 BCE, some Babylonians drew fantastical images of goats with fish tails. Other Babylonian myths related to the god Ea, or Enki, who was half fish, half human and brought wisdom to humanity. Half-goat, half-fish creatures also figure into ancient Sumerian tales.

Although the exact reason is unclear, many astronomers began to imagine the figure of Capricornus as a goat with a fish tail—a "water goat." This depiction, while unusual, fits with the other constellations in that part of the sky, many of which have been assigned water-themed descriptions. The water goat joins the water-bearer Aquarius, the fish Pisces, and the river Eridanus. The water theme may have begun because, in some lands, the appearance of these constellations in the sky corresponded to the rainy season.

Capricornus may also have had a more spiritual meaning. Because in ancient times the sun reached winter solstice while in Capricornus, ancient people may have associated the constellation with the end of the year.

Astronomical Importance

The people of the ancient world studied the stars and constellations for spiritual reasons as well as to help mark the seasons of the year. In the twenty-first century, many scientists turn to the constellations to help them study the various bodies of space. Like most other constellations, Capricornus includes many points of interest for modern astronomers and stargazers. It has also helped people study the path of the sun, which at one time reached its most southerly position in Capricornus at a spot still known today as the Tropic of Capricorn (though it is now in the constellation Sagittarius).

The stars of Capricornus are relatively faint and most are best viewed with binoculars or telescopes. Stargazers can find the constellation at its highest and brightest on September evenings around nine or ten p.m., though the stars are also readily spotted in October. Astronomers advise amateurs to locate Capricornus by finding three bright stars—alpha 2, beta, and omega Capricorni—that form an easy-to-spot triangle. Some of the most popular stars for stargazing include the optical double star alpha Capricorni, the binary star Prima Giedi, and the eclipsing binary star delta Capricorni.

Astronomers can study more than just stars in Capricornus. The constellation includes two stars with exoplanets as well as Messier 30, a globular cluster disk of many stars. Several notable galaxy groups, such as New General Catalog (NGC) 7103 and 6907, can be spotted with powerful telescopes in or near the constellation, and some of these have given astronomers valuable views of huge explosions called supernovas. Meteors are frequently spotted within Capricorn as well, particularly around July 30, when the Capricornid meteor shower passes through the constellation. Stargazers may spot about five to fifteen meteors each hour at the peak of the stream.

PRINCIPAL TERMS

• celestial equator: the imaginary line above Earth’s equator that halves the celestial sphere; it is equally distant from the celestial poles.
• constellation: a region of space defined by a pattern of stars that can be seen in the night sky from Earth.
• declination: the north-south position of a celestial body relative to the celestial equator expressed in degrees of arc.
• International Astronomical Union: an association of professional astronomers from all over the world who define astronomical constants while promoting research, education, and discussion on important astronomical topics.
• right ascension: the east-west position of a celestial body defined in relation to the celestial equator and expressed in hours and minutes, not degrees of arc.

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