Capricornus, often recognized as the “Sea-Goat,” is an ancient constellation with a rich history. Formally cataloged by Ptolemy in the 2nd century CE, its depiction as a mythical creature, part goat and part fish, dates back over 5,000 years to Babylonian and Sumerian civilizations. The constellation holds a significant place in the night sky, nestled among other water-related constellations.
Understanding the Stars of Capricornus
The number of stars visible in Capricornus varies with observational conditions. Under ideal dark skies, approximately 15 stars are discernible to the naked eye within its boundaries. However, the constellation is generally faint, with only one star brighter than magnitude 3, making it challenging to spot from light-polluted areas.
The International Astronomical Union (IAU) recognizes 49 Bayer/Flamsteed stars within Capricornus’s defined boundaries, including those visible only with telescopes, totaling 86 stars brighter than magnitude 6.5. A constellation is a formally defined region of the sky with precise boundaries, established by the IAU, encompassing all objects within its designated area. This differs from an asterism, which is a recognizable pattern of stars, such as the triangle or arrowhead shape often associated with Capricornus.
Notable Stars in the Sea-Goat
The constellation’s brightest star is Delta Capricorni, also known as Deneb Algedi, shining at an apparent magnitude of around 2.81 to 2.85. This multiple star system is an eclipsing binary, meaning its brightness changes slightly as one component passes in front of the other. Its name, “Deneb Algedi,” translates from Arabic to “tail of the goat,” marking a significant point in the constellation’s traditional depiction.
The second brightest star is Beta Capricorni, or Dabih, with an apparent magnitude of 3.05. Dabih is a complex multiple-star system believed to consist of five stars, though binoculars can resolve it into two main components. Its name “Dabih” comes from the Arabic word for “the butcher.” Alpha Capricorni, or Algedi, appears as a visible optical double star to the naked eye. Although Alpha1 and Alpha2 Capricorni appear close together, they are not physically related and are at different distances from Earth.
How to Spot Capricornus
For Northern Hemisphere observers, Capricornus is best viewed during late summer and early autumn. September is often optimal, as the constellation reaches its highest point in the evening sky around 9 p.m. local time. It is located in the southern sky, positioned between Aquarius and Sagittarius.
To find Capricornus, first locate the prominent Summer Triangle asterism, formed by Vega, Deneb, and Altair. From Altair, an imaginary line extending southward can lead to Capricornus, which often appears as a faint, triangular or arrowhead shape low on the horizon. Due to its faintness, viewing conditions are important. Light pollution, excessive artificial light in the night sky, can significantly obscure fainter stars. Seeking out locations with minimal light pollution will greatly enhance visibility.