Roman Astronomy

  • The greatest Roman Astronomer was Ptolemy (c.90-168 CE) who produced a revolutionary Star Catalogue called the Almagest. This predicted the position of any Planet at any time of day or night on any day of the year and listed 48 Constellations.
  • Despite modelling his System of the Planets with the Earth at the centre and not the Sun, Ptolemy’s ideas remained in force for 1,400 years until 1543 CE, when Copernicus proposed the Heliocentric System.

Astronomy in Everyday Roman Life

Roman Calendar

  • The Days of the Week and the Months of the Year were named after the Planets or the Gods they represented. The Romans originally divided the month into three sections of the Eight Phases of the Moon.
    • The Kalends:
      • This was the first observation of the visible new moon’s crescent.
    • The Nones:
      • This was the first quarter Moon, after counting down from the Kalends.
    • The Ides:
      • This was the full Moon. The longest countdown then started to the first observation of the visible crescent of the following month’s moon, the next Kalends.
  • However, in the fifth century BCE, the Romans ceased correlating the Moon Phases with the Calendar, and instead used fixed days of the month.

Julian Calendar

Roman Festivals

  • The dates of Roman Festivals were originally timed according to the positions of the Sun, Moon and Planets in the Calendar.

Ptolemy: The Great Roman Astronomer

  • Claudius Ptolemaeus (c.90-c.168 CE), known as Ptolemy, was a Greco-Roman Geographer, Mathematician, Astronomer and Astrologer who lived and worked in Alexandria, Egypt. He was able to study the Ancient Greek Astronomers and then produce his own Works.
  • He is best known for his Geographia, a Map of the World, and the Almagest, a listing of all the Stars and Constellations, amongst other works.
  • Ptolemy's Geographia
    • A Geography of the known World.
  • Ptolemy's Almagest
    • An astronomical Work in 13 books based on a Geocentric Universe.
    • Ptolemy states that Hipparchus was his source for calculating the Mean Synodic Month.
  • Ptolemy's Tetrabiblos
    • Also known as ‘Quadripartitum’ meaning ‘the four books’, is an Astrological Work combining Astrology with Philosophy of Aristotle.
  • Ptolemy's Planisphaerium
    • A description of the mathematical calculations needed to project the stars on to the celestial sphere. This appears to be a guide to constructing an Astrolabe, but there is no proof that he built one, although he did build an Armillary Sphere.
  • ‘Optics’
    • A Work describing the Reflection, Refraction and Colour of Light. It contains a Table giving the angle of refraction of light from air to water.

Sosigenes

  • Julian Calendar (45 BCE)
    • Sosigenes was an Astronomer at Alexandria, Egypt, who advised Julius Caesar on the creation of the Julian Calendar in 45 BCE. He was the author of 3 Treatises on the Calendar, now lost.

Ancient Greek Astronomers

  • Thales
    • Thales of Miletus (624-526 BCE) was a Greek Astronomer and one of the Seven Sages of Greece.
    • He wrote a Work, ‘Astronomy’, written in verse, according to Plutarch. This was the first Greek Work on Astronomy. He developed the theory known as Thales Theorem.
  • Hipparchus
    • Hipparchus (c.190-c.120 BCE) was a Greek Astronomer who is known as the ‘Father of Trigonometry’, because he compiled the first known Trigonometric Table. He used this to calculate the eccentricity of the Sun and Moon’s orbits.
    • He wrote a Work, the ‘Commentary on the Phaenomena (appearances) of Eudoxus and Aratus’, which showed how to calculate the rising and setting of the Constellations and the Milky Way.
  • Euclid
    • Euclid was a Greek mathematician who lived and worked in Alexandria around 300 BCE.
    • His main work was ‘Elements’, which holds all the theories of Euclidean Geometry including a description of Prime Numbers.

Astronomical Instruments

  • Armillary Sphere
    • The Armillary Sphere is a model of the sky centred on the Earth used to display the positions of the Planets and Stars.
  • Astrolabe
    • The Astrolabe was a device used in Celestial Navigation to calculate the user’s Latitude and local time. Astrolabe means ‘star-taker’ in Greek.
  • Dioptra
    • The Dioptra was a sighting line attached to a protractor, enabling the elevation of a celestial object to be read in degrees.
  • Gnomon
    • The Astrolabe was a device used in Celestial Navigation to calculate the user’s Latitude and local time. Astrolabe means ‘star-taker’ in Greek.
  • Roman Sundial
    • The Roman sundial was either a semicircular dial or a concave hemispherical surface with a nodus that measured the sun’s shadow.
    • Vitruvius in Book IX of De Architectura lists all known types of Sundial.

The Planets and Stars

Heliacal Rising and Setting

Eclipses

Equinoxes

Solstices

  • Summer Solstice
    • The Summer Solstice is the day the Sun ceases to rise in the Sky and starts to descend, which is the longest day of the year.
  • Winter Solstice
    • The Winter Solstice is the day the Sun ceases to descend in the sky and starts to rise again, which is the shortest day of the year.

The Two Definitions of the Seasons

  1. The Astronomical Seasons: (based on the Earth’s position relative to the Sun)
    •  (N. Hemisphere)
      • Spring: March 21st (Vernal Equinox)
      • Summer: June 21st (Summer Solstice)
      • Autumn: September 22th (Autumnal Equinox)
      • Winter: December 21st (Winter Solstice)
  2. The Meterological Seasons: (based on the annual temperature cycle)
    • (N. Hemisphere)
      • Spring: March 1st
      • Summer: June 1st
      • Autumn: September 1st
      • Winter: December 1st

Zodiac

  • The Zodiac is the area of the sky through which the Sun, Moon and Planets travel, which is a path eight degrees either side of the Ecliptic.

Geographical coordinate system

Celestial coordinate system

Celestial Navigation

  • Celestial Navigation is navigating without using a compass based on the known positions of the stars in the Night Sky.

 

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