Observing the Night Sky: The Birth of Astronomy

CHAPTER 2- OBSERVING THE NIGHT SKY: THE BIRTH OF ASTRONOMY

LEARNING OBJECTIVES

  • Define the main features of the Celestial Sphere and explain the system astronomers use to describe the sky.

    • The celestial sphere is an imaginary sphere surrounding Earth, where stars and celestial objects appear fixed as it rotates around Earth's axis.

  • Describe how the motions of the stars, Sun, Moon, and planets appear to us on Earth.

    • Due to Earth's rotation, celestial objects appear to move in circular paths about the celestial poles.

  • Understand the modern meaning of the word constellation.

    • A constellation is a recognized group of stars that are viewed as a pattern in the night sky, used to map regions of the sky.

  • Describe how the ancient Greeks were able to deduce that the Earth is spherical.

    • The Greeks observed various phenomena, such as the way ships disappear hull-first over the horizon and the round shadow of Earth during a lunar eclipse.

  • Summarize Eratosthenes’ method for calculating the circumference of the Earth.

    • Eratosthenes measured the angle of the shadow cast by a monument in two different cities and calculated Earth's circumference using the proportion of these angles.

  • Explain what precession is and who discovered this third motion of the Earth.

    • Precession is the slow, conical motion of Earth's axis. It was discovered by Hipparchus around 150 B.C.

  • Explain how Ptolemy was able to account for the retrograde motion of the planets within the geocentric model.

    • Ptolemy added epicycles within the geocentric model to explain the apparent retrograde motion of planets.

2.1 THE SKY ABOVE

GEOCENTRIC MODEL

  • Ancient Framework: Ptolemy’s Geocentric Model positioned Earth at the center of the universe, with celestial bodies orbiting in complex epicycles.

  • Observational Basis: This model was based on naked-eye observations, explaining planetary motion through deferents and epicycles to match the apparent retrograde motion.

AN INTRODUCTION TO THE CELESTIAL SPHERE

  • The Earth is not the center of the Universe or the Solar System, but our night sky observations can give this impression.

  • The sky appears as a vast, rotating sphere, with stars seemingly fixed within it.

  • The apparent motion of the sky is caused by the Earth’s rotation, not the stars’ movement.

THE SKY AROUND US

  • Horizon: Where the sky meets the ground.

  • Zenith: The point directly overhead for an observer.

CIRCLES ON THE CELESTIAL SPHERE

  • The celestial sphere is an imaginary sphere surrounding Earth, with stars appearing fixed on its surface as it rotates.

  • Earth's rotation on its axis creates the illusion that the sky revolves around us, making celestial objects appear to move in circular paths.

  • This tilted representation positions the observer at the top of the celestial sphere with the North Pole marked as "N."

TERMS TO KNOW

  • Celestial Sphere: An imaginary sphere surrounding Earth where celestial objects appear to be projected.

  • Zenith: The point directly above an observer.

  • Horizon: The line where the sky meets the ground.

CELESTIAL POLES, CELESTIAL EQUATOR, AND CIRCUMPOLAR ZONE

  • Celestial Poles: The north and south celestial poles are points in the sky that appear to rotate due to Earth's axial rotation.

  • Celestial Equator: An extension of Earth's equator onto the celestial sphere, dividing the sky into northern and southern hemispheres.

  • Apparent Motion: Stars near the celestial poles trace circular paths while those near the celestial equator rise and set along straight trajectories.

Circumpolar Zone

  • Definition: The circumpolar zone is the region of the sky surrounding the North or South Pole where circumpolar stars remain visible year-round.

  • Extent: This zone stretches from the horizon to the zenith in the observer’s respective hemisphere.

  • Observability: Circumpolar stars within this zone never set, making them useful for navigation and long-term sky observations.

TERMS TO KNOW

  • Celestial Poles

  • Celestial Equator

  • Circumpolar Zone

RISING AND SETTING STARS

  • Diurnal Motion: Refers to the daily motion of the Sun, Moon, and stars caused by Earth's rotation on its axis.

    • Stars appear to transverse arcs about the celestial poles.

  • Pole Star: For the Northern Hemisphere, Polaris is the pole star, located within 1° of the North Celestial Pole.

THE ECLIPTIC

  • The annual motion of the Sun traces a path along the celestial sphere known as the ecliptic.

  • Solar Time Shift: Due to its movement along the ecliptic, the Sun rises approximately 4 minutes later each day relative to the stars.

  • Ecliptic Tilt: The ecliptic is inclined at 23.5° to the celestial equator, causing seasonal variations.

THE MERIDIAN

  • In astronomy, the meridian is the great circle passing through the celestial poles and the zenith and nadir of an observer's location.

CONSTELLATIONS ON THE ECLIPTIC

  • The ecliptic circle intersects with a set of constellations believed to be special by ancient people, forming part of their astrology systems.

    • Some constellations are visible at certain times while others are hidden by the Sun's brightness in the day sky.

TERMS TO KNOW

  • The Ecliptic

  • The Meridian

  • Latitude

LATITUDE EFFECTS

  • The visible movement of stars across the night sky is influenced by the observer's latitude on Earth:

    • North Pole (90°N): All stars circle Polaris and none rise or set, making them circumpolar stars.

    • Equator (0°): All stars rise in the East and set in the West without circumpolar stars.

    • Zenith: The point directly overhead at the observer's location.

WHY WAS THE NORTH STAR SO USEFUL FOR NAVIGATION?

  • Historically, seafarers relied on Polaris for determining latitude because the altitude of the North Star corresponds directly to the observer's latitude.

  • Altitude of the Pole Star (Polaris): Always equal to the latitude of the observer.

THE CELESTIAL TILT

  • The celestial equator is tilted by 23.5° to the ecliptic.

  • North Americans and Europeans perceive the Sun positioned north of the celestial equator in June and south in December.

FIXED AND WANDERING STARS

  • Ancient observers distinguished between fixed stars that maintain their positions and wandering stars that include planets.

    • The recognized wandering bodies are Mercury, Venus, Mars, Jupiter, Saturn, as well as the Sun and Moon.

ANCIENTS SAW STARS

  • Constellations: Groups of stars named for gods, animals, or objects, varying by culture; 48 ancient constellations exist, now officially recognized as 88 by the International Astronomical Union since 1930.

  • Cultural Variations: Different names and patterns for constellations exist across cultures, notably Greek, Latin, and Arabic influences.

WHAT IS AN ASTERISM?

  • An asterism is a specific pattern of stars that does not constitute an official constellation and can be part of a recognized constellation.

2.2 ANCIENT ASTRONOMY

ANCIENT ASTRONOMY AND THE GEOCENTRIC MODEL

  • Several ancient cultures such as Babylonian, Chinese, and Mayan civilizations made astronomical observations, but the work of the Greeks is most recognized.

  • The Greeks adopted a geocentric view using philosophical, observational, and geometric approaches to develop their models.

THE GEOCENTRIC MODEL: ASSUMPTIONS

  1. The Greeks considered the circle a perfect shape; hence celestial bodies must move in circular orbits.

  2. Observers on Earth cannot sense its motion, leading to the conclusion that Earth is stationary at the center of the universe.

  3. Therefore, other celestial objects were viewed as moving around Earth.

ACCOMPLISHMENTS OF THE ANCIENT GREEKS

  • Aristotle (384-322 BC): Supported the geocentric view and documented known phases of the Moon, the roundness of Earth, and relative distances of the Moon and Sun.

  • Aristarchus of Samos (312-230 BC): Proposed a heliocentric model and made geometric calculations showing the relationship between the size and distance of the Sun compared to Earth and the Moon.

HOW ERATOSTHENES CALCULATED THE EARTH'S CIRCUMFERENCE

  • Eratosthenes of Alexandria (276-194 BC): First to accurately estimate Earth's size, arriving at 250,000 stadia (~39,400 km), closely matching the modern value of 40,075 km.

STELLAR BRIGHTNESS AND PRECESSION

  • Hipparchus (c. 150 B.C.): Established an observatory on Rhodes and created a system of stellar magnitudes to classify brightness from 1st (brightest) to 6th (faintest).

  • Discovered precession, the slow, wobbling motion of the celestial pole, observing its effects over a century.

PRECESSION

  • Precession involves Earth's axis wobbling in a 26,000-year cycle, shifting its north celestial pole's position over millennia; now near Polaris, it was near Thuban 5,000 years ago and will be close to Vega in 14,000 years.

ALMAGEST: CLAUDIUS PTOLEMY

  • Ptolemy (c. 140 AD): In his 13-volume work "Almagest", he compiled and expanded upon ancient Greek astronomical knowledge, refining the geocentric model.

RETROGRADE MOTION

  • The initial geocentric model struggled with explaining retrograde motion; Ptolemy's revisions effectively accounted for this phenomenon using a more intricate model of planetary movement.

PTOLEMY’S MODEL

  • Marketed the belief that celestial bodies must move in circles or combinations of circles (epicycles and deferents) to meet observational patterns, leading to the accurate depiction of retrograde motion.

2.3 ASTROLOGY AND ASTRONOMY

SCIENCE AND PSEUDOSCIENCE

  • Astrology: Originated in Babylonia about 2,500 years ago, asserting that planetary and stellar positions influence human actions, yet lacks a statistical foundation, relying on zodiacal constellations.

THE HOROSCOPE

  • Horoscope: A chart illustrating planet placements at an individual’s time of birth, derived from Greek words meaning "time" and "watcher", thus translating to "marker of the hour".

2.4 THE BIRTH OF MODERN ASTRONOMY

THE DARK AGES

  • Following Rome's fall around 400 AD, Europe saw a significant loss of astronomical knowledge; however, the geocentric model persisted for 13 centuries, with Arab scholars preserving ancient texts like Almagest, translating them into Arabic to prevent knowledge loss.

  • Many star names in modern usage stem from Arabic names due to this preservation effort.

COPERNICUS: A BREAK WITH THE PAST

  • Nicolaus Copernicus (1473-1543) of Poland introduced a heliocentric model, asserting that Earth is a planet, rotating internally while orbiting the Sun alongside other planets, solidifying modern astronomy's foundations in his work "De Revolutionibus".

RETROGRADE MOTION: A NEW EXPLANATION

  • Copernicus’ heliocentric model elegantly clarified retrograde motion, describing how Earth overtakes Mars in orbit, causing an apparent backward motion relative to the stars.

GALILEO GALILEI

  • Galileo Galilei (1564-1642) supported the heliocentric model, playing an essential role as one of the first modern scientists, conducting groundbreaking astronomical and physical research.

GALILEO’S TELESCOPES

  • While Galileo did not invent the telescope, he constructed one in 1609 soon after learning of existing designs, and he was the first to systematically apply it to astronomical studies.

GALILEO’S ASTRONOMICAL DISCOVERIES

  • Galilean Moons: He discovered Jupiter's four largest moons, showing celestial bodies could orbit something other than Earth.

  • Phases of Venus: Observing phases similar to the Moon, which contradicted the Ptolemaic model, which only predicted crescent phases.

GALILEO, THE “MODERN” SCIENTIST

  • Conducted numerous empirical experiments, challenging Aristotelian doctrines. He documented his research, enabling others to replicate and validate his discoveries.

SUMMARY

  • The geocentric model has been disproven; nevertheless, the concept of the Celestial Sphere remains useful for explaining apparent celestial motion.

  • Earth's Rotation: Causes daily motions for celestial bodies, determining diurnal rise and set times, directly influenced by observer latitude.

  • Orbital Motion: Causes annual movements, with seasonal variations linked to Earth's axial tilt and orbit.

  • The Greeks established foundational concepts regarding Earth's spherical shape, proximity of celestial bodies, and calculated Earth’s circumference accurately.

  • Astronomical observations were systematically documented by Hipparchus, leading to further advancements.

  • Ptolemy's contributions established a robust geocentric model integrating observations of retrograde motion.

  • Astrology remains a pseudoscience with no empirical backing; in contrast, Copernicus spearheaded the shift to heliocentrism, while Galileo's discoveries underscored this new perspective.