ASTR 1P01 - Lecture 3: Ancient Astronomy Summary

Astronomy and the Scientific Method

Astronomy has a history of replacing old theories with new ones based on evidence.
Astronomy provides an opportunity to learn about the scientific method, critical thinking, and skepticism.
These skills are important for understanding the universe and distinguishing truth from falsehood.
Modern astronomy provides an understanding of:
- Physical laws governing celestial objects.
- Details about celestial objects (distance, size, composition, evolution).
- Telescopes and tools for measuring properties beyond naked eye observation.
Modern humans evolved approximately $300,000$ years ago, and recorded history began approximately $5,000$ years ago.
Significant knowledge about the universe has been acquired in the last few centuries, with important discoveries made recently.
In 1923, it was discovered that the Milky Way is not the entire universe but just one galaxy among many.
Black holes were theorized over a century ago, with the first image captured in 2019.

Ancient Astronomy

In prehistoric times, humans lacked understanding of celestial bodies and often associated them with "gods."
Early astronomers were priests of ancient religions who believed celestial objects influenced their lives, leading to the origin of astrology.
Astronomy was linked to religion and astrology for millennia across various ancient cultures.
Eventually, humans realized celestial objects are governed by natural laws like those on Earth.
Astronomy requires data collection and analysis, necessitating writing and mathematics, which emerged around $5,000$ years ago.
Ancient civilizations like the Babylonians and Egyptians recognized the periodicity of astronomical phenomena, using them to track time.
Calendars were developed to predict seasonal changes, aiding agriculture.
This need was a main driver for advancements in mathematics.
Babylonian astronomers recorded positions of the Moon and planets for centuries.
By approximately 400 BC, they identified regular patterns, enabling predictions of celestial positions and lunar eclipses, but accurate solar eclipse prediction remained elusive.
Ancient Chinese astronomers also kept astronomical records.
The oldest written solar eclipse record dates back to around 2000 BC in China.
Chinese astronomical catalogs spanning 3,000 years contain details of eclipses, comets, meteors, exploding stars, and sunspots.
This historical data is used by modern astronomers.

Axial Precession

Ancient Greek astronomer Hipparchus built an observatory around 150 BC on Rhodes.
He measured celestial object positions accurately and created a star catalog with approximately 850 entries with celestial coordinates.
Hipparchus categorized stars by apparent magnitude based on brightness; brighter stars have smaller magnitudes.
The term "magnitude" is still used today but with a more precise definition.
Apparent brightness differs from actual brightness (luminosity).
Stars with equal luminosity at different distances have different apparent magnitudes.
Hipparchus compared his data with older observations and discovered that the north celestial pole's position changes over time through axial precession.
The Earth's rotation resembles a spinning top with two types of rotation:
- Fast spin around the axis.
- Slow precession of the axis itself.
The Earth bulges at the equator.
Gravitational forces from the Sun and Moon act on this bulge, causing the axis to precess, completing a full circle in roughly $25,700$ years.
Precession causes the north and south celestial poles to change.
Currently, the north celestial pole is near Polaris, the "North Star."
Around 14,000 years ago, Vega was the North Star, and it will be again in about 11,700 years.

The Spherical Earth

Many ancient cultures initially believed in a flat Earth.
However, evidence easily proves Earth is spherical.
Ancient Greeks knew this as early as Pythagoras's time, 2,500 years ago.
Aristotle collected evidence around 330 BC, leading to widespread acceptance among Greek scholars.
Evidence for a spherical Earth:
- Lunar Eclipses: Earth's round shadow is visible on Moon during lunar eclipses.
- Only spherical objects produce round shadows regardless of orientation.
- If Earth were not spherical, lunar eclipse shadows would vary with the relative positions of the Sun and Earth.
- Visibility of Stars: Travelers moving south see new stars not visible in the north, and the North Star's height decreases southward.
- This occurs because different parts of the sky are visible from different points on a sphere; a flat Earth would allow everyone to see the same stars.
- Photos from Space: Modern technology provides direct evidence via satellite and astronaut photos, consistently showing a spherical Earth.
- Disappearing Ships: Ships sailing into the horizon disappear gradually from bottom to top.
- This occurs because ships go down the curve of the Earth, similar to an ant walking on an orange.
- A flat Earth would show the ship becoming smaller but not disappearing.
- View from Different Heights: Higher altitudes allow seeing farther distances.
- The Earth's curvature obstructs views from lower altitudes.
- A flat Earth would allow seeing the same distance from any height.
- Circumnavigation of the Earth: Magellan-Elcano expedition (1519-1522) circumnavigated the globe without finding an edge.
- Numerous subsequent circumnavigations have occurred by boat and plane.
- Circumference of the Earth: Eratosthenes measured Earth's circumference around 240 BC using sticks in Syene and Alexandria, approximately $800$ km apart.
- Different shadow lengths indicated varying angles of sunlight due to Earth's curvature. If the Earth was flat, the stick would have cast the same shadow.
- In Syene, the Sun was at the zenith (0° angle), while in Alexandria, it was ~ $7.2$ ° south.
  $360 / 7.2 = 50$ , so the circumference is 50 times the distance between the cities.

The Geocentric Model

Before the 17th century, the geocentric model (Earth-centered) prevailed due to:
- Lack of awareness of Earth's movement.
- Religious views emphasizing Earth's central role.
Over time, astronomers realized the geocentric view was incorrect.
The current understanding is the heliocentric model (Sun-centered).
The Sun is one of trillions of stars, each with its own solar system, planets, and potential for life.
Earth and humans are relatively insignificant in the context of the universe.

The Heliocentric Model and Parallax

Aristarchus of Samos (310-230 BC) proposed the first heliocentric model, but it was rejected by most Greek scholars.
One argument against it was the lack of observed stellar parallax.
Parallax describes that closer objects appear to move relative to distant objects as the observer's position changes.
In the context of Earth's orbit, nearby stars should shift positions relative to distant stars over the year (stellar parallax).
The Greeks couldn't detect stellar parallax, leading them to two conclusions:
- Earth doesn't revolve around the Sun, or
- Stars are too distant for parallax to be measurable.
The latter is true; stars are light-years away, distances inconceivable to the ancient Greeks.
Stellar parallax exists but requires precise modern instruments to detect.
Friedrich Bessel measured the stellar parallax of 61 Cygni in 1838, estimating its distance at approximately $11.4$ light-years.

Ptolemy

Ptolemy (2nd century) wrote Almagest, a comprehensive astronomy treatise.
The book compiled existing knowledge and introduced a geocentric model predicting planetary positions based on Hipparchus's data in addition to his own.
This model was used for over 1,400 years.
Planetary motion results from:
- The planet's motion around the Sun.
- The Earth's motion around the Sun.
Planets move along the zodiac, and the Sun drifts eastward relative to constellations over the year.
Planets generally move eastward (prograde motion) but sometimes appear to move westward (retrograde motion).
Retrograde motion occurs when Earth passes a planet, creating the illusion of backward movement. Ptolemy's model explained retrograde motion using:
- Epicycles: small circles planets orbit on.
- Deferents: large circles epicycles orbit on.
- Earth is off-center from the deferent.
- Equant: point opposite Earth used to maintain constant epicycle speed.

Science and Simplicity

The Sun is at the solar system's center, and planets move in ellipses, not epicycles.
The epicycle model is not an accurate depiction of planetary orbits, yet it predicted planetary motion.
With enough epicycles, any shape can be approximated mathematically, including ellipses.
Ptolemy's geocentric model accurately predicted but lacked explanatory power because it simply moved complexity around instead of explaining it.
Scientific models should offer simple mechanisms for explaining complex results.
The heliocentric model with elliptical orbits and gravity laws is a simple model that accurately predicts planetary motion.
Occam's razor states that simpler theories are better in terms of predictive value.
Effective scientific theories generate significant output based on minimal input.
The heliocentric model provides much output for little input, while Ptolemy's geocentric model requires as much input as it gives output, making it unsuitable for explaining planetary motion.

Modern "Flat-Earthers" and Geocentrists

Geocentrism and flat Earth theories have been disproven by science.
Despite evidence to the contrary, some people still believe in these theories.
Literal interpretations of religious scriptures and misinformation spread through conspiracy theories often motivate these beliefs.
Conspiracy theorists claim that:
- The Earth is flat.
- This truth is hidden by a global conspiracy.
- Millions of scientists are keeping this secret.
Belief in geocentrism or flat Earth is relatively rare, but irrational beliefs like astrology are common.

Conclusions

Concepts like axial precession and parallax are still used today.
Ancient ideas such as a geocentric universe are obsolete due to modern scientific advances.