ASTR 1P01 - Lecture 5: Modern Astronomy Summary

Modern Astronomy

Beginning of Modern Astronomy

• The beginning of modern astronomy occurred in the 16th and 17th centuries.
• A pivotal discovery was heliocentrism.
• An important question addressed was why we don’t feel the Earth moving.
• Key to understanding planetary motion are Kepler's laws.

Ptolemy’s Geocentric Model

• Recall of Ptolemy’s geocentric model.
• Planets orbit in a small circle called an epicycle.
• Each epicycle orbits in a large circle called the deferent.
• The Earth is not at the exact center of the deferent, but slightly off-center.
• The equant is on the opposite side of Earth, serving as the point with respect to which the epicycles move at a constant speed.

After Ptolemy

• Ptolemy’s model was accepted for 1,400 years.
• Ptolemaic geocentrism was around ~150 CE.
• Copernican heliocentrism was around ~1550 CE.
• During the Middle Ages in Europe (5th to 15th century), there wasn't much progress in astronomy.
• People were preoccupied with plagues, wars, and crusades, leaving little focus on scientific advancement.

Islamic and Persian Contributions

• Islamic and Persian astronomers and mathematicians made significant progress.
• They improved astronomical measurements and developed new mathematical methods.
• They also constructed the first observatories.

Islamic Observatories

• The first Islamic observatory was built around 1074 in present-day Iran.
• The great observatory of Maragheh was built around 1260, also in Iran.
• It featured high-quality instruments and a substantial library with 400,000 manuscripts on astronomy.
• Similar observatories were built in Uzbekistan in 1420 and Istanbul in 1575.

The Renaissance and Scientific Revolution

• The Renaissance period started in the 15th century in Europe.
• New developments in mathematics, physics, and astronomy emerged.
• These developments led to the scientific revolution, fundamentally changing humanity's understanding of nature and the universe.

Nicolaus Copernicus

• In the 16th century, Nicolaus Copernicus, a Polish astronomer, developed a heliocentric model of the solar system.
• The Earth was repositioned as one of several planets orbiting the Sun, rather than the center of the universe.
• In 1543, the year Copernicus died, his model was published in "On the Revolution of the Celestial Spheres".
• This publication is often considered the start of the scientific revolution.

Copernicus's Heliocentric Model

• Copernicus correctly placed the Sun at the center with the Earth and other planets orbiting it.
• He correctly stated that the celestial sphere stays fixed, appearing to rotate due to the Earth’s rotation around its axis.

Imperfections in Copernicus's Model

• The model still contained inaccuracies:
  • Planets moving in epicycles.
  • Deferents as circles.
  • Epicycles moving at a constant speed.
  • Celestial bodies embedded on actual celestial spheres.
• Today, we know orbits are ellipses without epicycles or celestial spheres, and planetary speed varies along the orbit.

Prior Heliocentric Ideas

• Heliocentrism was not new; Aristarchus of Samos initially suggested it in ancient Greece.
• Heraclides Ponticus proposed the Earth rotates around its axis a century before Aristarchus.
• However, these ideas were not seriously considered until Copernicus's time, 1,800 years later.

Copernican Revolution

• Copernicus's work sparked over a century of scientific progress known as the Copernican revolution.
• Outcomes:
  • Practical: A correct understanding of the solar system's structure.
  • Philosophical: The Copernican principle, which states that humans and the Earth are not special or the center of the universe.
  • Earth is just one planet in one solar system, in one galaxy, among trillions of others, suggesting potential for other life forms.

Objections to Heliocentrism

• Religious objections arose from contradictions with a literal interpretation of the Bible.
• Religious texts reflect beliefs at the time of writing, before the scientific method was available, and are therefore not reliable scientific sources.
• Consistency of the Bible with a geocentric model is because its authors believed in geocentrism.

Copernicus's Response to Religious Objections

• Copernicus responded that those claiming his model contradicts the Bible are interpreting it to benefit their argument.

Scientific Objections to Heliocentrism

• Absence of stellar parallax was a significant scientific objection, raised by the ancient Greeks against Aristarchus.

Stellar Parallax

• The apparent shift in the position of a nearby star when viewed from different points in Earth's orbit around the Sun.

Contradiction with Daily Experience

• The claim that Earth is moving contradicted daily life experiences.
• Questions included:
  • Why we don't feel the motion.
  • Why objects not bolted down don't fly away.
  • Why a dropped ball hits the ground directly below instead of at an angle.

Earth's Motion

• The Earth rotates around its axis at approximately $0.46 \, \text{km/s}$ at the equator and orbits the Sun at about $30 \, \text{km/s}$.
• Ancient Greeks only considered these motions.

Current Understanding of Motion

• The entire solar system, including Earth, moves around the center of the Milky Way galaxy at $220 \, \text{km/s}$.
• The galaxy itself moves towards the Great Attractor at approximately $1,000 \, \text{km/s}$.

Copernicus's Lack of Answers

• Copernicus did not have definitive answers to these objections.
• People believed in Aristotle's theory that being at rest is the