Astronomy Topic 4

Astronomy Through the Ages

Two Eyed Seeing

Viewing the universe through two lenses: western eyes and indigenous eyes to gain a more holistic understanding of knowledge and perspectives.

Wurdi Yuang

Aboriginal Stonehenge near Glenrowan, Australia, aligning with astronomical events.

Mayan Astronomy

~2000 BC - ~1000 AD

This civilization's extensive study of celestial bodies, involving calendars and aligning structures.

Indigenous Astronomy

Incans: Machu Piccu, ~1400-1600 AD

A study of celestial phenomena

use of astronomical alignments and rituals in alignment with agricultural cycles.

Indigenous Astronomy in the Americas

Eratosthees and Size of the Earthn

He concluded that Alexandria and Syene are 7° apart

on the Earth, and hence ~1/50 of Earth’s

circumference apart. 276-195 BC

Using geometry, he estimated the Earth's circumference to be about 39,375 kilometers.

Aristarchus and Distances in Solar System

he found that the Sun is  ~20x further away from Earth

than the Moon is, with angle between Sun-Moon 87° (310-230 B.C.)

Aristarchus and Size of Moon

He estimated Moon as ~ ½ size of Earth (more like ¼)

Aristarchus and Size of Sun

He argued that Sun is ~20 times bigger/further than the Moon (in reality ~400 times). Combined with Eratosthenes, could then determine sizes of Moon and Sun, and distances to them.

Geocentric Cosmology

The Earth is at the centre, unmoving

The planets, sun and moon orbit us

All motion is in the form of perfect circles

All motion is at uniform speed

The Greek Geocentric Model

An Earth-centered, or geocentric,model of the universe that posits the Earth as the stationary center around which the Sun, Moon, and stars orbit.

The First Problem with Geocentric Model

The Sun and Moon change their apparent sizes!

Second Problem with Geocentric Model

The “merry-go-round” model doesn’t explain retrograde motion―periods when the planets appear to move backwards in the constellations.    Biggest problem

The Ptolemaic System

To explain retrograde motion, he created a complicated system of spheres on spheres: epicycles and deferents that allowed for the apparent backward motion of planets.

Third Problem with Geocentric Model

Why are they always near the Sun? 

  (Visible in the twilight or dawn sky, never at midnight) This observation raises questions about the orbits of planets and their position relative to the Earth and Sun, indicating inconsistencies in the geocentric model.

Copernicus

He Wanted to simplify; disliked the contrived Ptolemaic model.

Hesitated to publish. 

But places Sun at the center!    *Not the first … (1473-1543)

remember Aristarchus

heliocentric model

An astronomical model that places the Sun at the center of the universe, with planets, including Earth, orbiting around it, contrasting with the geocentric model.

retrograde motion

The apparent movement of a planet in the opposite direction to its usual orbit, often observed from Earth. This phenomenon is explained by the relative positions and motions of Earth and the other planets.

superior planet

A planet outside Earth's orbit, such as Mars, Jupiter, or Saturn, which appears to move more slowly in the sky compared to inner planets.

inferior planet

A planet that orbits inside Earth's orbit, such as Mercury or Venus, which appears to move more quickly in the sky compared to outer planets.

inferior conjunction

The alignment of an inferior planet with the Sun, where the planet is directly between the Earth and the Sun.

superior conjuction

The alignment of a superior planet with the Sun, where the planet is positioned directly opposite the Earth from the Sun.

opposition

The position of a planet when it is directly opposite the Sun from Earth, maximizing visibility and brightness. This alignment occurs for superior planets and is an important event for observation.

Elongation

the angle between the Sun and a planet, as viewed from Earth.

Conjunction

a planet and the Sun lining up, as viewed from the Earth.

Sidereal Period of Inferior Planet to Synodic Period

1/P = 1/E + 1/S

P=sidereal period S = synodic period

E=Earth’s sidereal period = 1 year = 365.25days

The time it takes for an inferior planet to complete one orbit around the Sun relative to the distant stars, typically shorter than its synodic period.

Sidereal Period of Superiors Planet to Synodic Period

1/P = 1/E – 1/S

P=sidereal period S = synodic period

E=Earth’s sidereal period = 1 year = 365.25days

The time it takes for an inferior planet to complete one orbit around the Sun relative to the distant stars, typically shorter than its synodic period.

Galilea’s important discoveries

First 4: Features on the moon, Sunspots and their motions,Extensions on Saturn, and Stars in the Milky Way

included the 4 moons of Jupiter, the phases of Venus, sunspots, and the rings of Saturn.

Phases of Venus

This discovery of Galileos was key to support the heliocentric model of the universe

Galileo’s Trial and House Arrest

After opposing the Catholic Church's geocentric views, Galileo was tried by the Inquisition in 1633 and found guilty of heresy. He spent the remainder of his life under house arrest, continuing his scientific work. 

moons of Jupiter

Discovered by Galileo, these four objects—Io, Europa, Ganymede, and Callisto—provide critical evidence for the heliocentric model of the solar system.

Tycho Brahe and Kepler’s Laws of Planetary Motion

three laws of planetary motion, describing the elliptical orbits of planets and how they move around the sun

parallax

the apparent shift in position of an object viewed from different angles, used to measure distances to nearby stars.

Keplers 3 questions

1. What shapes are planetary orbits? Are they really circles?

2. Do planets move at constant speed in their orbits?

3. How do the different planets compare in their speeds and/or orbital periods?