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Geometric Universe




Slide 18

Heliocentric Model of Planetary Motion (Copernicus, 1473-1543 CE)

Heliocentric Model of Planetary Motion (Copernicus, 1473-1543 CE)

In the heliocentric model, the Earth and other planets orbit the Sun.

The prograde and retrograde motions are apparent effects due to relative motions of the Earth and the planets.

Slide 19

Advantages of the Heliocentric Model

Advantages of the Heliocentric Model

The heliocentric model of Copernicus is not intrinsically more accurate.

Calculation is easier with the Copernicus model.

Copernicus was able to determine the orbital radii (relative to Earth orbit) of all six planets, while in Ptolemy model the lengths are incorrect.

Heliocentric models predict stellar parallax, while geocentric models predict otherwise.

Slide 20

Further Developments

Further Developments

A schematic heliocentric model is shown on the right. The heliocentric model would later be a great help to Kepler (1571-1630 CE) in finding his laws of planetary motions empirically.

Later Newton (1643-1727 CE) gave the model a firm physical basis using law of gravity and motion would

Stellar parallax, hence distance, was first measured in 1838 (Bessel).

In Copernicus’s theory, the Sun is at the center of the universe, while the Earth is merely a planet.

We now know that Sun is just one of the stars in one of the galaxies (Milky Way Galaxy) in one of the group of galaxies (Local Group) in one of the superclusters (Virgo/Local Supercluster) in the universe.

Slide 21

Modern View of the Solar System

Modern View of the Solar System


Terrestrial planets


Gas Giants (outer planets)

Trans-Neptunian Objects (TNO)

Kuiper Belt

Scattered Disc

Oort Cloud (hypothetical)


Note: Dots represent objects. Someone looking at the solar system at this scale shouldn’t see asteroids and the Oort cloud with naked eyes. Much of the Solar System is empty space.

Slide 22

Orion Arm

Orion Arm

Note: nebulae are usually in spiral arms.

Milky Way Galaxy (2-4x109 stars)

Note: globular clusters (105-106 stars) orbit the galactic core as satellites.

Solar Neighborhood

Beyond the Solar System (Hierarchy of Objects) [pictures from]

Slide 23

Beyond the Milky Way [pictures from]

Beyond the Milky Way [pictures from]

Local Group (30+ galaxies)

Neighboring Superclusters

(100 superclusters shown)

Virgo Supercluster

(100 groups/clusters of galaxies)

Visible Universe

(107 superclusters)

* visible ≠ whole

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