The Copernican (Heliocentric, or Sun-centered) Model of the Solar System

Nicolaus Copernicus
Polish astronomer, mathematician, and canon
Modeled the movements of the Sun, the Moon, and the five known planets in the skies above, using a heliocentric system of orbits.
Born: 1473 in Torun, Poland
Died: 1543 in Frombork, Poland

Quotation: Finally we shall place the Sun himself at the center of the Universe. All this is suggested by the systematic procession of events and the harmony of the whole Universe, if only we face the facts, as they say, `with both eyes open'.

The advances in cosmology of the sixteenth century were built upon a strong framework. They combined the mathematical advances of the Greeks with the significant advances in observation and instrumentation of the Islamic world. Al-Haitham (965 – 1040) was born in Basrah, part of present-day Iraq. He promoted the scientific method of inquiry, and developed the field of optics in its infancy. His astronomical work included

In the heliocentric system, the Sun is considered to be the center of the solar system. All of the planets rotate about the Sun. The Moon rotates around the Earth, and thus moves around the Sun with the Earth.

An illustrationof the heliocentric model of the solar system, from the Harmonia Macrocosmica star atlas of Dutch-German cartographer Andreas Cellarius, circa 1660.

Tycho Brahe (1546 – 1601) developed a comprehensive method of recording precise observations of the skies, before the advent of the telescope. His many detailed observations included:

Tycho proposed an intermediate model of the universe, where the Earth was still the center point, but the other planets were allowed to rotate around the Sun as it rotated around the Earth. This was an intermediate solution, designed to explain the observational evidence that the planets moved around the Sun while preserving the sacrosanct notion that the Earth was the center of the Universe. Copernicus was willing to move one step further away from orthodoxy. By removing the Earth from its central position he freed the orbital paths of the planets from the extremely convoluted models of the Ptolmeic system, paving the way for Kepler's laws of elegant symmetry.

Galileo Galilei (1564 – 1642) was the first to provide strong, compelling evidence in favor of this system. He made a number of observations with the (newly invented) telescope, and discovered:

What Are the Possible Phases of Venus?
Heliocentric ModelGeocentric Model
[NMSU, N. Vogt]

The diagram drawn above shows Venus as it could appear in the heliocentric system and in the geocentric system. The magenta portion of each image of Venus (in positions 1 through 12, forming a circle) represents the side which faces the Sun, while the green line divides the side which faces toward Earth (the side that we see) from the side which is hidden from us. The row of images across the top of the frames shows Venus as it would appear from Earth if visible (note: in positions 1 and 5 through 9, Venus would be lost in the sunlight if viewed by eye).

In the heliocentric system, the twelve positions cover an entire range of the possible locations of Venus around the Sun as viewed from Earth. In the geocentric system, the twelve positions cover an entire range of the possible locations of Venus on its epicyclic path as it travels around the Earth.

Under the geocentric model, could Venus ever appear even near to fully illuminated (as a filled magenta circle) to us on Earth? Because this model requires that Venus always lie closer to the Earth than the Sun does (Venus can never pass behind the Sun), Venus can never be illuminated with more than a crescent portion.