Mars Timeline of Discovery:
1570BC thru 1799

By Jim Plaxco

Last Updated: December 1999

The Mars Timeline of Discovery is a work in progress. Its goal is to give the reader a general feel for the progress in humankind's understanding of the planet Mars.

The timeline is divided into two parts. Part 1, this document, covers the years 1570BC thru 1799. Part 2 covers the years 1800 thru 1962.

As early as the 28th Dynasty of the New Kingdom, Egyptians refer to Mars as "Horus of the Horizon", a God whom they depicted as a human with the head of a hawk. They also spoke of the planet travelling backwards, a reference to its retrograde motion.

Aristotle (384-322 BC), the Greek philosopher, happens to observe an occultation of Mars by the Moon. From seeing the Moon pass in front of Mars, Aristotle concludes that Mars is higher up in the heavens than the Moon.

Claudius Ptolemy (120-180) publishes his book, Almagest sometime between 127 and 141. The title means "The Greatest Compilation". This book presents Ptolemy's ideas of how the Universe worked. This came to be known as the Ptolemaic System. It made use of epicycles and equants to describe the orbits of the planets. In this system, the hierarchy of the Universe was Earth - Moon - Mercury - Venus - Sun - Mars.

Tycho Brahe (1546-1601), the Danish astronomer and master of Uraniborg (an observatory built before telescopes but containing instruments to accurately determine the position of objects in the night sky), makes the most detailed measurements to date of the location of Mars in the night sky. He carries out these measurements over 9 apparitions of the planet.

Johannes Kepler (1571-1630), a German astronomer hired by Tycho Brahe as an assistant, publishes his first two laws of planetary motion in his book Astronomia Nova... Commentaries on the Motions of Mars. These laws are

1. The orbit of each planet is an ellipse with the Sun at one of its foci.
2. Each planet will sweep out equal areas in equal time.

Kepler's discover was made possible largely through his use of Tycho Brahe's accurate measurements of the position of Mars in the night sky. It has been postulated that Kepler might not have made his discoveries had he chosen to use a planet other than Mars for his analysis - the nature of the Martian orbit lent itself admirably to Kepler's attempt to create a better model for predicting the positions of the planets.

Galileo Galilei (1564-1642), a professor at the University of Padua in Venice, Italy, uses the telescope, invented in 1608, to observe Mars, a first.

Galileo Galilei (1564-1642) reports in a letter, dated December 30, 1610, to a friend that "I dare not affirm that I was able to observe the phases of Mars; nevertheless, if I am not mistaken, I believe that I have seen that it is not perfectly round."

Johannes Kepler (1571-1630) publishes his 3rd law of planetary motion in his bookHarmonice mundi (The Harmony of the Worlds). This law states that the square of the orbital period is proportional to the cube of the body's mean distance from the Sun. Again, Kepler used the data for the orbit of Mars to discover this law.

Francisco Fontana, an amateur astronomer from Naples, Italy, makes the earliest known drawing of Mars based on his telescopic observations. He observes that "the disk of Mars is not uniform in color."

Francisco Fontana of Naples, Italy produces the second known drawing of Mars on August 24. This drawing, made when Mars was at quadrature, shows Mars in a gibbous phase.

Christiaan Huygens (1629-1695), a Dutch astronomer and discoverer of the rings of Saturn, first describes a feature on Mars that we believe to be what we call Syrtis Major. The feature is present in a drawing he made on October 13, 1659. Huygens describes this feature's appearance as like a large bog.

Christiaan Huygens (1629-1695), a Dutch astronomer, estimates that Mars is about 60% the size of the Earth. He makes this determination by:

• Observing Mars telescopically with an eyepiece that has a micrometer installed;
• Determines the angular size of Mars based on micrometer measurements;
• Estimates the distance from Earth to Mars;
• Using geometry to estimate the true size of Mars from the size as measured in his eyepiece.

Christiaan Huygens (1629-1695), a Dutch astronomer and discoverer of the rings of Saturn, estimates the Martian day to be 24 hours long. He does this based on his observations of the rotation of the albedo feature Syrtis Major.

Gian Domenico Cassini (1625-1712), a French astronomer, observes that Mars has a polar cap.

Gian Domenico Cassini (1625-1712), a French astronomer, states that he has determined that the length of a Martian day is 24 hours, 40 minutes long. This is based on his observations of the time it takes Martian surface features to complete one rotation.

Christiaan Huygens (1629-1695), a Dutch astronomer and discoverer of the rings of Saturn, makes a drawing of Mars that includes the south polar cap.

Cassini sends fellow astronomer Jean Richer to Cayenne, French Guiana in South America. His mission is to measure the position of Mars in the night sky. At the same time, Cassini, from Paris, France, would also measure the position of Mars in the night sky. By combining their measurements, Cassini was able to determine the parallax of Mars. This made it possible for him to then determine the distance from the Earth to Mars at the time of their measurement. The result of this calculation was then used to help determine the size of the Astronomical Unit.

Christiaan Huygens (1629-1695), publishes Cosmotheoros, a book about whether or not there is life on Mars.

Maraldi observes both polar caps of Mars. He describes them as "taches blanches" (white spots). Further, he observes that the cap at the south pole is not centered on Mars' axis of rotation. He also notes that the north pole was bordered by a dark band. Maraldi interpreted this to be melt water.

William Herschel (1738-1822), a British astronomer and discoverer of Uranus, discovers that the inclination of Mars axis of rotation is approximately 24 degrees. He makes this determination based on his observation of the way in which Martian surface features rotate.

William Herschel (1738-1822), a British astronomer, observes the white areas around the poles of Mars and deduces that they are polar ice caps. While others had previously observed the caps, none had gone so far as to claim that they were made of ice.

William Herschel (1738-1822), a British astronomer, in an address to the Royal Society has the following to say about Mars:

It appears that this planet is not without considerable atmosphere; for besides the permanent spots on the surface, I have often noticed occasional changes of partial bright belts; and also once a darkish one... These alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapors floating in the atmosphere of the planet... Mars has a considerable but modest atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to our own.

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