Early Greek astronomers tried to explain the motion of the five then-known planets: Mercury, Venus, Mars, Jupiter, and Saturn. Most people at that time held a geocentric view of the universe: Based on the observed motion of the celestial sphere, they believed that the Sun, the Moon, the stars, and the planets revolved around Earth. A theory of the overall structure and evolution of the universe is called a cosmology, so the prevailing Earth-centered cosmology was called geocentric. Geocentric cosmology, consistent as it is with casual observation of the sky, held sway until the sixteenth century, when Copernicus revolutionized our understanding of the cosmos.

Explaining the motions of the five planets in a geocentric universe was one of the main challenges facing the astronomers of antiquity. The Greeks knew that the positions of the planets slowly shift relative to the “fixed” stars in the constellations. In fact, the word planet comes from a Greek term meaning “wanderer.” They also observed that planets do not move at uniform rates through the constellations. From night to night, as viewed in the northern hemisphere, the planets usually move slowly to the left (eastward) relative to the background stars. This movement is called direct motion or prograde motion. Occasionally, however, a planet seems to stop and then back up for several weeks or months. This reverse movement (to the right or westward relative to the background stars) is called retrograde motion. Both direct and retrograde motions are best observed by photographing (Figure 2-2a) or plotting (Figure 2-2b and 2-2c) the nightly position of a planet against the background stars over a long period.

All planetary motions on the celestial sphere are much slower than the apparent daily movement of the entire sky caused by Earth’s rotation. Throughout a single night it is very difficult to detect motion of the planets among the stars. Therefore, the planets always rise with the stars in the eastern half of the sky and set with them in the western half.

The effort to understand planetary motion—and especially to explain retrograde motion—in a geocentric cosmology resulted in an increasingly contrived and complex model (see Discovery 2-1: Earth-Centered Universe). The ancient Greek astronomer Aristarchus of Samos proposed a more straightforward explanation of planetary motion, namely, that all of the planets, including Earth, revolve around the Sun. The retrograde motion of Mars in this heliocentric (Sun-centered) cosmology occurs because the faster-moving Earth overtakes and passes the Red Planet (Figure 2-3). The occasional retrograde movement of a planet is merely the result of our changing viewpoint as we orbit the Sun—an idea that is beautifully simple compared to the geocentric system with all of its complex planetary motions. (The word heliocentric is misleading. Although the local planets, moons, and small pieces of space debris do orbit the Sun, the stars and innumerable other objects in space do not. In fact, the Sun and the bodies that orbit it all orbit the center of our Milky Way Galaxy.)

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Over the centuries following Aristarchus, increasingly accurate observations of the planets’ locations revealed errors in the predictions of geocentric cosmology. To reconcile that cosmology with the data, more and more complex motions were attributed to the planets. By the mid-1500s, the geocentric cosmology had become truly unwieldy in its efforts to predict the motions of the planets accurately. It was then that the Polish mathematician, lawyer, physician, economist, cleric, and artist Nicolaus Copernicus resurrected Aristarchus’s theory. Copernicus was motivated by an effort to simplify the celestial scheme, and eventually his heliocentric cosmology did just that.

Pursuing the heliocentric cosmology, Copernicus determined, by observations, which planets are closer to the Sun than Earth and which are farther away. Because Mercury and Venus are always observed fairly near the Sun, he correctly concluded that their orbits must lie inside Earth’s orbit. The other planets visible to Copernicus—Mars, Jupiter, and Saturn—can sometimes be seen high in the sky in the middle of the night, when the Sun is far below the horizon. This placement can occur only if Earth comes between the Sun and a planet. Copernicus therefore concluded (also correctly) that the orbits of Mars, Jupiter, and Saturn lie outside Earth’s orbit. Today, Mercury and Venus are called inferior planets, while all the planets farther from the Sun than Earth are called superior planets.

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The geometric arrangements among Earth, another planet, and the Sun are called configurations. For example, when Mercury or Venus is directly between Earth and the Sun (Figure 2-4), we say the planet is in a configuration called an inferior conjunction; when either of these planets is on the opposite side of the Sun from Earth, its configuration is called a superior conjunction.

The angle between the Sun and a planet as viewed from Earth is called the planet’s elongation. A planet’s elongation varies from zero degrees to a maximum value, depending upon where we see it in its orbit around the Sun. At greatest eastern or greatest western elongation, Mercury and Venus are as far from the Sun in angle as they can be. This position is about 28° for Mercury and about 47° for Venus. The superior planets all have maximum elongations of 180°. When either Mercury or Venus rises before the Sun, the planet is visible in the eastern sky as a bright “star” and is often called the “morning star.” Similarly, when either of these two planets sets after the Sun, the planet is visible in the western sky and is then called the “evening star.” Because these two planets are not always at their greatest elongations, they are often very close in angle to the Sun. This positioning is especially true of Mercury, often making it hard to see from Earth. Venus is often nearly halfway up the sky at sunrise or sunset and therefore quite noticeable during much of its orbit. Because they are so bright and sometimes appear to change color due to the motion of Earth’s atmosphere, Venus and Mercury are often mistaken for UFOs. (The same motion of the air causes the road in front of your car to shimmer on a hot day.)

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Planets farther from the Sun than Earth have different configurations. When one of these planets is located behind the Sun, as seen from Earth, that planet is said to be in conjunction. When a planet is opposite the Sun in the sky, that planet is at opposition. It is not difficult to determine when a planet happens to be located at one of the key positions in Figure 2-4. For example, when Mars is at opposition, it appears high and bright in the sky at midnight.

It is relatively easy to follow a planet as it moves from one configuration to another. However, these observations alone do not tell us the planet’s actual orbit, because Earth, from which we make the observations, is also moving. Copernicus was therefore careful to distinguish between two characteristic time intervals, or periods, of each planet.

Recall from Chapter 1 that a planet’s sidereal period is the time it takes that body to make one complete orbit of the Sun as measured with respect to the distant stars; an observer fixed at the Sun’s location watching that planet would see it start at one point on the celestial sphere, go around the sphere, and end at the same place it started from. The sidereal period or orbit is the length of a year for each planet.

The other useful time interval that Copernicus used is the synodic period. The synodic period is the time that elapses between two successive identical configurations as seen from Earth. This period can be from one opposition to the next, for example, or from one conjunction to the next (of the same type, Figure 2-5). The synodic period tells us, among other things, when to expect a planet to be closest to Earth and, therefore, most easily studied.

Thus, nearly 500 years ago, Copernicus was able to obtain the first six entries shown in Table 2-1 (the others are contemporary results included for completeness). Copernicus was then able to devise a straightforward geometric method for determining the distances of the planets from the Sun. His answers turned out to be remarkably close to the modern values, as shown in Table 2-2. From these two tables it is apparent that the farther a planet is from the Sun, the longer it takes to complete its orbit.

Copernicus presented his heliocentric cosmology, including supporting observations and calculations, in a book entitled De revolutionibus orbium coelestium ( On the Revolutions of the Celestial Spheres), which was published in 1543, the year of his death. His great insight was the conceptual simplicity of a heliocentric cosmology compared to geocentric views, especially in explaining retrograde motion. However, Copernicus incorrectly assumed that the planets travel along circular paths around the Sun. Without using epicycles similar to those used in geocentric theory (see Figure D2-1), many of his predictions were no more accurate than those of the earlier theory, as we will see shortly.

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In November 1572, a bright star suddenly appeared in the constellation Cassiopeia. At first, it was even brighter than Venus, but then it began to grow dim. After 18 months the star faded from view.

Modern astronomers recognize this event as a supernova explosion, the violent death of a certain type of star (see Chapter 11). In the sixteenth century, however, the prevailing opinion was quite different. Teachings dating back to Aristotle and Plato argued that the heavens were permanent and unalterable. From that perspective, the “new star” of 1572 could not really be a star at all, because the heavens do not change. Many astronomers and theologians of the day argued that the sighting must be some sort of bright object quite near Earth, perhaps not much farther away than the clouds overhead. A 25-year-old Danish astronomer named Tycho Brahe realized that straightforward observations might reveal the distance to this object.

Consider what happens when two people look at a nearby object from different places—they see it in different positions relative to the things behind it. Furthermore, their heads face at different angles when looking at it. This variation in angle that occurs when viewing a nearby object from different locations is called parallax (Figure 2-6).

Tycho reasoned as follows: If the new star is nearby, its position should shift against the background stars over the course of a night (Figure 2-7a). His careful observations, done in the spirit of the scientific method, failed to reveal any parallax, and so the new star had to be far away, farther from Earth than anyone had imagined (Figure 2-7b). Tycho summarized his findings in a small book, De nova stella (On the New Star), published in 1573.

From 1576 to 1597, Tycho made comprehensive observations, measuring planetary positions with an accuracy of 1 arcminute (arcmin; see Section 1-5), about as precise as is possible with nontelescopic instruments (the telescope was invented in 1608). Upon Tycho’s death in 1601, most of his invaluable astronomical records were given to his gifted assistant, Johannes Kepler.

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