| Overview of the Sky and Planets |
| | Much of our initial discussion of Astronomy will concern the motion of objects in the sky. Therefore, we shall introduce some terminology and a coordinate system that allow us to specify succinctly the location of particular objects in the heavens. For a more extensive discussion, see Astronomy without a Telescope. |
| | The "Road of the Sun" on the Celestial Sphere |
| | Another important imaginary object on the celestial sphere is the "ecliptic" or "Road of the Sun", which is the imaginary path that the Sun follows on the celestial sphere over the course of a year. As the diagram at left indicates, the apparent position of the sun with respect to the background stars (as viewed from Earth) changes continuously as the Earth moves around its orbit, and will return to its starting point when the Earth has made one revolution in its orbit. |
| | East and West on the Celestial Sphere |
| | It is useful to define east and west directions on the celestial sphere, as illustrated in the following figure. |
| | Thus, objects to the west of the Sun on the celestial sphere precede the Sun in the diurnal motion of the celestial sphere (they "rise" before the Sun and "set" before the Sun). Likewise, objects to the east of the Sun trail the Sun in the diurnal motion (they "rise" after the Sun and "set" after the Sun). Generally, one object is west of another object if it "rises" before the other object over the eastern horizon as the sky appears to turn, and east of the object if it "rises" after the other object. |
| | Celestial Coordinate Systems |
| | We can define a useful coordinate system for locating objects on the celestial sphere by projecting onto the sky the latitude-longitude coordinate system that we use on the surface of the earth. As illustrated in the adjacent figure, this allows us to define "North and South Celestial Poles" (the imaginary points about which the diurnal motion appears to take place) and a "Celestial Equator". |
| | The figure illustrates that these imaginary objects are the exact analogs of the corresponding imaginary objects on the surface of the earth. Thus, we shall be able to specify the precise location of things on the celestial sphere by giving the celestial analog of their latitudes and longtitudes, or something related to those quantities. |
| | It is clear after only minimal observation that objects change their position in the sky over a period of time. This motion is conveniently separated into two parts: |
| | Diurnal motion at different latitudes Actually, all objects are slowly changing their relative positions on the celestial sphere, but for most the motion is so slow that it cannot be detected over timespans comparable to a human lifetime; only the "wanderers" have sufficiently fast motion for this change to be easily visible. |
| | The entire sky appears to turn around imaginary points in the northern and southern sky once in 24 hours. This is termed the daily or diurnal motion of the celestial sphere, and is in reality a consequence of the daily rotation of the earth on its axis. The diurnal motion affects all objects in the sky and does not change their relative positions: the diurnal motion causes the sky to rotate as a whole once every 24 hours. |
| | Superposed on the overall diurnal motion of the sky is "intrinsic" motion that causes certain objects on the celestial sphere to change their positions with respect to the other objects on the celestial sphere. These are the "wanderers" of the ancient astronomers: the planets, the Sun, and the Moon. |
| | It is useful in discussing objects in the sky to imagine them to be attached to a sphere surrounding the earth. This fictitious construction is called the celestial sphere. At any one time we see no more than half of this sphere, but we will refer loosely to the imaginary half-sphere over our heads as just the celestial sphere (see adjacent figure). |
| | The point on the celestial sphere that is directly over our heads at a given time is termed the zenith. The imaginary circle passing through the North and South points on our horizon and through the zenith is termed the celestial meridian. We will introduce additional terminology associated with the celestial sphere later. |
| | Aspects and Phases of the Planets |
| | The planets, as viewed in the sky, exhibit characteristic aspects and phases. "Aspects" refers to the location of the planet with respect to our overhead sky reference (objects on the celestial sphere); "phases" refers to the fact that the planets, through a telescope, exhibit phases (differing amounts of lighted hemispheres as viewed from the earth). The terminology associated with these aspects and phases is different, depending on whether we refer to an inferior planet or a superior planet. |
| | Aspects and Phases of the Inferior Planets |
| | The inferior planets exhibit the aspects and phases illustrated in the following diagram. |
| | Gibbous phases are phases between quarter and full phases. Greatest Elongation refers to the largest separation of the planet from the Sun in our sky, either to the East, or to the West. Thus, we see that the inferior planets exhibit a complete set of phases (just like the Moon) as viewed from the earth, and can never be further from the Sun than the angles defined by greatest elongation. |
| | Aspects and Phases of the Superior Planets |
| | The aspects and phases of the superior planets differ from those of the inferior planets because of geometry: their orbits are outside that of the Earth. These aspects and phases are indicated in the following diagram. |
| | When a superior planet is at quadrature, it is on our celestial meridian at sunrise or sunset. Comparing with the preceding diagram for the inferior planets, we notice two basic differences: (1) The superior planets do not exhibit a full range of phases; they are always gibbous or full. (2) The superior planets can be located at any distance East or West of the Sun in our sky, unlike the inferior planets where there is a limiting angle away from the Sun (greatest elongation). |
| | Classification of the Planets |
| | Much of our concern this semester will be with the development of our present understanding of the Solar System. We begin with a brief overview of the modern and ancient classifications of the planets. |
| | The 7 Planets of the Ancients |
| | The term "planet" originally meant "wanderer": it was observed long ago that certain points of light wandered (changed their position) with respect to the background stars in the sky. In ancient times, before the invention of the telescope and before one understood the present structure of the Solar System, there were thought to be 7 such wanderers or planets: Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the Sun. This list is different in several respects from our modern list of planets: |
| | The Earth is missing, because it was not understood that the points of light wandering on the celestial sphere and the Earth on which we stood had anything in common. |
| | Uranus, Neptune, and Pluto are missing because they would only be discovered when the telescope made them easily visible. |
| | Uranus is barely visible to the naked eye; it was discovered in 1781. |
| | Neptune and Pluto are too faint to see at all without a telescope; they were discovered in 1846 and 1930, respectively. |
| | The Sun and the Moon were classified as planets because they wandered on the celestial sphere, just like Mars and Jupiter and the other planets. |
| | Stars Look Different from Planets |
| | Planets (and the Sun and Moon) have some observational characteristics that distinguish them from what we would now call the stars: |
| | The planets move relative to stars on celestial sphere |
| | The nearer and larger planets appear as disks in telescope |
| | The brighter planets do not "twinkle" |
| | The planets are always near the imaginary yearly path of the Sun on the celestial sphere (the ecliptic) |
| | The relative positions of the stars are fixed on celestial sphere |
| | The stars appear as "points" of light, even through the telescope |
| | The stars appear to "twinkle" |
| | Stars can be anywhere on the celestial sphere |
| | The planets of the modern solar system are grouped into several different and sometimes overlapping classifications, as illustrated in the following figure: |
| | The planets inside the orbit of the earth are called the Inferior Planets: Mercury and Venus. |
| | The planets outside the orbit of the earth are called the Superior Planets: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. |
| | The planets inside the asteroid belt are termed the Inner Planets (or the Terrestrial Planets): Mercury, Venus, Earth, and Mars. |
| | The planets outside the asteroid belt are termed the Outer Planets: Jupiter, Saturn, Uranus, Neptune, and Pluto. |
| | The planets sharing the gaseous structure of Jupiter are termed the Gas Giant (or Jovian) Planets: Jupiter, Saturn, Uranus, and Neptune. |
| | Thus, the Sun traces out a closed path on the celestial sphere once each year. This apparent path of the Sun on the celestial sphere is called the ecliptic. Because the rotation axis of the Earth is tilted by 23.5 degrees with respect to the plane of its orbital motion (which is also called the ecliptic), the path of the Sun on the celestial sphere is a circle tilted by 23.5 degrees with respect to the celestial equator (see diagram at right). |
| | The ecliptic is important observationally, because the planets, the Sun (by definition), and the Moon are always found near the ecliptic. As we shall see later, this is because all of these objects have orbits that lie nearly in the same spatial plane. |
| | Overview of the Sky and Planets |
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