CS_Ch11_Astronomy 2/28/05 4:01 PM Page 689 Activity 3 Orbits and Effects The precession of Geo Words 41,000 year the Earth’s axis is cycle orbital plane: (also 25˚ axis now axis in one part of the 23.5˚ called the ecliptic or 22˚ range of 13,000 years plane of the ecliptic). precession cycle. axial tilt A plane formed by Another part is the path of the Earth the precession of around the Sun. the Earth’s orbit. orbital orbital inclination: the angle As the Earth plane plane between the orbital plane of the solar moves around the system and the actual Sun in its elliptical orbit of an object Obliquity Cycle Axial Precession orbit, the major around the Sun. axis of the Earth’s orbital ellipse is January July rotating about the conditions now Sun. In other Sun words, the orbit January July itself rotates conditions in about around the Sun! Earth 11,000 years These two Orbital Precession Combined Precession precessions (the Effects axial and orbital precessions) Figure 3 The tilt of the Earth’s axis and its orbital path about the combine to affect Sun go through several cycles of change. how far the Earth is from the Sun during the different seasons.This combined effect is called precession of the equinoxes, and this change goes through one complete cycle about every 22,000 years.Ten thousand years from now, about halfway through the precession cycle, winter will be from June to September, when the Earth will be farthest from the Sun during the Northern Hemisphere winter.That will make winters there even colder, on average. Inclination When you study a diagram of the solar system, you notice that the orbits of all the planets except Pluto stay within a narrow range called the orbital plane of the solar system. If you were making a model of the orbits of the planets in the solar system you could put many of the orbits on a tabletop. However, Pluto’s orbit, as shown in Figure 4, could not be drawn or placed on a tabletop (a plane, as it is called in geometry). Pluto’s path around the Sun is inclined 17.1° from the plane described by the Earth’s motion around the Sun.This 17.1° tilt is called its orbital inclination. 689 Coordinated Science for the 21st Century.