1. Ancient Greeks 22.1 Early Astronomy  Geocentric Model In the ancient Greeks’ geocentric model, the moon, sun, and the known planets—Mercury, Venus, Mars, and Jupiter—orbit Earth.  Heliocentric Model In the heliocentric model, Earth and the other planets orbit the sun.

2. The Birth of Modern Astronomy 22.1 Early Astronomy  Johannes Kepler Kepler discovered three laws of planetary motion: 1. Orbits of the planets are elliptical. 2. Planets revolve around the sun at varying speed. 3. There is a proportional relationship between a planet’s orbital period and its distance to the sun.

3. The Birth of Modern Astronomy 22.1 Early Astronomy  Johannes Kepler An ellipse is an oval-shaped path. An astronomical unit (AU) is the average distance between Earth and the sun; it is about 150 million kilometers.

4. Planet Revolution

5. The Birth of Modern Astronomy 22.1 Early Astronomy  Sir Isaac Newton Although others had theorized the existence of gravitational force, Newton was the first to formulate and test the law of universal gravitation.  Universal Gravitation Gravitational force decreases with distance. The greater the mass of an object, the greater is its gravitational force.

6. Gravity’s Influence on Orbits

7. Motions of Earth 22.2 The Earth–Moon–Sun System  Rotation Rotation is the turning, or spinning, of a body on its axis (day/night).

8. Motions of Earth 22.2 The Earth–Moon–Sun System  Revolution Revolution is the motion of a body, such as a planet or moon, along a path around some point in space (once/year). Perihelion is the time in January when Earth is closest to the sun. Aphelion is the time in July when Earth is farthest from the sun.

9. Earth-Sun Relationships 17.1 Atmosphere Characteristics  Earth’s Motions Earth has two principal motions—rotation and revolution.  Earth’s Orientation Seasonal changes occur because Earth’s position relative to the sun continually changes as it travels along its orbit (23.5 degree tilt).

10. Tilt of Earth’s Axis

11. Earth-Sun Relationships 17.1 Atmosphere Characteristics  Solstices and Equinoxes The summer solstice is the solstice that occurs on June 21 or 22 in the Northern Hemisphere and is the “official” first day of summer. The winter solstice is the solstice that occurs on December 21 or 22 in the Northern Hemisphere and is the “official” first day of winter.

12. Earth-Sun Relationships 17.1 Atmosphere Characteristics  Solstices and Equinoxes The autumnal equinox is the equinox that occurs on September 22 or 23 in the Northern Hemisphere. The spring equinox is the equinox that occurs on March 21 or 22 in the Northern Hemisphere.

13. Length of Daylight 17.1 Atmosphere Characteristics  The length of daylight compared to the length of darkness also is determined by Earth’s position in orbit.

14. Solstices and Equinoxes

15. Motions of Earth 22.2 The Earth–Moon–Sun System  Precession, Nutation, and Barycenter Precession- change in direction of the axis, but without change in tilt. This changes the stars near the Pole, but does not affect the seasons. Over a period of 26,000 years. Nutation- wobbling around the precessional axis. This is a change in angle- ½ degree one way or another. This occurs over an 18 year period. And is due to the Moon exclusively. This would very slightly increase or decrease the amount of seasonal effects.

16. Precession

17. Precession and Nutation

18. Motions of Earth 22.2 The Earth–Moon–Sun System  Precession, Nutation, and Barycenter Barycenter- the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other.

19. 22.2 The Earth–Moon–Sun System Two bodies with the same mass orbiting a common barycenter.

20. Motions of Earth 22.2 The Earth–Moon–Sun System  Precession, Nutation, and Barycenter When a moon orbits a planet, or a planet orbits a star, both bodies are actually orbiting around a point that lies outside the center of the larger body. For example, the moon does not orbit the exact center of the Earth, but a point on a line between the Earth and the Moon approx. 1,710 km below the surface of the Earth, where their respective masses balance. This is the point about which the Earth and Moon orbit as they travel around the Sun.

21. 22.2 The Earth–Moon–Sun System Two bodies with a major difference in mass orbiting a common barycenter internal to one body (similar to the Earth– Moon system)Earth Moon

22. Motions of Earth 22.2 The Earth–Moon–Sun System  Precession, Nutation, and Barycenter This means that the Sun is not stationary in our solar system. It actually moves as the planets tug on it, causing it to orbit the solar system’s barycenter. The Sun never strays too far from the solar systems barycenter.

23. 22.2 The Earth–Moon–Sun System Two bodies with an extreme difference in mass orbiting a common barycenter internal to one body (similar to the Sun– Earth system)Sun Earth