Introduction to Astronomy

Orbits and Gravity  

 Introduction to Astronomy

    Orbits and Gravity

         Contents

 Introduction to Astronomy

    Orbits and Gravity

Copernicus 

       A historical perspective

Tycho

Keppler

Galileo

 Introduction to Astronomy

    Orbits and Gravity

Copernicus played a central role in the emergence of modern Astronomy, shifting from the geocentric model to the heliocentric model.

       A historical perspective

 Introduction to Astronomy

    Orbits and Gravity

Tycho Brahe collected meticulous data about the location of stars and planetary motion

       A historical perspective

 Introduction to Astronomy

    Orbits and Gravity

Kepler found a hidden pattern in the data collected by Brahe, and formulated an enduring model of planetary motion.

       A historical perspective

 Introduction to Astronomy

    Orbits and Gravity

       A historical perspective

For those interested in a deeper dive in the historical perspective:

 Introduction to Astronomy

    Orbits and Gravity

       The Laws of Planetary Motion

Planetary Orbits

This video shows a summary of all three laws of planetary motion and the events leading to their discovery. 

In the slides below, you will dive into the details of each law.

 Introduction to Astronomy

    Orbits and Gravity

       Kepler's Laws of Planetary Motion

Kepler’s first law: Each planet moves around the Sun in an elliptical orbit, with the Sun at one focus of the ellipse.

 Introduction to Astronomy

    Orbits and Gravity

       Kepler's Laws of Planetary Motion

• Why are the orbits elliptical?

 Introduction to Astronomy

    Orbits and Gravity

       Kepler's Laws of Planetary Motion

Kepler’s second law: The straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time.

 Introduction to Astronomy

    Orbits and Gravity

       Kepler's Laws of Planetary Motion

Notice how the eccentricity affects the orbital speed.

• The planet speeds up in its closest approach and slows down when it is the furthest.

• Higher eccentricity causes more variability in the orbital speed.

 Introduction to Astronomy

    Orbits and Gravity

       Kepler's Laws of Planetary Motion

Kepler’s third law:

The square of a planet’s orbital period (P) is directly proportional to the cube of the semimajor axis of its orbit (a).

Note: Some resources use the symbols T for period and d or r for average distance

 Introduction to Astronomy

    Orbits and Gravity

       Kepler's Laws of Planetary Motion

Kepler’s Laws examples:

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Laws of Motion

Introduction to Newton's Laws of Motion

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Laws of Motion

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Laws of Motion

• Newton’s first law: Every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force.

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Laws of Motion

Newton’s second law: The change of motion of a body is proportional to and in the direction of the force acting on it, and inversely proportional to its mass.

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Laws of Motion

• Newton’s third law: The mutual actions of two bodies upon each other are always equal and act in opposite directions.

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Laws of Motion

Three more videos about the three laws (different perspectives for deeper understanding.)

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Laws of Motion

Solved Examples

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Universal Law of Gravitation

• Newton's insights
• Newton's Universal Law of Gravitation
• Earth's Gravity

In this video

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Universal Law of Gravitation

Even though the force of gravity depends on the masses of both objects, the rate at which an object falls only depends on the mass of the object attracting it.

In this video

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Universal Law of Gravitation

 Introduction to Astronomy

    Orbits and Gravity

       Newton's Universal Law of Gravitation

 Introduction to Astronomy

    Orbits and Gravity

       How to get into orbit!

• Mass
• Gravity
• Types of Orbits
• Escape Velocity
• "Weightlessness"

In this video

 Introduction to Astronomy

    Orbits and Gravity

       How to get into orbit!

• "Weightlessness" starting @4:00

In this video

 Introduction to Astronomy

    Orbits and Gravity

       How to get into orbit!

Gravity & Orbital Motion

In this video

 Introduction to Astronomy

    Orbits and Gravity

       How to get into orbit!

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