Orbits of Solar System Bodies

1.4.1 Kepler's Laws of Planetary Motion

Each planet orbits the Sun as shown in plan view in Figure 1.5. As a crude approximation, the planetary orbits can be represented as circles centred on the Sun, with all the circles in the same plane, and each planet moving around its orbit at a constant speed; the larger the orbit, the slower the speed. A far better approximation is encapsulated in three empirical rules called Kepler's laws of planetary motion. These were announced by the German astronomer Johannes Kepler (1571-1630), the first two in 1609, the third in 1619.

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

Kepler's second law As the planet moves around its orbit, the straight line from the Sun to the planet sweeps out equal areas in equal intervals of time.

We shall come to the third law shortly.

Focus 1

Focus 1

Centre

Centre

Figure 1.7 An ellipse, though far more eccentric than the orbit of any planet. This is the shape of the orbit of the comet 21P Giacobini-Zinner (Table 1.4).

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment