Astronomy Chapter 2

Retrograde motion

When something slightly moves off its usual path

Geocentric model

When the Earth is in the center

Heliocentric model

When the Sun is in the center

Inferior planets

The planet’s orbits are smaller than Earth’s

Superior planets

The planet’s orbits are bigger than Earth’s

Astronomical unit

The distance between the Sun and the Earth

Escape speed

The speed something has to travel to escape the orbit

Why do we see phases of Venus, while Mars always appears to be full?

Venus’s orbit is smaller than the Earth’s, Mars’s orbit is larger, so we can only see the lit-up portion

How does a geocentric model attempt to explain retrograde motion?

by putting circle upon circle, each planet is orbiting the Earth, while the planet itself is also moving in a circle

How do we know the geocentric model is wrong?

-the speed of faraway stars would be too high

-the universe is infinite, anything could be the center

-cannot explain why some planets have phases while others don’t

-it’s too complicated to explain retrograde motion

What are Kepler’s Laws? What is the significance of Kepler’s Laws?

1. Planets travel in an elliptical motion around the Sun

2. the closer to the sun, the faster it travels, and the further from the sun, the slower it travels.

3. p^2/a^3=1. P= orbital period while a= the semi-major axis (longest point of the ellipse).

   significant because they were the first laws to describe how the planets moved