What is the universe Part 2 Motion in Space

The Heliocentric Model From 5,000 ago until the 1500s, a geocentric view of planetary motion was widely accepted Polish astronomer Nicolas Copernicus developed the first heliocentric model in which the planets orbit the sun in circles

Problems with the Geocentric Model Assumed that planets and stars orbited a stationary Earth Could not explain the apparent retrograde motion of some planets (Mars) Retrograde motion= planets further from the sun move slower in their orbit due to less gravitational pull and appear to move backwards

From Circles to Ellipses Danish astronomer Tycho Brahe studied the movement of Mars taking nightly measurements WITHOUT a telescope Accurately measured the elliptical orbit within a matter of degrees Died in 1602, seven years before the telescope was invented

Brahe’s Apprentice Johannes Kepler (Brahe’s apprentice) was in the right place at the right time and used Brahe’s data to proposed the first of a set of three laws that are named after him

Kepler’s Laws The planets move around the sun in ellipses The imaginary lines connecting the sun to each planet sweeps equal areas in equal time The relationship between a planet’s ellipse and its orbital period is: P2 = a3 (where P equals the orbital period and a equals the length of the semi-major axis)

Eccentricity Due to its elliptical orbit, a planet is not at a constant distance from the Sun Perihelion = closest to Aphelion = farthest from

Kepler’s Second Law

Kepler’s Third Law The farther a satellite is from the object it is revolving aroung: 1. The longer it will take to complete an orbit 2. The greater the distance it will travel to complete the orbit 3. The slower its average speed of motion

Newton’s Laws (physical science review!) An object in motion tends to stay in motion UNLESS an outside forces acts upon it An object accelerates in the same direction of the force applied For every action, there is a equal and opposite reaction

Kepler + Newton Gravity pushes planets towards the sun The gravitational force holding the planets on their orbit is continuously changing the planets’ velocity creating elliptical orbits When planets are closer to the sun, they speed up!

Why does the night sky change? Parallax Precession An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight. Earth’s wobble on its tilted axis It takes the Earth about 26,000 years to wobble It is caused by a tug of war on Earth by the Sun and Moon, but the Moon’s pull is stronger due to proximity

Our Motion & the Moon Nutation Barycenter