Aim: How do planets move? Notepack 25 Aim: How do planets move? Do Now: What are the names of the planets that orbit around the sun?

Observing the Heavens Early observers of the night sky saw stars, Sun and Moon, move across the sky each night and believed that the objects move around the Earth. This is called the geocentric model; which means, that the Earth is the center of the universe and everything revolves around it. Geo – means Earth

Observing the Heavens (cont) It wasn’t until the 16th century, the astronomer Nicolaus Copernicus made detailed observations and realized that the Sun was the center and that everything thin the sky revolves around it.. This is called The heliocentric model. Helo – means SUN.

Motion in Space Objects are constantly moving through space: The universe is expanding Galaxies are moving away from each other. Red Shift Galaxies are spinning Spiral galaxies have arms that spin around a nucleus Planets, stars, moons, and comets revolve around objects Moons orbit planets Planet and comets orbit stars Stars orbit the nucleus of a galaxy Planets and stars rotate Planets and Stars spin on their axis

Planetary Orbits There are 8 planets in our solar system the revolves around the Sun. Planets revolve on a path called an orbit. In our solar system, the planet orbit the Sun in a counter clockwise direction. Planets closer to the Sun have a smaller orbit than planets further away from the Sun.

Planetary Orbits (cont) Because planets in small orbits take less time for them to revolve around the Sun than planets with large orbits, inner orbit planet seem to “catch up” and pass outer planets.

Planetary Orbits (cont) When looking up at the night sky for a series of days, planets appear to be moving in one direction, than backward, and than forward again. This is because the Earth is in a smaller orbit that the planet being observed. The Earth is “catching up and then passing it.” This is known as retrograde.

Jupiter and Saturn retrograding in the night sky

Planetary Orbits (cont) Planetary orbits are not circular in shape, instead they are elongated circles called ellipse. Elliptical orbit. An ellipse is a long oval. The Sun is not in the center, instead it is closer to one end called the focus.

Planetary Orbits (cont) When the planet is at its furthest to the Sun, it is said to be at its aphelion (furthest) The Sun will appear small When the planet is at its closest to the Sun, it is said to be at its Perihelion (closest) The Sun will appear large

Planetary Orbits (cont) As a planet moves closer to the Sun, the more gravitational pull there is between the Sun and the planet, so the planet speeds up. (the planet travels a longer distance) As a planet moves farther away from the Sun, the less gravitational pull there is between the Sun and the planet, so the planet slows down. (the planet travels a shorter distance) Example: The Earth takes the same amount of time to travel all three distances A,B, and C; even through each distant is different.

ECCENTRICITY refers to how elongated the ellipse is

The smaller the Eccentricity number is, the more circle like it is The smaller the Eccentricity number is, the more circle like it is. (Closer to ZERO) The larger the Eccentricity number is, the more oval like it is. (Closer to 1)

Calculate the eccentricity of the ellipse below: length of major axis Formula: eccentricity = distance between foci length of major axis

Lab