7.4 Meet Your Solar System The ancient Greeks noticed that five objects appeared to move through the constellations They were observing the five planets that you are able to see with the unaided eye: Mercury Venus ----->>> Mars Jupiter Saturn What are constellations again? A group of stars that seem to form a distinctive pattern in the sky They thought that these objects were special because they were not fixed against the background sky like how the stars appear to be, for this reason, they were labelled as planets, planet is Greek for wanderer.

The Planets Planet: an object that orbits one or more stars (and is not a star itself), is spherical and does not share its orbit with another object The planets, the Sun and other smaller objects make up the solar system The Sun’s gravitational pull keeps the objects in the solar system in orbit around the Sun

Asteroid Belt

Models of the Solar System Geocentric Model: Earth is the centre of all planetary motion, with the planets and the Sun travelling in perfect circles around Earth Heliocentric Model: The Sun is the centre of the solar system, and the planets orbit the Sun in perfect circles Revision made to Heliocentric Model: The orbits of the planets are ellipses Geo means Earth, Helio means Sun Remember difference between circle and ellipse?

Classification of the Planets The planets Mercury, Venus, Earth and Mars are called the inner planets Sometimes they are called terrestrial planets = Earth-like They are relatively small and have solid cores and rocky crusts

Farther away, Jupiter, Saturn, Uranus and Neptune are located and they are the outer planets These planets are known for their large gaseous bands and cold temperatures They are also called the gas giants

Planetary Motion Mercury and Venus stay near the Sun They can be see only in the early evening or morning In comparison, on any given night, Mars, Jupiter and Saturn move westward along with the fixed stars due to Earth’s rotation

However, at certain times these three planets “wander” against the backdrop of stars in a slow, looping motion that lasts several weeks This apparent change in direction is called retrograde motion Retrograde motion is produced when Earth catches up with and passes an outer planet in its orbit

Earth is on an inside track and moves faster than the outer planets Every time it catches up to an outer planet and moves between the outer planet and the Sun, the planet appears to make a looping motion

Distance Between The Planets Large distances keep the planets well separated from each other The planets lie so far apart that measuring them in kilometres would result in values that are too large to comprehend For this reason, astronomers created a unit for measuring distances in the solar system: astronomical unit (AU)

The AU is equal to the average distance between Earth and the Sun - about 150 million kilometres! Therefore, the Earth is 1 AU from the Sun The average distance between the Sun and an object orbiting the Sun is called the orbital radius - measured in AU The planets share many similarities with one another, but also have many differences

Earth: LIFE, H2O in three states of matter, atmosphere Mercury: No atmosphere = huge differences between day and night temperatures, freezing and thawing results in immense cracks on surface Venus: Earth’s sister planet = similar composition and size, has atmosphere with lots of S = acid rain, also has CO2 and N2 Earth: LIFE, H2O in three states of matter, atmosphere Mars: Red = Fe rocks oxidizing (rust), thin atmosphere, has a canyon 8km deep and as wide as the distance from Vancouver to Toronto Jupiter: Largest planet, Great Red Spot is the size of three Earths = storm in atmosphere, very thin rings Saturn: Lots of rings made of ice particles, atmosphere Uranus: Rings, blue-green atmosphere = CH4, rotates on 90o angle Neptune: Blue atmosphere, very thin rings Which ones are inner and outer? Note that, as well as what the chemical symbols represent