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GEOCENTRIC AND HELIOCENTRIC MODELS There are two ways the Earth’s position was explained by scientific cultures. Geocentric means Earth-centred Heliocentric means Sun-centred Scientists used to accept the geocentric model, but shifted to accepting the heliocentric model, which led to a major shift in scientific world views. Notice how many planets are shown in the heliocentric model. Why are Uranus and Neptune missing? This is a great example of how scientific knowledge has evolved in light of new evidence, and the role of technology (telescope) in this evolution. Here are some telescope images of planets: Saturn Jupiter Venus Back to Calculations… As we touched on previously, astronomers have developed convenient units of measure to accommodate and reduce large distances to manageable numbers. Interstellar (distances between the stars) are measured using the light-year (l.y.). Since light travels about 9.5 trillion km per year, this distance is considered one light year. Astronomers have developed another useful unit for smaller distances in space. In the solar system, for instance, the standard unit of measure is the astronomical unit (A.U.), which is the average distance between the Sun and the Earth (about 1.5 x 108 km). This unit is better for measuring distances between planets. For example, the distance from the Sun to Jupiter is about 5.0 A.U. Do the practice questions below (keep them, don’t hand them in). Here are some formulas to help you out: To get light-years from km: light-years = km ÷ (9.5 x 1012) To get km from light-years: km = light-years x 9.5 x 1012 To get A.U. from km: A.U. = km x 1.5 x 108 To get km from A.U.: km = A.U. ÷ (1.5 x 108) 1. A star is 6 light years away. How many km away is it? 2. A star is 25 trillion km away. How many light years away is it? 3. Uranus is 19.218 A.U. from the Sun. How many km is that? 4. Mars is 225 billion km away from the sun. How many A.U. is that? 5. Venus is 0.7233 A.U. away from the sun. How many km is that? 6. The closest Venus and Earth ever get is 39,630,000 km away. How many A.U. is that? 7. Pluto is 5,916,000,000 km away from the Sun on average. a. How many A.U. is that? b. How many l.y. is that? 8. The average distance between Neptune and the Sun is 30.06 A.U. How many light-years is that?