How Do We Know How Far That Planet Is

How Far Is That Planet?

The relative distances to the planets were first determined by Johannes Kepler in the 17th century. Working from Tycho Brahe’s observational data over many years, he devised his third law which relates a planets distance from the sun to its period of revolution (how long it takes to go around the sun, in other words, its planetary year). Kepler’s genius was that he realized planets revolve in ellipses around the Sun (not perfect circles as Aristotle had stated), and his equation is built around this critical fact. In the 18th century, Edmund Halley (of Halley's comet fame) suggested a method using triangulation to observe a transit of Venus (when a planet moves across the face of the Sun) to determine the first absolute value of the AU. Although Halley never lived to see the attempt, in 1761 astronomers fanned out around the globe to measure an AU at 95 million miles, respectively close to today's value of 92,955,807.267 miles. Once the distance of the Sun to the Earth was known all the planet distances could be worked out since Kepler's Laws allowed for the distances to all the planets to be worked out in AU's.

Today astronomers bounce radar, moving at the speed of light, off the planets and measure the time it takes the radar to go there and back. Then they can find the distance by multiplying ½ that time in seconds by the speed of light. speed of light = 3 x 105 km/sec.

seconds Here is the formula: (3 x 105 km/sec) = Distance (km) 2

Try this problem: It takes a radar signal 11 minutes to make the round trip to Mars from Earth. How far is Mars from Earth? (After you try to solve this problem, check out the answer below.)

First divide 11 minutes by 2 (to find the one way distance to Mars) = 5.5 minutes. Convert 5.5 minutes into seconds by multiplying by 60 = 330 seconds, and multiply that by 3 x 105 km/sec = 9.9 x 107 km.

1. Did you do the problem correctly? Now convert your answer to AU’s. (1 AU = 150,000,000 km.). Notice that we're working in SI units and so we use km instead of miles. Divide your distance to Mars by 150,000,000. 2. Find the distance to Venus in km & AU’s. The time it takes radar to bounce round trip to Venus is 5 minutes.

3. Find the distance to the Moon in km. & AU’s. It that takes 2 seconds for radar to bounce round trip.

Answer the questions below. 4. Who first figured out the relative distances to the planets? Do the planets move in circles or ellipses? Who had said that the planets moved in circles originally?

5. Who devised a way to figure out an AU? What is a transit? Which planet was measured as it transited across the Sun to determine the AU?

6. What distance does an AU measure (from where to where)? What is an AU in miles?

7. How do we measure the distance to planets today? How fast is the speed of light?