| MadSci Network: Astronomy |
The distance to the Sun is about 93 million miles. We can start to imagine the size of this by building a scale model. Let's begin by shrinking the Sun down to only 4 1/2 feet across, about the 3/4 the height of Mr. Carothers. On this scale, the Earth would be the size of a marble, one mile away! To actually measure this distance is not an easy job.
The first person to try and measure the distance to the Sun was the Greek astronomer, Aristarchus, who lived in the 3rd century, BC. He measured the angle between the Sun and Moon, when the Moon was in either first or third quarter (when it's a "half moon"). The angle is just less than 90 degrees, and he used simple trigonometry to determine the relative distances between the Earth and Moon, and the Earth and Sun. Unfortunately, the angle between the Sun and Moon is very hard to measure accurately, and Aristarchus measured a distance to the Sun that was 20 times too small.
More accurate measurements have only been made within the last hundred years. We cannot measure the distance to the Sun directly. Instead, we have looked at other planets, or even other stars. One way that was used was triangulation, looking at either asteroids or Mars. Two astronomers, on different parts of the Earth, will see Mars at a slightly different position in the sky. With the different angles of Mars in the sky, and the distance between the astronomers known, simple trigonometry gives the distance to Mars. It's very similar to the way surveyors work.
Once the distance between the Earth and Mars is determined, we know the distance between the Earth and the Sun. This is because the relative sizes of the planetary orbits are known very accurately.
Astronomers have also looked at other stars. The Earth will move first towards and then away from stars in the sky, as it orbits around the Sun. Because of this motion, the light from the stars shows a Doppler Shift (this means that the light from the stars appears redder when we move away from them and bluer when we move towards them). The shift tells us how fast the Earth is moving. Because we know how long it takes to orbit the Sun, we then know the size of the Earth's orbit.
Since 1958, we have measured the distance to the Sun using Venus. Radio astronomers use radar to determine the distance to Venus. They send radio waves to Venus, and measure how long it takes for the waves to bounce back to Earth. Radio waves travel at the speed of light, which we know very accurately, and so we then know the distance to Venus. Again, because we know the relative sizes of the orbits of Venus and the Earth, knowing the distance to Venus tells us the distance to the Sun.