Monday, August 13, 2012

It's Summer Because Earth is Closer to the Sun, Right? WRONG! It's Further Away!

Earth's orbit around the sun is not a perfect circle; it is shaped more like an oval, or an ellipse. Over the course of a year, Earth moves sometimes closer to the sun and sometimes farther away from the sun. Earth's closest approach to the sun, called perihelion, comes in early January and is about 91 million miles (146 million km). The farthest from the sun Earth gets is called aphelion. It comes in early July and is about 94.5 million miles (152 million km).
It is now Summer in the northern Hemisphere because of the tilt of Earth's axis.

Earth's average distance from the sun (93 million miles) is known as an astronomical unit (AU). Astronomers use that scale for measuring distances throughout the solar system. Jupiter, for example, is 5.2 AU from the sun. Neptune is 30.07 AU from the sun. On the outer edges of the solar system, the Oort Cloud, where comets are thought to originate, is 100,000 AU from the sun. The distance to the nearest star, Proxima Centauri, is about 250,000 AU. However, to measure longer distances, astronomers use light-years, or the distance that light travels in a single Earth year, which is equal to 63,239 AU. So Proxima Centauri is about 4.2 light-years away.

Finding the distance

Historically, the first person to measure the distance to the sun was Aristarchus around the year 250 BC. In more recent times, astronomer Christiaan Huygens calculated the distance from Earth to the sun in 1653. He used the phases of Venus to find the angles in a Venus-Earth-Sun triangle. For example, when Venus appears half illuminated by the sun, the three bodies form a right triangle from Earth's perspective. Guessing (correctly, by chance) the size of Venus, Huygens was able to determine the distance from Venus to Earth, and knowing that distance, plus the angles made by the triangle, he was able to measure the distance to the sun. However, because Huygens' method was partly guesswork and not completely scientifically grounded, he usually doesn't get the credit.
In 1672, Giovanni Cassini used a method involving parallax to find the distance to Mars and at the same time figured out the distance to the sun. He sent a colleague, Jean Richer, to French Guiana while he stayed in Paris. They took measurements of the position of Mars relative to background stars, and triangulated those measurements with the known distance between Paris and French Guiana. Once they had the distance to Mars, they could also calculate the distance to the sun. Since his methods were more scientific, he usually gets the credit.

— Tim Sharp, Reference Editor

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