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National Aeronautics and Space Administration

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Sun-Earth Day 2009: Total Solar Eclipse, China

Sun-Earth Day 2009: Total Solar Eclipse, China

Amateur Astronomers

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Lunar Parallax Challenge
The Lunar Parallax Challenge is a calculation you can do using a few easy steps, some math, and a lot of fun. If you think you're up to the challenge, download the instructions. If you happen to get your own photos of the eclipse, don't forget to send them in to the email within the pdf document.

The size of the universe has been the subject of great scientific campaigns for thousands of years. Distances to the sun, moon, planets, and stars were debated heavily up until recent times. Even before the invention of the telescope, astronomers had a powerful tool at their disposal for deriving distances within the solar system. This tool was geometry.

Armed with geometry as well as planer and spherical trigonometry, around 190 BC, the Greek astronomer, Hipparchus knew that the position of the moon appeared to change against the background of the heavens depending on ones location on Earth. This effect is called lunar parallax. Hipparchus assumed that the sun was at an infinite distance. He then observed the solar eclipse of March 14, 190, which was total in his birth place of Nicaea. Hipparchus watched as the moon covered the sun. He combined his observations with observations of the eclipse from Alexandria, approximately 9 degrees of latitude south where the moon was reported to cover 80% of the sunís disk. From this information, he concluded that the moon was between 71 and 81 Earth radii from the Earth. This is close to the modern values of 56.9 and 63.6 Earth radii. Hipparchus later calculated values of 62 Ė 72 Earth radii for the distance to the moon. Claudius Ptolomy (100-178 AD) and others followed, measuring lunar parallax to derive the distance to the moon achieving more and more accurate results. For the August 1 solar eclipse, you will replicate these historic observations and find the distance to the moon for yourself.

On our web site, you will be able to get information on many other important eclipses over the past 2,000 years. In "*/Eclipses through Time/*", you will find out about eclipses that changed the course of history and made significant impacts on society, politics, and science. Wars were won or lost, peace treaties signed, kings rose or were deposed, and many new discoveries about our universe were made just by observing a solar eclipse.