Measuring the size of the Solar System is remarkably hard. We can’t just roll out a tape measure and check our numbers twice. We can’t even just use the surveyor’s parallax method on any old day of the week. Until the advent of the laser and radar, the only way we could measure distances was to wait for specific alignments to occur, specifically, we had to wait for 1761 and 1769 Venus to Transits in front of the Sun. This year, you have the chance to take advantage of the Opposition of Eros on Jan 30-Feb 1, 2012 and the Transit of Venus on June 5/6, 2012 to help verify existing measurements of the Solar System’s size. Details here, and here, and keep reading to learn more. This is a project being supported by Astronomers Without Borrders. It actually wasn’t until 1771 that the Earth’s distance from the Sun was determined with better than 1% accuracy. During the 1969 Venus Transit, astronomers travelled to locations all across the globe – from St Petersburg in the north to Tahiti in the south – and very carefully measured where and when they saw Venus pass in front of the Sun. Since the astronomers knew where they were on the planet Earth, and since Kepler’s laws (and Newton’s confirming physics) told them the ratios of the distances of the planets from the Sun, they were able to use like triangles to accurately measure the distance from the Earth to Venus and the Earth to the Sun. In 1771, Jerome Lalande was able to use the combined measurements taken in 1761 and 1769 to determine the Earth is 153 million kilometers (plus or minus 1 million km). This same type of measurement can be made using the asteroid Eros. When the Sun, Earth, and Eros align – a geometry called the Opposition of Eros – it is possible to measure how Eros aligns with the background stars from different locations on Earth, and use those differences in alignment to determine the precise distance to Eros. Most of the time, this type of measurement isn’t practical. As it orbits, Eros’s distance from the Sun varies. When Eros is far from the Sun and the Earth, it is harder to use this method because the observed alignments with the stars don’t vary a lot from place to place and Eros is just a whole lot fainter and thus harder to see. The only time that Eros can be effectively used to measure solar system distance is when it is very close to Earth at opposition. The Earth and Eros were last close together at opposition in 1901 and 1931. These past oppositions allowed astronomers to refine their knowledge of the size of the Solar System. This year, on Jan 30 – Feb 1,  Eros is passing through opposition, and re-enacting its 1931 opposition. Today, we can (and will) be using radar to accurately measure its distance by measuring how long it takes the radar beam to travel to and return from the asteroid. But radar isn’t a good way to teach kids about the process of science, and how historically we came to better understand our universe. This is where you come in. Steve van Roode and Michael Richmond are orchestrating a campaign to get people like you outside, looking up, and using photography and an accurate clock (phone apps available to help) to document the opposition. You’ll need some sort of a camera + telescope or camera + telephoto lens to do this well, but it doesn’t have to be a really fancy set up. They’ve documented everything for you here. From your images, you’ll be able to measure Eros’s position (and if you take more than one image over several hours, you can measure it’s movement too!). By submitting your position on Earth, the time of your observations, and your observed position of Eros, your data becomes part of a global dataset that anyone – you, your kid, classrooms and the curious around the world – can use to determine for themselves the size of the solar system. Have you been looking for an excuse to try astrophotography ? Let this be your excuse to go out, look up, click a few frames, and help people repeat historical scientific measurements.