Using only the sunlight striking the Earth and a wooden dowel, students can measure the circumference of the earth. Eratosthenes did it over 2,000 years ago. In Cosmos, Carl Sagan shared the process by which Eratosthenes measured the angle of the shadow cast at local noon when sunlight strikes a stick positioned perpendicular to the ground. By comparing his measurement to another made a distance away, Eratosthenes was able to calculate the circumference of the earth. (See Carl Sagan’s depiction of this process at (https://www.youtube.com/watch?v=G8cbIWMv0rI.) We will provide an online learning environment where students will be able to do science the same way Eratosthenes did. A notable project in which this was done was The Eratosthenes Project, conducted in 2005 as part of the World Year of Physics; in fact, we will be drawing on the teacher’s guide (http://www.physics2005.org/projects/eratosthenes/TeachersGuide.pdf) developed by that project. The difference between that activity and How Big Is Earth? is the online learning environment provided by the iCollaboratory where teachers and students can collaborate, share data, and reflect on their learning of science and astronomy. We will also maintain and share an ongoing database of student measurements. We will collect data on both teacher and student learning from surveys, discussions, and self-reflection done online. Students in each school will collaborate with students around the world; during the first year, schools in six countries will participate. We will share our research about the kinds of learning that takes place only in global collaborations. This is a sustainable project. Another ongoing annual project in the iCollaboratory, The Moon Over Us, has grown from 5,000 students in 2 countries to 6,000 students in 6 countries; total number of students participating has been 15,000. How Big Is Earth? has the same potential to impact the learning of students globally.