Citizen Scientists Map Venus Transit

Esri Builds Astronomy Observations Map Application


  • Astronomers Without Borders and Esri built an application that instantaneously displayed participants' observations.
  • The transit data was collected on a central server and displayed just seconds later.
  • Powered by GIS, the 2012 Transit of Venus project may well be one of the largest crowdsourced mapping projects to date.

During a Venus transit, the planet appears as a small dot moving across the sun, an event lasting approximately six hours. Transits of Venus occur in pairs eight years apart, separated by more than a century. Since the last Venus transit was seen in 1882, no one living today had experienced one until 2004, the first of a pair culminating in 2012.

Crowdsourced observations are published on the ToV2012 mapping application.

Crowdsourced observations are published on the ToV2012 mapping application.

Working with Astronomers Without Borders, Esri built a web map application that instantaneously displayed participants' transit of Venus observations as part of a historic project. Under the aegis of Astronomers Without Borders, Dutch high school physics teacher Steven van Roode developed an iPhone application to allow people to contribute their observations of the transit to a central database. Esri designed the web map application ToV2012, which worked in conjunction with the iPhone application. On June 5 and 6, 2012, 25,000 amateur astronomers from around the world used the free application on their smartphones to record important timings during the transit.

The transit data was collected on a central server and displayed just seconds later via the ToV2012 web map application running on ArcGIS for Server. The dynamic map was published on the Esri and Astronomers Without Borders websites. The Esri application included an animation showing how Venus would appear as it traveled across the sun, as well as which regions of the earth would be able to view it and at what times. As the earth rotated, these citizen scientists' observations lit up a separate map in near real time.

For 500 years, the transit of Venus has been a marker of technological advancement. In the 17th century, two people witnessed the event by using the newly invented telescope. In the 18th century, astronomers' measurements of the transit of Venus were used to calculate the distance between the earth and the sun.

This image of Venus eclipsing the sun was taken with an Apple iPhone 4.

This image of Venus eclipsing the sun was taken with an Apple iPhone 4.

Edmond Halley (think Halley's Comet) proposed the idea of using the transit of Venus as a means to measure the size of the solar system. Observers at widely separated locations could record the exact time Venus moved from edge to edge across the sun's disk. The observers, seeing the sun at different angles, would record two distinct paths of Venus due to parallax. Mathematicians could use these observation times to geometrically calculate the distance from the earth to the sun.

During the 1761 and 1769 transit of Venus, astronomers and explorers recorded the times and location of their sightings. Fifty years later, a German astronomer determined the solar parallax to be 8.57 arc seconds, giving a distance from earth to the sun of 95,000,000 miles (152,887,680 kilometers).

The 1874 and 1882 transits were recorded using the newly invented camera. Ten years later, the director of the United States Naval Observatory determined the solar parallax and calculated the earth-to-sun distance to be 92,797,000 +/- 59,700 miles (149,342,295 +/- 96,007 kilometers).

By the time the transit of Venus came around in 2004, use of broadcast media and the Internet was global. Telescopes on earth and in space were trained on Venus, and the transit was televised around the world. In the eight years between then and the 2012 transit, technology has advanced to include GIS, smartphone and tablet applications, social media, and light clients built on a superfast Internet infrastructure.

For more information about Astronomers Without Borders, the 2012 Transit of Venus project, and other galactic marvels, visit

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