The Venus transit phone app is now available in iTunes for free download. While the Android version is still being worked on, users of an iPhone can already enjoy the phone app. This app will be of great help in your observation of the transit of Venus on June 5 and 6, when the planet Venus is seen in front of the Sun for the very last time this century.
The main aim of the app is to allow you to participate in our international outreach project determining the distance to the sun using the transit of Venus, but there are a couple of other features that come in handy for all observers.
Transits of Venus can be used to find the distance to the sun. It was this utilisation, proposed by Edmond Halley in 1716, that inspired the eighteenth and nineteenth century astronomers to travel all across the globe to watch the transit from far flung places. Halley described that the required observations could already be secured without
any other instruments (…) than common telescopes and clocks, only good of their kind; and in the observers, nothing more is needful than fidelity, diligence, and a moderate skill in Astronomy.
All they had to do was to measure the times the limb of Venus’ silhouette would touch the solar disk on the inside. When in 2004 the transit of Venus occurred again after an absence of a hundred and twenty-two years, this experiment was repeated by thousands of diligent amateurs like you, watching how the disk of Venus slowly approached the solar limb and timing the exact instant of contact. Submitting the data and letting a website do the math, all observations combined into an accurate distance to the sun, a distance that until the 1900s couldn’t be measured accurately otherwise. This historical re-enactment in 2004 was a great success, but there were also issues. That’s where the Venus Transit phone app comes in.
One problem was the determination of time: if you’re on a parking lot or on a sports field observing the transit, where do you get your time accurate to the second? Back in 2004, this was a problem for many one-time observers. Not anymore. The phone app makes use of a GPS clock, allowing you to make accurate time measurements wherever you are: if you have a an unobscured view of the sky, you’re okay.
Another hindrance was the determination of geographic location. In 2004, you had to resort to an atlas or a website which listed the coordinates of major places. A 2004 map of submitted data shows how things went wrong sometimes: a couple of European observers are found in the Atlantic! The phone app will find your location for you using GPS.
A third obstacle was the black drop effect that had troubled historical observers as well: the exact instant of contact between the limbs of Venus and the sun is obscured by a greyish fuzziness. In the eighteenth century there wasn’t really a workaround: every observer made judgements on his own. In the nineteenth century astronomers were trained on beforehand with a mechanical model representing a simulated transit. In 2004, we were back in the eighteenth century; nobody really knew how to deal with the black drop, resulting in times that differed up to 10 seconds for observers watching the transit side by side. The phone app features a realistic simulation of the transit, so you can practise the timing of the contacts over and over again, until you are a skilled transit observer!
And lastly, the phone app will take care of the submission of the data. In 2004, you had to go to a computer, fill in a digital form by hand and send your observations to a database. Now, all you have to do after you finished your observation is press the submit button, and you’re done!
Download the app for free and get actively engaged in the observation of the transit of Venus!