Phone app

By Steven van Roode

New technologies, like the VenusTransit phone app, will allow individuals to send their observations of the 2012 transit of Venus to a global experiment to measure the size of the solar system. The free phone app has been developed by Norbert Schmidt of DDQ in the Netherlands. For more information, you can contact Steven van Roode: s.vanroode@transitofvenus.nl.

Features of the phone app

In centuries past, explorers traveled around the globe to time the transit of Venus to determine the size of the solar system. We invite you to inspire international collaboration during the 2012 transit of Venus by enabling a digital re-creation of those global expeditions. The phone app will allow citizens around the world to witness this rare phenomenon and to contribute their observation to a collective experiment to measure the sun’s distance. This will literally be the last such opportunity in your lifetime.

To have extended utility, the phone app will serve you before, during, and after the transit. Prior to the transit, you can practice timing the interior contacts using a simulation of the transit. Additionally, you can see predicted times of contact for your location. During the transit, the phone app will assist you in measuring the time of the interior contacts. After the transit, you can access your data on a map on our website. Make sure to have read the instructions below, especially about how to measure the times of contact.

Practicing the timing of the contacts
At the start and end of the transit, Venus will touch the limb of the sun on the inside. The exact times of these contacts vary according to your location on earth as a result of parallax. By combining the contact times measured all across the world, the distance to the sun can be determined.

The instant the limbs of Venus and the sun touch isn’t readily recognised because of the black drop effect: a greyish hue that obscures the exact time of contact. To get accustomed to the appearance of the black drop effect, the phone app has a realistic simulation that let’s you practise the timing of the contacts.

The simplest method is to tap the phone’s touch screen at the time you think the limb of Venus touches the solar limb on the inside. The app will evaluate your try and indicates if you tapped the screen too early or too late. Going through a couple of practise runs, you start to learn how true contact should look like – and you will get it right on the day of the transit.

The simulation also supports a more advanced method to record the times of contact, used by professional astronomers in 1882. This method involves timing two instants (which means tapping the touch screen twice before evaluating your try).
Just before interior contact at ingress the sun will show two fine sharp horns of light, gradually approaching each other to meet around the dark body of the planet. These horns of light don’t meet instantly. Very generally there will be a period of several seconds during which the light will glimmer and disappear by turns. Tap the touch screen at the start and at the end of this period, of which the middle is that of true contact.
At egress, the band of light in front of the planet very gradually grows fainter until it is lost in vision. From the commencement of the intermittently darkening until the definite disappearance of the sun’s light from view, there is a series of progressive changes, which may extend over a period of several seconds. Again, tap the touch screen at the start and at the end of this period, of which the middle is that of true contact.

Both methods should yield the same times of contact.

Predicted times of contact
When the time of the transit draws near, you would like to know the time when you should be ready at your eyepiece. The phone app has a tab which gives you the predicted times of contact for your location, as well as the times of sunset and sunrise. This way, you know at what time the transit starts or ends. Contacts that cannot be viewed from your location (because the sun is below the horizon) are indicated in grey.

The exterior contacts refer to the moments Venus touches the solar limb from the outside at the start of the transit (ingress) and the end of the transit (egress). Likewise, the interior contact refer to the moments Venus touches the solar limb at the inside.

The phone app selects your current GPS location, but you can select other places as well. Just move the map to the new location, hold you finger on the touch screen and the times of contact for the new location are shown. Tap the blue compass icon at the top right of the map to return to your current location.

On the day of the transit: timing the contacts
Using the timer, just tap the phone’s screen at the moment of internal contact (when Venus appears within the sun, just touching the sun’s edge). As you are looking through the eyepiece and not at the screen, the phone will vibrate when you log your times to let you know the time was successfully recorded. The app will record the exact GPS time and your location, which then will be sent to the global database. When submitting your results, you are also asked to enter your email address. Although this is optional, you will be able to add more info to your entry later if you send your address along with the data.

With the Venus Transit app, it’s like you’re holding an almanac, an artificial transit device, a pendulum, a personal assistant to read the clock and a mailman – all needed by the eighteenth and nineteenth century astronomers observing the transit of Venus – in one hand!

Creation of an interactive depository
After the transit, you can access your data on an interactive map on our website, which will also show all social media items associated with the transit of Venus. This way, a depository of our transit experiences is created: the one place to look for results, pictures and personal stories of the 2012 transit of Venus, and a unique tool for science classes as well! At the same time, it will provide for a time capsule for our descendants to discover when the twenty-first century’s transits of Venus are taking place.