Measure the Sun’s distance

Steven van Roode By Steven van Roode

The observations of the transit of Venus, made at different points of the Earth, may be used to determine the Sun’s distance from the Earth. This possibility, once proposed by Edmond Halley, induced the famous voyages astronomers made in the eighteenth and nineteenth century to observe the transit of Venus from the ends of the Earth, and caused the digital re-creation of those global expeditions in 2004. In 2012 this collective experiment will be repeated again on a large scale, making use of the latest technologies available to us. This will be a project everyone can easily join in to!

The effect of parallax causes the transit to look slightly different for two observers at different sites on Earth: Venus doesn’t enter or leave the Sun’s disc simultaneously, and, observed at the same moment, Venus’ position in front of the sun is not quite the same. These effects give rise to basically two ways to obtain the Sun’s distance from observations of the transit of Venus. The first is timing the start and end of the transit from two stations, Halley’s original 1716 method. The second is photographing the Sun at the same moment from two stations and measure the displacement of Venus due to parallax. Make your observations useful: contribute to the collection of data!

Find a partner to collaborate with

To join the project, it is sufficient to submit your own observations of the transit of Venus using the phone app. But it may be interesting to have an actual partner to collaborate with. You can then gear your observing techniques to each other, exchange your data and work out the results together. Especially for schools this is a way to get students actively engaged in an exchange project. Our Partner Search Tool allows you to find a suitable partner and get in touch. To contact any of the observers, you should first register as an observer yourself.

Timing the transit

In his famous proposal submitted to the Royal Society in 1716, Edmond Halley explained how the Sun’s distance could be calculated from the transit of Venus or Mercury. Because of the effect of parallax and the Earth’s diurnal rotation, the duration of the transit of Venus, observed from two widely separated places, will differ from each other by a small amount of time. If this observed difference is found to be greater or less than the difference obtained theoretically from an assumed value of the Sun’s distance, then, according to Halley, the measured Sun’s distance will be greater or less in the same proportion.

Halley's method

Halley provided for an explanatory though inaccurate geometric construct to arrive at the transit’s duration from an assumed value of the Sun’s distance. The main principle of the calculation already set out and warmly recommended by Halley, the primarily work left to astronomers of a next generation therefore was to find the duration of the transit more accurately from theory.
On the occasion of the 1753 transit of Mercury, French astronomer Joseph-Nicolas Delisle noted that Halley’s reasoning with regard to the proportionality between the Sun’s distance and the duration of the transit could equally be applied to the moments of interior contact at either ingress or egress. The advantage of this alternative, which now bears Delisle’s name, was that observations from places where the transit would only be partially visible, could also be used to establish the Sun’s distance, thus increasing the number of potential observing sites.

The charm of Halley’s proposal is its simplicity of execution: all that is needed are “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 that is required is to find the times of contact, your position on Earth and submit these values to the project’s global database.

Phone App

A free phone app will allow you to contribute your observation to the collective experiment. Basically, the phone app will facilitate the timing of the internal contacts (when Venus appears within the Sun, just touching the Sun’s edge) using one button, and submitting all the essential data using another. All you have to do is watching the transit carefully and tapping twice with one finger! Furthermore, the app will be practical in many other ways: it will allow you to compute the theoretical times of contact for your location and there will be a feature helping you to practise with the determination of the contact times. If you are interested in the mathematical background of the measurement of the sun’s distance using the timings of the transit of venus, visit the website of Udo Backhaus and Stephan Breil,

Simultaneous observations

From two simultaneously taken photographs of the Sun during the transit of Venus, the Sun’s distance can be deduced as well. The two pictures, when layed over each other, show at once the parallactic shift of Venus. Obviously, the pictures should be orientated the same way and be of the same size.

Simultaneous method

This method is advantageous from a didactic point of view. The effect of parallax is clearly visible and corresponds to our direct experience of parallax in daily life. Also, active involvement is required in the adjustment of the pictures and the handling of the data.
The key requirement is that the photographs are taken at the same time. Here, the phone app will turn out to be useful as well. It will assist you in taking pictures of the Sun at a couple of fixed, predetermined times during the transit of Venus and help you find corresponding pictures taken from other places. For more information about this particular method as well as a platform for data exchange, see the website of Udo Backhaus and Stephan Breil,

Share your data and experiences

Once you submitted your observations to the project’s database, your involvement in the project doesn’t end. On the contrary, it’s the start of sharing and exchanging your experiences with other observers around the world. You can access your data on a map, edit your entry, and upload descriptions, text, images, or movies. You may comment on other’s entries and they can leave a comment at yours.

Presentation of collected data

You will also be able to repeat Halley’s experiment yourself by selecting timings from different sites, or find the Sun’s distance by pairing pictures of others with those of your own. The possibilities will be endless!

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