transit calculator
"No living person can lay claim to having seen a Venus transit..." Dennis di Cicco
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Transit  Calculator

The Transit Calculator allows you to use your own observations of the transit of Venus across the face of the Sun to calculate the distance from the Earth to the Sun – this is known as the astronomical unit (au). This is just what Horrocks tried to do in 1639. The au is of enormous scientific importance as it is fundamental in establishing astronomical and cosmological distances throughout the entire universe.

To participate in the project you are first recommend to read:

What will you need

The transit calculator provides a straight-forward method for you to take some simple observations during six hour journey that Venus will make across the disc of the Sun.  In particular, the calculator uses a specially designed UK transit chart.

In a nutshell, you need to do the same observation on at least two separate occasions during the transit of Venus. If you are able to measure as accurately as you can on the exact time at which Venus sits on a circle marked on the UK transit chart and input these times into the web-based calculator, then your own personal estimate of the Sun-Earth distance will be automatically calculated, returned to you and compared with the accepted value (149.598 million km). Providing more timing on more circles in the path or timings that are further apart in time will help to improve the accuracy of your result.

Remember this is a tricky observation to do and if your estimate is within 30 million km, then you have done very well indeed, in fact, you will have done better than Jeremiah Horrocks!

In summary:-

Quick Start
  1. safe observing facilities - see events database
  2. accurate watch (accurate to 1 second)
  3. your exact Longitude and Latitude
  4. UK observing chart: Equatorial | Elevation-Azimuth
  5. web form to submit your observations

More details are:-

[1] To be able to make your own estimate of the astronomical unit you will need to have access to safe observing facilities on the morning of the transit. Many schools and local astronomical societies will be providing this service, if the weather permits, and some of these are listed in the events database

[2] Your observations will require you to make a record of the time at which the planet reaches the points marked on the chart. This can be reliable achieved by using the same time piece for all your observations and setting it to the nearest second just before you start observing using the teletext service broadcast on the television.

[3] The exact path of Venus across the Sun depends on your location so you will need to find your longitude and latitude. The best estimate can be found using a GPS system or failing this use a large scale atlas

[4] The University of Central Lancashire and Starlink have provide a chart (select the correct chart from those displayed on the right) for you to follow the path of the planet as it crosses the Sun between 6 am and noon on the 8th of June. To use this chart properly you will need to be able to project the image of the Sun onto the chart. The correct scale can be achieved by adjusting the distance between the eyepiece projecting the image and the chart itself. This can be done in advance to minimise the setup time.


[5] The traditional observations to make are the four contact times which would require you to start observing at approx 0600 BST. We have also included 7 additional points that are marked A, B, C, - G on the charts to enable you to start observing at a later time. Better results will be possible if the observations are further a part.

Report your observations

On the day you will be able to submit your observations to this website. The transit calculator will provide you with your estimate of the au and add your observations to the national database of result. The link below allows you to see the form in advance so you can plan you observing programme. Updates to the form will be published in the next week.

report your observations







report your observations






Equatorial mount path

downloads: pdf | gif


Elevation-Azimuth mount path

downloads: pdf | gif