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Mercury and the AU: Measuring the AU with a Transit of Mercury

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#1 astro151


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Posted 24 October 2019 - 04:05 PM

For anyone interested in trying to measure the Astronomical Unit (AU) for yourself using Sir Edmund Halley’s technique…


The principles are simple.  You need two observers spaced far apart on the globe (the further the better) to take simultaneous images of the transit.  Technically you can team-up with anyone.  However, finding someone with the same equipment as you can make things easier.


Basic Data Collection Principles


Ultimately, you need to take images from the two locations and align them as precisely as possible to see how the silhouette of Mercury’s disc shifts due to parallax.


There are several factors involved:


1.) Image scale – You can always re-size images so that the Sun’s disc is the same diameter.  Telescopes with the same focal length and accessories (flatteners, filters, reducers, etc.) should produce images where the sun is the same size.  To potentially avoid the need to re-size images, find someone with the same focal length and accessories as you.


2.) Image alignment – centering the Sun’s disc in the final picture is fairly easy and can be done in any number of ways.


3.) Image rotation – the chances of having cameras at different locations oriented at the same angle is very small.  At some point you will probably need use software to rotate one of the images to match the other so that the same features (except for Mercury) overlap.


NOTE: Mercury’s disc will only appear to shift by a very small amount!


Finding an Imaging Partner


If you do not already know someone at great distance from you, you can use this very basic Mercury and the AU Observers List (Google Sheet).  Enter your Cloudy Nights username, approximate location, the telescope and camera you plan to use.  Then look for others to partner-up with.  The further apart you are, the better (as long as you can still take simultaneous images).  Contact each other using Cloudy Nights

Personal Messages (PMs).


Google Sheet Rules:


1.) This is just intended to help people self-organize.  I will not be managing or maintaining the list or answering questions.



2.) Share only the info you feel comfortable with.



3.) The Google Sheet will be deleted on Monday, Dec. 2nd (three weeks after the transit) so your info is not permanently “out there”.




You and your imaging partner(s) need to take simultaneous images.  Depending on where you are on the globe, you may not both be able to see the entire transit.  Choose times which you can both see and at which the Sun is as high in the sky as possible.  Coordinate this amongst yourselves.


Image Processing


Everyone likes to post-process images in their own way.  The recommendation is that you share your unprocessed images with each other.  Re-size images so that the Sun has the exact same diameter in all images.


Aligning images will be the difficult part.  The best is if you know how to use software to align the images so you can measure Mercury’s shift by counting the pixels rather than printing images and measuring by hand.  Physical measurements done very carefully can still yield good results.


Data Analysis


Now that you have a pair of images, how do you analyze them and calculate the AU?  Here is a slightly stripped-down version of the AU Lab I designed for my astronomy students.

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