Today I would like to share another picture from the May 9, 2016, Mercury transit. This is a composite created by Monica Bobra at Stanford University. It's an excellent view of the transit from 2nd contact to 3rd contact, with an image every 20 minutes. Creating these composites takes a good bit of art. You can't just add the images together as that blurs the background image of the Sun. You have to take cutouts around the planet and reinsert them in a selected background image.
It turns out that MDI on SOHO was turned on for the transit. Bobra did the same analysis to produce this image from MDI. The data near 2nd contact is not available.
You can see the paths of Mercury are slightly different in the two images. The MDI path is closer to the solar equator and tilted at an angle to the path in the HMI image. These differences are caused by the different viewpoints of the transit from the two spacecraft. Similar differences are what were measured to determine the size of the astronomical unit (AU) during the Venus transits of the 1700 and 1800's.
Thanks, Monica!
