IRIS Movie of the Day
At least once a week a movie of the Sun taken by NASA's Interface Region Imaging Spectrograph (IRIS) is posted by one of the scientists operating the instrument.
Orbital motions and Mercury
Credit: IRIS, LMSAL/NASA, Wei Liu and Bart De Pontieu
This movie illustrates again the parallax effect caused by the different vantage points of IRIS and SDO but now with an SDO/AIA background image showing the transition region plasma at 100, 000K. We are actually seeing some neat orbital dynamics in action: of the satellites observing the Sun and Mercury that is! The Mercury along the white line shows where IRIS saw Mercury (it is not a real image of Mercury!) while the other Mercury is as observed by SDO. Notice how the SDO Mercury is initially "behind" the IRIS Mercury (along the white line) and catches up by the time the transit ends. Also, the IRIS Mercury wobbles up and down. The wobble of IRIS is caused by the excursions of IRIS to the Earth's north and south pole which it flies over every 97 minutes. SDO's Mercury catching up with IRIS' Mercury is because by the end of the transit SDO is hovering over a spot on Earth (with the longitude of New Mexico where its downlink station is) where it is almost local noon, i.e., SDO is located almost exactly between the center of Earth and the Sun. That means by that time it has almost the same vantage point as IRIS: the orbit of IRIS is symmetric around the Earth center-Sun line.