Astro
Senior Member
STEREO is a NASA mission consisting of two satellites, one ahead of the Earth's orbit, and one behind. They both have a variety of instruments, including five cameras, two of which are specifically for photographing the Sun's corona, COR1 and COR2
Earth has been visible on STEREO Ahead in the COR2 imager since December 18th. Since that time it has moved very slowly across the image, moving only about half a degree in the course of a month. This caused ThorNews on YouTube to claim that Earth "stood still" for a month and that this was highly unusual.
In fact it's perfectly normal. I created a spreadsheet to calculate where Earth should be in the COR2 images relative to the sun. The images from STEREO Ahead are currently all beacon quality (256x256 native resolution) due to bandwidth limitations imposed by the side-lobe operations of its antenna as it orbits to the opposite side of the sun (heating of the feed horn required off-pointing the antenna to lower the temperature since it has to increasingly point towards the sun to also point towards the earth). I calculated the sun-earth separation as seen by STEREO Ahead's position in orbit around the sun and then converted this to a pixel value based on astrometry of an old COR2 image ( http://nova.astrometry.net/user_images/566023#annotated ) accounting for the 8 fold decrease in current resolution.
http://dropcanvas.com/nhcs7
The spreadsheet correctly predicts the location of Earth relative to the sun in the COR2 images and proves that the motion is normal and expected given the orbits of earth and STEREO:
This makes intuitive sense when you realize that STEREO Ahead is in a very earth-like orbit. It originated from earth and it only has a slightly smaller semi-major axis than earth. It therefore orbits the sun nearly at the same speed as earth, only slightly faster. Earth will therefore exhibit a very slow motion relative to the sun as seen from STEREO Ahead. You can double check the functionality of the spreadsheet by substituting the orbital elements of other planets farther from the sun than STEREO Ahead (earth falls into the category of an "outer planet" to STEREO Ahead ever so slightly) into the place of earth's orbital elements in the spreadsheet. I was able to correct predict the entry of Jupiter into the COR2 field of view using the spreadsheet. Jupiter will proceed quickly across COR2's field of view, passing the earth, because it is in a vastly different orbit with a much larger semi-major axis, therefore STEREO orbits the sun much faster than Jupiter.
Earth has been visible on STEREO Ahead in the COR2 imager since December 18th. Since that time it has moved very slowly across the image, moving only about half a degree in the course of a month. This caused ThorNews on YouTube to claim that Earth "stood still" for a month and that this was highly unusual.
In fact it's perfectly normal. I created a spreadsheet to calculate where Earth should be in the COR2 images relative to the sun. The images from STEREO Ahead are currently all beacon quality (256x256 native resolution) due to bandwidth limitations imposed by the side-lobe operations of its antenna as it orbits to the opposite side of the sun (heating of the feed horn required off-pointing the antenna to lower the temperature since it has to increasingly point towards the sun to also point towards the earth). I calculated the sun-earth separation as seen by STEREO Ahead's position in orbit around the sun and then converted this to a pixel value based on astrometry of an old COR2 image ( http://nova.astrometry.net/user_images/566023#annotated ) accounting for the 8 fold decrease in current resolution.
http://dropcanvas.com/nhcs7
The spreadsheet correctly predicts the location of Earth relative to the sun in the COR2 images and proves that the motion is normal and expected given the orbits of earth and STEREO:
This makes intuitive sense when you realize that STEREO Ahead is in a very earth-like orbit. It originated from earth and it only has a slightly smaller semi-major axis than earth. It therefore orbits the sun nearly at the same speed as earth, only slightly faster. Earth will therefore exhibit a very slow motion relative to the sun as seen from STEREO Ahead. You can double check the functionality of the spreadsheet by substituting the orbital elements of other planets farther from the sun than STEREO Ahead (earth falls into the category of an "outer planet" to STEREO Ahead ever so slightly) into the place of earth's orbital elements in the spreadsheet. I was able to correct predict the entry of Jupiter into the COR2 field of view using the spreadsheet. Jupiter will proceed quickly across COR2's field of view, passing the earth, because it is in a vastly different orbit with a much larger semi-major axis, therefore STEREO orbits the sun much faster than Jupiter.
Last edited by a moderator: