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 Subject: astronomy question Category: Science > Astronomy Asked by: kclou-ga List Price: \$15.00 Posted: 20 Jul 2005 21:38 PDT Expires: 28 Jul 2005 19:41 PDT Question ID: 546070
 ```Two astronomers, Joe and Jane, from different planets in their solar system meet at an astronomer's convention. Joe tells Jane that her planet always appers as gibbous or full phase when viewed from his planet. What phases will Joe's planet show when viewed from Jane's world? Which planet in their solar system, aside from Joe's home planet, cannot be Jane's home?```
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 ```Wish I could answer this one. Here are some thoughts: Joe's and Jane's planets are in synchronous orbit around their star. Joe's planet orbits inside the orbit of Jane's planet. Joe's planet appears to eclipse their star at all times. From the night side of Joe's planet, Jane's planet will always appear to be full, but with the shadow of Joe's planet cast upon it. Joe's planet will be in a new phase just as the moon is during a solar eclipse. Jane's planet cannot be a planet inside the orbit of Joe's planet. I'm no astronomer, but to achieve the phases between full and gibbous, I imagine the answer involves orbiting around a twin star such that the orbit of Joe's planet is accelerated and decelerated to present those phases. This would cause Joe's planet to appear to be crescent or new.```
 ```I wish I'd had homework questions like that! Have a search for info on the phases of Jupiter and Venus, and then apply that to Joe and Jane's observations. Ian G.```
 ```This is an easy one! Jane's planet must always be of higher orbit(further from the star) than Joe's. Then light from the central star reflected on Jane's planet as viewed on Joes planet will always appear in Full phase. (with the rare exception of when the star comes between Joe's and Jane's planets). If I could draw a diagram, this would be very obvious. Your questions. what phases? All phases, the full spectrum of phases just as the moon is viewed from Earth. what planet cannot be Janes home? Well any interior planet to Joe's else all the phases and not just one will be viewable. Therefore for example the closest planet to the star cannot be Jane's planet. Regards, MSc Astronomy.```