Expert: Jayendra Upadhye - 10/27/2004

Question
I would like to ask some other questions about twin star system. From the planet observer view.
Are there two equators for these planet, one for star A and second for star B? Are there different ecliptics for star A and B? Is any place on planet where day and night is permanent because of stars positions?

Answer
Hi,
In continuation of my previous answer..
from any planet's point of view..
1 - If one took equator to mean "midway" between the poles, there would be one equator corresponding to our 0 deg latitude, irrespective of the number of suns.
trying to get the equator as the midpoint of the binary's apparent northward / southward travel during the planetary year would confuse things as the binary orbital period would not be in synch with that of the planets, and avrages would have to be used, and these would change and repeat over many planetary years.
2 - The ecliptic again would have to be an "average" path followed by the binary in the planet's sky, thru the year.
(the path of their center of mass) if the stars were in tight orbit. else one would have situations where the ecliptic "precessed" as the stars slowly completed one orbit over many planetary years (where the planet happened to have a complex path "between the 2 stars".Such a path is possible).
3 - That is possible only where the planet is close to a binary system and the planet's orbit encloses the stellar orbits around each other. In such a case, like mercury (though it is not completely phase locked), the planet would over time like our moon present only one face all the time to the binary stars. {tidal locking]
Jayen