Comments on How do seasons work in a binary system (planet orbits one star, not both)?
Parent
How do seasons work in a binary system (planet orbits one star, not both)?
I have a planet orbiting one star in a binary system. When the planet is exactly between the two stars it will experience a double day; when the primary sun sets the secondary one rises, no overlap. (My second star sheds enough light to make a difference on the planet.) When the planet is on the opposite point in its orbit the primary star occludes the secondary, so it's as if there were one star, lighting-wise. I'm trying to figure out the stuff in between.
The planet orbits A at a distance of 1AU. Answers on the linked question suggest that the distance between A and B should be 10-20AU for this to be viable. The planet's orbit is meant to be viable; feel free to treat it as circular despite the drawing.
I'm having trouble working out what days look like on the planet for the points in between the two marked positions. I think at the halfway points it'll get overlapping days, but I don't know how long (as a ratio of the rotation period). It's probably a simple matter of geometry, but adding the rotation of the planet to the orbit is causing me problems.
What I'd really like is a chart showing the progression of the day -- time of first sunrise, second sunrise, first sunset, second sunset -- for the four main points and perhaps the four in between those (so I can understand the transitions), at the equator and at what we'll call 45deg N. Treat times as relative to star A -- noon is when A is directly overhead, regardless of where B is.)
I know we're going to need some axial tilt to make this planet have seasons; pick and declare any reasonable-seeming number that makes your calculations easy, or default to Earth's for the sake of comparison. I'm trying to visualize what days and nights look like on this planet; I don't have precise numbers in mind.
We're also going to need a rotation period. For the sake of the question let's assume 24 hours like on Earth. In practice, once I know what the proportions look like, I'll adjust the rotation to suit the needs of my inhabitants (because we're worldbuilders and we can do that :-) ).
Post
I see you've got some detailed answers already, although they are a bit dense. I'll try to give a simple conceptual answer instead.
From the point of view of being on the ground on the planet, star A will appear just like our sun on earth. Star B will be a smaller and significantly dimmer dot that follows in the same "track" as the sun, but at an angular offset that varies slowly. It takes a whole year for star B to go around a whole circle. This is a lot like our moon, except that it goes around in 28 days instead of a year.
For simplicity, I'll call it "summer" when the planet is between the two stars, and "winter" when it is way to the left in your diagram.
Starting in winter, only the sun (star A) is apparent in the sky since star B is behind it. After the winter point, star B will appear to lead the sun across the sky. It will rise a little above (earlier) than the sun and set a little below (earlier) than the sun. This leading angle will increase a little every day.
By spring, star B is leading the sun 90°. At that point, the first half of the night is dark, then the second half lit by star B. The effect is like the moon being out the second half of the night, although significantly brighter than our full moon.
By summer, the angle of star B has gotten all the way to 180°. In other words, star B is now opposite the sun. The sun illuminates the day, and star B illuminates the whole night. From this point until winter, star B will appear to lag the sun.
By fall, star B is trailing the sun by 90° and is illuminating the first half of the night. Over the next ¼ year, star B will get closer to the sun until it is right behind it (off by 0°) in winter again.
Another point to consider is the brightness of star B. It's not unusual for binary systems to have stars of different sizes. To make your world more stable and plausible for supporting life, I'd make star B as small and far away as reasonable for a binary system.
Illumination from a source falls off with the square of the distance. For simplicity, let's say both stars are the same and like our sun. If the two stars are 20 AU apart, then star B varies from 19 to 21 AU from the planet over a year. That makes it 360 times dimmer in summer to 440 times dimmer in winter.
Full sunlight on a really bright and clear day here on Earth is about 100k lux. That means summer nights on your planet will be lit with about 280 lux under clear sky. That's about typical indoor supermarket lighting. You would be easily able to read a newspaper, for example.
1 comment thread
0 comment threads