# Are there such circumbinary planet configurations that two suns move towards each other by the sky?

Is it possible that suns move towards each other (not necessarily in opposite directions)? Such a configuration would cause interesting dusk and midday light cycles, in my opinion.

Basically the question is about whether it is possible that one of the suns has a clockwise orbit (relative to the planet's pole) while the second sun has the counterclockwise orbit, which will be seen on the sky as follows:

• The can't only move towards eachother because sooner or later they would hit each other. And then you'd have a very interesting dusk... Please explain what you mean more clearly May 29 at 7:19
• @JamesK surely you understand that when seen from a planet, two celestial objects move toward each other they don't generally hit each other. They generally eclipse or occult. Mars doesn't hit the Moon twice a year for example.
– uhoh
May 29 at 7:26
• Yes, but in that case the objects would have to subsequently move away from each other. And in this configuration we would normally say that that the move around each other. I don't think the OP is asking if the stars sometimes move around each other. But I don't know what they are asking that's why the question is unclear. May 29 at 7:36
• Binary stars stay in line with their centre of mass (barycentre), so they can't orbit in opposite directions. Please see my answer on our sister site: physics.stackexchange.com/a/558847/123208 May 29 at 14:34

Are there such circumbinary planet configurations that two suns move towards each other by the sky?

Is it possible that suns move towards each other (not necessarily in the opposite directions)? Such a configuration would cause interesting dusk and midday light cycles, in my opinion.

Yes sure, absolutely!

If the circumbinary planet's orbit is in the same plane as the the binary's orbit, there will be regular eclipses as each passes behind the other. If one is bright and small, and the other dimmer and large, you may have a dim day, or weeks or months of them, depending on all the details (relative sizes and their period.

You may also have contributions from light reflected from one star off the other star! In other words, if they are really close and even if there is no occultation/eclipse, you'll get color gradients across each star's disks.

Basically "the sky's the limit" :-)

• No more of I think this answer is not useful, I'm not certain what the OP is asking, it's not clear, but this seems to answer the question "can two suns move around each other. I don't think it is useful to speculate on what you think the question ought to have been May 29 at 9:01
• @JamesK I'll wager 100 quatloos that I've understood the "newcomer"'s question correctly. :-)
– uhoh
May 29 at 10:07
• @uhoh that's a one interesting configuration, although something exotic and not strictly related to the question in general; i mean, obviously some geometrically bizarre configurations are possible, but the question is whether this is possible with normal conditions and normal suns May 29 at 13:48
• @JamesK i have made a clarifying update, as requested May 29 at 13:51
• @ivan866 the picture perfectly illustrated the part of the answer directly above it: "You may also have contributions from light reflected from one star off the other star! In other words, if they are really close and even if there is no occultation/eclipse, you'll get color gradients across each star's disks."
– uhoh
May 29 at 22:32

No,

In a circumbinary configuration the two stars are close together and close to the common centre of mass, and the planet moves in a wide orbit about both.

From the perspective of a planet, the motion of the star (or stars) is due to the rotation and orbit of the planet, and secondarily due to the actual motion of the stars relative to their centre of mass. Think how the sun moves daily from East to West (due to the rotation of the Earth) and yearly relative to the background stars (due to the orbit of the Earth). Your two suns will have the same basic motions.

The two stars will have similar motions, daily (due to the rotation of the planet) and annually (due to the orbit of the planet).

In addition the two stars will be in orbit around each other, as a result of this orbit they will move sometimes further apart from each other in the sky and sometimes closer together.

So if the planet has an Earth-like rotation, you would see the two suns rising, moving across the sky and setting close together. If you watched over several days you would notice that they moved closer together and then further apart. The length of this cycle could be more or less, but would typically be about a week. Over the course of the planet's "year" the two stars would move together across the background stars.

The further apart the two stars are, the slower their orbits will be. So if the two stars are able to move quickly in the sky, they would have to be very close to one another. And so could not be widely separated. Indeed, if the two stars appear widely separated, it makes the circumbinary orbit difficult to maintain. Stability of a circumbinary orbit depends on the two stars being close together.

A configuration in which the two stars are widely separated in the sky and then, over the course of a day come closer together and set together is not dynamically possible.

• is such a system theoretically possible that the planet's orbit CROSSES the center of masses between the two suns? May 29 at 16:25
• No. That is not theoretically possible in a stable circumbinary configuration. May 29 at 16:48
• I wonder what I'll spend the quatloos on.... May 30 at 20:52

At least one theoretical solution exists, involving a triple star orbiting a figure-8 shape. And from the P-type planet one of the suns will be seen as setting in a different direction.