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Part One: A Multiple star.

One possibility of having a planet with with an orital period of 365 Earth years, would be to have the planet orbit around a group of stars instead of a single star.

For a circumbinary planet, orbital stability is guaranteed only if the planet's distance from the stars is significantly greater than star-to-star distance.

The minimum stable star-to-circumbinary-planet separation is about 2–4 times the binary star separation, or orbital period about 3–8 times the binary period. The innermost planets in all the Kepler circumbinary systems have been found orbiting close to this radius. The planets have semi-major axes that lie between 1.09 and 1.46 times this critical radius. The reason could be that migration might become inefficient near the critical radius, leaving planets just outside this radius.[9]

Added 11-19-2022.

Part Two: Tidal heating of an exomoon.

Theoretical studies of the possibiity of habitable exomoons of giant exoplanets show that the extent of tidal heating on those exomoons depends on their distance from their planets, among other factors. As a rule the closer to the planet, the greater the tidal heating on the moon.

If the moon orbits too close to the planet, the tidal heating will be enough to initate a runaway greenhouse effect, turning all the surface water into atmospheric water vapor. If the moon orbits even closer, the tidal heating will cause excesse vulcanism on the moon, making it a volcanic hell like Io.

There would still be a problem with the moon getting enough light from the star for photosynthisis on the surface of the moon for plants to produce an oxygen atmosphere.

Fortunately, a small increase in the mass of a star will cause a greater increase in the luminosity of the star. Thus the correct distance to receive a specific amount of radiation from the star will increase more than the star's mass and graitational pull on a planet. So the more massive and luminous the star is, the longer the orbital period of a planet receiving exactly as much radiation from the star as the Earth will be.

With years 365 Earth years long, the astronomical seasons would each be 91.25 Earth years long. And some of the comments express fear that such long hot summers and long cold winters would be deadly for life on the planet, or in this case, exomoon.

The way to handle this is to give the planet an a vert low axtial tilt, so that the inclination of incomming solar rays is almost exactly the same in every season of the year.

And in the case of a giant habitable exomoon orbiting around a giant planet and getting most of its heat from tidal heating, the astronomical seasons wuld not cause climataollgical seasons on the Moon. The temperatures would be quite constant all year.

So why would the people on a planet or moon with such even and unnoticable seasons brother with counting orbital periods around the star and making them the years that their calendars are based on?

Because of the changing night time stars. At any given moment one side of a planet or a planet's moon will be facing toward the star of the system, and the atmosphere on sthat will scatter the bright star light and making the sky appear opaque and hide the stars. And on the opposite side of the planet from the star, the atmosphere will have light from it to scatter, and the sky will be transparent, and the stars will be seeen. The stars which are in the opposite direction to the star in that system.

On Earth, as the Earth slowly orbits the Sun, the direction opposite to the
slowly changes, and so the stars which are visible at night slowly change over the year.

And so the stars visible at night on that planet will slowly change over the course of a 365 Earth year orbit. So posssibly the stargazers on that planet treasure star charts handed down from earthly decades and centuries earlier during the 365 year long orbit and calculate how long wit will be until eachs uch set of stars is visible again.

So that might give the natives of he planet a reason to have a calendar period equal to 365 Earth years, or one orbital period of the planet, even without dramatic seasons.

Part One: A Multiple star.

One possibility of having a planet with with an orbital period of 365 Earth years, would be to have the planet orbit around a group of stars instead of a single star.

For a circumbinary planet, orbital stability is guaranteed only if the planet's distance from the stars is significantly greater than star-to-star distance.

The minimum stable star-to-circumbinary-planet separation is about 2–4 times the binary star separation, or orbital period about 3–8 times the binary period. The innermost planets in all the Kepler circumbinary systems have been found orbiting close to this radius. The planets have semi-major axes that lie between 1.09 and 1.46 times this critical radius. The reason could be that migration might become inefficient near the critical radius, leaving planets just outside this radius.

Part Two: Tidal heating of an exomoon.

This part was added the 11-19-2022.

Theoretical studies of the possibiity of habitable exomoons of giant exoplanets show that the extent of tidal heating on those exomoons depends on their distance from their planets, among other factors. As a rule the closer to the planet, the greater the tidal heating on the moon.

If the moon orbits too close to the planet, the tidal heating will be enough to initate a runaway greenhouse effect, turning all the surface water into atmospheric water vapor. If the moon orbits even closer, the tidal heating will cause excesse vulcanism on the moon, making it a volcanic hell like Io.

There would still be a problem with the moon getting enough light from the star for photosynthesis on the surface of the moon for plants to produce an oxygen atmosphere.

Fortunately, a small increase in the mass of a star will cause a greater increase in the luminosity of the star. Thus the correct distance to receive a specific amount of radiation from the star will increase more than the star's mass and graitational pull on a planet. So the more massive and luminous the star is, the longer the orbital period of a planet receiving exactly as much radiation from the star as the Earth will be.

With years 365 Earth years long, the astronomical seasons would each be 91.25 Earth years long. And some of the comments express fear that such long hot summers and long cold winters would be deadly for life on the planet, or in this case, exomoon.

The way to handle this is to give the planet a vert low axial tilt, so that the inclination of incoming solar rays is almost exactly the same in every season of the year.

And in the case of a giant habitable exomoon orbiting around a giant planet and getting most of its heat from tidal heating, the astronomical seasons wuld not cause climatological seasons on the Moon. The temperatures would be quite constant all year.

So why would the people on a planet or moon with such even and unnoticeable seasons bother with counting orbital periods around the star and making them the years that their calendars are based on?

Because of the changing night time stars. At any given moment one side of a planet or a planet's moon will be facing toward the star of the system, and the atmosphere on that will scatter the bright star light and making the sky appear opaque and hide the stars. And on the opposite side of the planet from the star, the atmosphere will have light from it to scatter, and the sky will be transparent, and the stars will be seen. The stars which are in the opposite direction to the star in that system.

On Earth, as the Earth slowly orbits the Sun, the direction opposite to the slowly changes, and so the stars which are visible at night slowly change over the year.

And so the stars visible at night on that planet will slowly change over the course of a 365 Earth year orbit. So possibly the stargazers on that planet treasure star charts handed down from earthly decades and centuries earlier during the 365 year long orbit and calculate how long wit will be until eachs uch set of stars is visible again.

So that might give the natives of the planet a reason to have a calendar period equal to 365 Earth years, or one orbital period of the planet, even without dramatic seasons.

Two to four times one hundred twenty five million kilometers would be two hundred fifty billion to five hundred million kilometers.

So the giant planet with the habitable moon could orbit the star at the corret distance to give it an orbital period of 365 Earth years base don the mass of the star (or stars).

If only there was some other potential sources of heat for giant moon orbiting a giant planet.

Fortunately, a small increase in the mass of a star will cause a greater increase in the luminosity of the star. Thus the correct distance to receive a specific amount of radiation from the star will increase more than the star's mass and graitaitnal pull on a planet. So the more massive and luminous the star is, the longer theorbital period of a planet receiving exactly as much radiation from the star as the Earth will be.

Using the most massive and short lived star that I dare two, onteh more massive end of spectral class F stars, I found that a planet at the Earth Equivalent Distance from such a star would have an orbital period several Earth years long. I also found that putting two stars at the center of the star system would inrease Earth Equivalnet distance, but it would increase the combined stellar gravity more and would make the orbital period at the Equivalent Distance shorter than for a single star of that mass.

Two to four times one hundred twenty five million kilometers would be two hundred fifty million to five hundred million kilometers.

So the giant planet with the habitable moon could orbit the star at the corret distance to give it an orbital period of 365 Earth years based on the mass of the star (or stars).

If only there was some other potential sources of heat for a giant moon orbiting a giant planet.

Fortunately, a small increase in the mass of a star will cause a greater increase in the luminosity of the star. Thus the correct distance to receive a specific amount of radiation from the star will increase more than the star's mass and graitational pull on a planet. So the more massive and luminous the star is, the longer the orbital period of a planet receiving exactly as much radiation from the star as the Earth will be.

Using the most massive and short lived star that I dare to, the more massive end of spectral class F stars, I found that a planet at the Earth Equivalent Distance from such a star would have an orbital period several Earth years long. I also found that putting two stars at the center of the star system would inrease Earth Equivalent distance, but it would increase the combined stellar gravity more and would make the orbital period at the Equivalent Distance shorter than for a single star of that mass.

One possibility of having a planet with with an orital period of 365 Earth years, would be to have the planet orbit around a group of stars instead of a single star.

So that might give the natives of he planet a reason to have a calendar period equal to 365 EArth years, or one orbital period of the planet, even without dramatic seasons.

Part One: A Multiple star.

One possibility of having a planet with with an orital period of 365 Earth years, would be to have the planet orbit around a group of stars instead of a single star.

So that might give the natives of he planet a reason to have a calendar period equal to 365 Earth years, or one orbital period of the planet, even without dramatic seasons.