I am working on a world where, instead of 24 hours, a day lasts 9 years. On this world the land far exceeds the oceans, to the point where the bodies of water are simply massive lakes (a la the Caspian Sea). The inhabitants of the world are forced to endlessly migrate across the world as a result of this.

Other interesting features of this world are it's two small moons and its large ring going around the equator. Assume that all else in this world is Earth-like, such as it's AU, size and gravity.

On a world with a 9 year day, what temperatures could I expect to find at various longitudes and latitudes, regardless of environmental pressures?

  • $\begingroup$ By AU do you mean Astronomical Unit? $\endgroup$
    – sphennings
    May 30, 2017 at 14:50
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    $\begingroup$ You may find this helpful in tuning your question. "Because of Mercury's unusually eccentric orbit, in which it ranges between 29 and 43 million miles (46-70 million km) from the Sun, the tidal lock between it and the Sun takes the form of a 3:2 resonance. It rotates on its axis three times for every two times that it orbits the Sun, completing one rotation in about 59 days, and one orbit of the Sun in about 88 days. The 1:1 resonance is much more likely to occur between bodies in a near-circular orbit, like the Earth and Moon, or Pluto and Charon, who have tidally locked each other." $\endgroup$ May 30, 2017 at 14:51
  • $\begingroup$ Meaning there may be some constraints demonstrated and some idea of the temperature distribution. What wont be there is the atmospheric effect of distributing the temperature. What may be helpful there is to look at the debate on 1:1 resonance planets Hot side vs Cold side with a Terminus zone VS winds that circulate the heat and cold around the planet. $\endgroup$ May 30, 2017 at 14:54
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    $\begingroup$ "The inhabitants of the world are forced to endlessly migrate across the world as a result of this." - Maybe they should consider living in the poles in constant temperate twilight instead ? Your idea is more interesting but you'll have to cover this plothole somehow. $\endgroup$
    – Keelhaul
    May 30, 2017 at 14:55
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    $\begingroup$ AU is a unit of distance not a property of a planet. $\endgroup$
    – sphennings
    May 30, 2017 at 15:06

2 Answers 2


With an Earth-like atmosphere and distance from the sun, the planet would most likely have an "average" surface temperature similar to Earth's, but with worse extremes.

On the equator

The light side would be locked into a Sahara desert temperature (40 C), but even hotter because there's no night-time to cool off. The dark side would be locked into something like a Gobi desert temperature (-40 C), but even colder because there's no day-time to warm up.

Getting closer to the poles

The temperatures would be more moderate -- any range is possible, based on distance from the equator. Temperature would be pretty constant because of the lack of daytime/nighttime.

On the poles

Fortunately for us, Earth's poles already almost have this condition north of the Arctic Circle. The sun never goes completely down for several months of the summer, and never comes down for several months of the winter. For comparison of a habitable area near the poles, we can look to the city of Barrow, Alaska, where temperatures range from 26 C in the summer to -50 C in the winter.

What does this mean for inhabitants?

Similar to the suggestion from this answer, someplace near the poles would likely be the most habitable place to live for human-like beings. Being near the poles would also make the distance you need to migrate each year much shorter. Staying on the daytime side near the latitude of Barrow, Alaska (about 70 degrees) would yield very pleasant temperatures (Barrow's summer temperatures are around 25 C) and would mean you'd only have to travel around 1000 miles per year to keep up with the sun.

Further research

As Enigma Maitreya mentioned in the comments, this condition would be somewhat similar to the orbit of Mercury and Venus. However, Mercury and Venus are both much closer to the Sun, and Mercury has no atmosphere, while Venus has an atmosphere 100x more dense than Earth's.

Mercury's temperature gets as hot as 425 C in the daytime & summer, and as cold as -175 C in the nighttime & winter. By comparison, Venus is 2x further away from the sun, and therefore receives only 25% of the solar irradiance of Mercury, yet its heavy atmosphere makes it have an average surface temperature of around 435 C. (I'm taking all this information from Wikipedia.)

It's difficult to translate this exactly to an Earth-like planet that has years-long days, but it could be done if you are sufficiently math-inclined. You'd need to look up formulas for the magnitude of the greenhouse effect and for the temperature caused by solar irradiance and apply those to the differences between Earth and the comparison planets to obtain a reasonable guess.

  • $\begingroup$ I would have hypothesized that temperatures on the sunny side near the equator would soar high above your estimate of a somewhat-uncomfortable 40 C. It's close to solar noon for almost a year, meaning areas near the equator will be absolutely baking. Temperatures in the desert don't peak just because, things only cool off because the sun goes down. $\endgroup$ May 31, 2017 at 15:20
  • $\begingroup$ Right, it's definitely going to be hotter than 40 C, hence why I said "even hotter because there's no night-time to cool off." I just hesitated to give an exact number of how much hotter it'd be, which hopefully isn't too misleading. $\endgroup$
    – Jan
    May 31, 2017 at 15:34

How hot/cold would it get?

The extremes in seasonal temperatures on our planet are caused by relatively minor differences in solar exposure times between summer and winter. With exposures as long as 9 years, expect much greater extremes: both an uninhabitably cold night side and an uninhabitably hot day side.

Mercury serves as a good baseline because it is tidally locked but has no atmosphere. You should then adjust your numbers upward to account for the warming of an atmospheric greenhouse effect. Mercury has a surface temperature of 700 K (800 °F/427 °C) on the sunny side and 100 K (-280 °F/-127 °C) on the dark side.

Your absence of oceans will exaggerate the temperature difference between day and night that might otherwise be mildly offset by transferring heat through ocean currents (thermohaline circulation). Furthermore, larger land masses allow a planet's surface to get hotter more quickly, and increase thermal reflection, which is why the northern hemisphere of earth (which has more land masses) is hotter on average during its summer than the southern hemisphere is during its.

Fewer liquid oceans will probably correlate to lower atmospheric humidity in general, suggesting a diminished greenhouse effect such that the cold side will be exceptionally cold (colder than the coldest surface temperatures on Earth). For reference, the coldest temperature recorded on Earth was −94.7 °C (−138.5 °F).

You can safely assume there'll be an absence of liquid water on the surface of both the day and night side, but with the potential for liquid water around the twilight areas.

How would the temperature and rotation affect the climate?

A recent blog post on the Universe Factory guesses that a tidally locked planet would transfer heat from the day side to the night side via planet-wide hurricanes.

I don't entirely agree with that assessment; the center of the baked, low-pressure, zero humidity desert on the daytime side could become a meteorological "dead zone" with little wind and no capacity to support condensation or precipitation. Further, a tidally locked planet does not rotate and thus could not produce the Coriolis effect, so the Earthly process of cyclogenesis would not occur.

However, the twilight areas of the planet would see an exchange of hot and cold air from the day/night sides through the process of convection, and from the interactions of cold high-pressure systems with the warm low-pressure areas.

convection/precipitation diagram

Now, your hypothetical planet is not tidally locked, so it would have a weak Coriolis effect, leading to a disparity between the weather systems at the equatorial twilight and the weather systems at the polar twilights.

Cyclones would form along the twilight areas and generally migrate toward the poles, although the lack of oceans on your planet could slow their advance; terrain features like mountains could also stop or divert the advancing winds.

enter image description here

Assuming no significant terrestrial features prevent them from doing so, the two major streams of air to the poles coming from opposite sides of the hemisphere could combine to influence the convection at the poles to generate monstrous hurricanes there.

More salient details about the weather patterns will depend on your geology and terrain.


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