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Picture a tidally locked planet that is also the closest planet to the star (~0,17 AU from a star with a luminosity of ~1,5 times that of the Sun).

The side of the planet that faces the star would surely be a scorched wasteland, not sure about the dark side of the planet though, I've thought of the possibility that the dark side of the planet is host to frozen, icy reliefs, but haven't found info to sustain whether this would be plausible or not.

In any case the frozen side, even if plausible, would have little to no light to reflect since it would always point outwards of the system. This leaves us with nothing but one scorched, half surface to reflect light towards the civilisation observing the stars over at the ringed planet ~1,24 AU away from the star. Now, the rings are low albedo, but still make an impact on the visibility of the night sky (an impact I'm happy to dim considerably if needed).

So the question is: How advanced need the civilisation be in order to notice their warmest planetary neighbour is tidally locked to the star?

Are we talking huge observatories? Very developed telescopes? Simple telescopes? Maybe even bare eye because of some reason that escapes my understanding of it all?

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If we can use the history of observation of Mercury as reference, they would need something akin to the tech level of our 1800

The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639, Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.

The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800, Johann Schröter made observations of surface features, claiming to have observed 20-kilometre-high (12 mi) mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.

If they are able to spot some surface feature, like reliefs at the terminator, and use it to determine the rotation of the planet, they might be able to infer the tidal locking.

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  • $\begingroup$ According to THIS nineplanets.org/mercury They didn't accurately work out/prove the 3:2 spin state of Mercury until 1965 using radar, although they might have been able to earlier if it had been really important. $\endgroup$
    – DWKraus
    Jun 28 at 17:34
  • $\begingroup$ I was also thinking about the surface detail piece. Astronomers were able to see Jupiters Great Red Spot in 1665 and that is a surface detail. In a tidally locked planet the terminator and a given surface feature will always be the same distance apart. $\endgroup$
    – Willk
    Jun 28 at 21:59
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The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800, Johann Schröter made observations of surface features, claiming to have observed 20-kilometre-high (12 mi) mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.[164] In the 1880s, Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury's rotational period was 88 days, the same as its orbital period due to tidal locking.[165] This phenomenon is known as synchronous rotation. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations.[90] Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map.[166]

In June 1962, Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences, led by Vladimir Kotelnikov, became the first to bounce a radar signal off Mercury and receive it, starting radar observations of the planet.[167][168][169] Three years later, radar observations by Americans Gordon H. Pettengill and Rolf B. Dyce, using the 300-meter Arecibo radio telescope in Puerto Rico, showed conclusively that the planet's rotational period was about 59 days.[170][171] The theory that Mercury's rotation was synchronous had become widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.[172]

Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.[173] Data from Mariner 10 subsequently confirmed this view.[174] This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.[164]

So if Mercury actually was tidally locked in a 1:1 ratio, it would have been discoverable with 1880s and later instruments. Astronomers were only fooled into thinking that Mercury was tidally locked with a 1:1 ratio because of unusual circumstantes which fooled them.

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