Can early astronomers determine the gravity of their planet's "moon" without ever going there?

As I was thinking about space flight for my world, I thought about how they would need to know a planet's gravitational force before they could land, and found that humans figured out the moon's gravity using its orbital period, tides, etc.

However...the situation that native astronomers find themselves in is an interesting one, as the "moon" in question is actually the home planet's binary twin. Since the two planets are similar in mass, both planets are tidally locked to each other which means that, from a stationary point on land, the moon would not move in the sky and there would be no tides, thus our human methods are useless.

Would it be possible for pre-space age astronomers to figure out the "moon's" gravity?

In my mind, a really easy way would be to send a satellite to the planet and find how long it takes to orbit at a distance, but this question assumes no such technology is possible.

Also, if anything here is important, the data of the planets are as follows:

Home Planet:

• ~0.7x Earth mass
• ~0.9x Earth radius & gravity
• **density of both planets are the same as Earth

Moon Planet:

• ~1.84x Earth mass (technically this makes Home planet the moon, but unimportant)

• ~1.26x Earth radius & gravity

• Distance between planets is about 129,000km

• Day length (both planets) is 79.9 hours

posted over 5 years ago

Foosic17‭

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