# Will Lunar Months be the same length Millions of Years in the Future?

If I remember correctly, the amount of time it takes for the moon to rotate around Earth is getting longer over time. Is this true, and if so or the opposite is true and a lunar month is getting shorter, is there a way to estimate how long a lunar month will be in the far future?

Doing a worldbuilding project that takes place 53 million years in the future, and trying to figure out how many months they'd put in a year.

Edit: Specifically trying to figure out the length of the synodic month, not the sidereal month.

• astronomy.stackexchange.com/questions/49039/… This question is relevant. It gives that in just over 70 million years, the rotation of the Earth will have slowed to 360 days in a year, and at the same time the orbital time of the moon will have increased by about 8 hours. So in 40 million years you might get 4 to 5 hour increase in the length of the month. Commented Oct 15, 2023 at 21:49
• Curious, do you mean the length of a synodic or sidereal month? Would the increase in time be about the same amount of hours? Commented Oct 15, 2023 at 23:03
• *same amount of hours for each type of month? Commented Oct 15, 2023 at 23:14
• I think that is about sidereal but the synodic month willy increase by the same amount, more or less, since the earth orbit about the sun isn't going to change substantially over such a short time. Commented Oct 16, 2023 at 5:22
• Nice question! (+1). I think that since the Moon is receding away from us at a rate of 3.8 cm per year, it would be continuously increasing as gravity is an inverse square force field, so accounting for that increasing rater, by inserting the rate into Kepler's 3rd law of planetary motion along with masses of the primary and secondary body, we could get to know the sidereal month, which would give a very close answer to the actual synodic month as @JamesK suggested. Perhaps you could account for the milankovitch eccentricity cycles to precisely know the change in Earth's radius vector. Thanks! Commented Oct 16, 2023 at 16:55