Why is Earth's day 24 hours long (and how did the sun keep it from being longer)?

[Admin]

Thermal tides in Earth's atmosphere have counterbalanced gravitational tides from the moon, bringing a pause to the slowing down of Earth's rotation.

Why is Earth's day 24 hours long (and how did the sun keep it from being longer)? : Read more

 

[rod]

The space.com article states. "The length of Earth's day is only 24 hours, rather than over 60 hours, thanks to a temporary balance between the gravitational tidal forces from the moon and the sun. When the moon formed about 4.5 billion years ago, probably from a giant impact, it was much closer to Earth than it is now and our planet was spinning much faster, with a day length of less than 10 hours. Since then, the moon has been gradually moving outward, stealing some of Earth's angular momentum, with the result that Earth's rotation has slowed. Today, as we all know, a day on Earth lasts 24 hours. However, at the rate at which the moon is moving away from us — measured to be 1.49 inches (3.78 centimeters) per year by experiments using laser reflectors left on the moon by Apollo astronauts — our planet should have slowed to the point of having 60-hour-long days. So what slowed the slowing down?"

That is very interesting Using the Moon orbit at 6 earth radii distance (not a mean near 60.3 today), the lunar month is < 21 hour orbital period around the Earth, not to mention the very large angular size in the sky we would see While simulations can be run calculating such physical changes in the Earth-Moon system using a 4.5 Gyr time period, demonstrating that the Earth-Moon system existed like this in nature is more challenging to show. Various giant impact models for the origin of the Moon present various parameter differences too like the original rotation period for the proto-earth before Theia slams into the proto-earth to create the Moon. Was it a very slow rotation and then after impact, rapid rotation spun up? Some reports I have in my home database suggest the proto-earth after Theia created the Moon, had a 3–5-hour day too, not 10 hours.

 

[rod]

The reference paper cited is very good reading, https://www.science.org/doi/10.1126/sciadv.add2499, Why the day is 24 hours long: The history of Earth’s atmospheric thermal tide, composition, and mean temperature

It is focused on the geologic time scale from 2.2 Gyr to 600 Myr. I note from the abstract, "...We use geologic data, a dynamical model, and a Monte Carlo sampler to find possible histories for the Earth-Moon system. In the most likely model, the lod was fixed at ≈19.5 hours between 2200 and 600 Ma ago, with sustained high T and an increase in the angular momentum LEM of the Earth-Moon system of ≈5%."

My note. I would think the *geologic data* used *must be very reliable* to calculate the Earth's LOD as 19.5-hour day for the period 2.2 Gyr to 600 Myr ago. After 600 Myr until present, Earth's Day slows down to the present 24-hour period. Extrapolating Earth's changing LOD back 4.5 Gyr, looks challenging

 

[rod]

I reviewed the 13-page PDF in the report cited, very interesting reading as I previously stated. I note this from the DISCUSSION portion near the end (page 10).

"If we integrate the initial conditions from our best-fit thermal tide model but neglect the thermal tide, we find a lod of about 65 hours at the current epoch. The period of the normal mode of Earth’s atmosphere is set primarily by R⊕ and the mean surface temperature, which combine to give Pres ≈ 9 to 11.5 hours. The data shown in Fig. 3 indicate that the lod was roughly constant over the boring billion, consistent with capture into a state in which T☾ ≈ Tth. The long duration and relatively recent occurrence of this resonant state may be responsible for the fact that the day is currently 24 hours long."

So, it seems we have a 19.5-hour LOD for much of the period 2.2 Gyr to 600 Myr, and then the Earth slows at a faster rate, and we have the 24-hour day today. Otherwise, we could have a 65-hour LOD today. Modeling Earth-Moon system and Sun changes (example, Faint Young Sun) to fit such evolutionary changes for Gyr periods - remains hard and tough, starting with the proto-earth rotation period, after giant impact with Theia said to create the Moon, and what happens after that The problem is clearly wrapped up with the entire geologic time scale used for Earth IMO.

 

[PabloMB]

Hi
I have read that Venus has an over-rotating athmosphere. Would it be the case of the young Earth during this sun-resonant age? Perhaps I'm oversimplifying, but an athmosphere helping the earth rotation is the best (and simpler) explanation I can think of.
Thanks. Regards - Pablo

 

[Pogo]

Yikes! A 20 hour workday?