The Moon is slowly receding from Earth. This gradual separation means that millions of years ago, during the Age of Dinosaurs (the Mesozoic Era, 252 to 66 million years ago), our satellite was significantly closer. Scientists can estimate this paleodistance and the resulting effects on the Earth-Moon system.
The Physics Driving Lunar Recession
The mechanism responsible for the Moon’s increasing distance is a constant exchange of energy between Earth and its satellite. This interaction begins with the gravitational pull of the Moon, which raises massive bulges of water on the Earth’s oceans.
Because the Earth rotates much faster than the Moon orbits, the planet’s spin drags these tidal bulges slightly ahead of the Moon’s direct gravitational line of sight. This misalignment means the Moon’s gravity exerts a backward pull on the bulges, which acts as a brake, slowing the Earth’s rotation. This braking effect is known as tidal friction.
According to the conservation of angular momentum, the total rotational energy of the Earth-Moon system must remain constant. As the Earth loses rotational angular momentum due to this braking effect, that momentum is transferred to the Moon’s orbit. This transfer of energy forces the Moon into a slightly higher orbit.
A higher orbit requires the Moon to move at a slower speed, causing it to spiral outward at a current rate of about 3.8 centimeters per year. This process ensures that as long as the Earth continues to rotate faster than the Moon orbits, the Earth’s day will continue to lengthen, and the Moon will continue its outward migration.
Measuring Paleodistance Through Geological Records
To determine the Moon’s distance during the Mesozoic Era, scientists use a specialized rock record called tidal rhythmites. These layered sedimentary rocks preserve cyclical patterns of ancient tidal and rotational forces. By analyzing the thickness and composition of these layers, researchers can count the number of days that occurred in an ancient month and year.
A detailed study focused on the fossilized shells of the extinct rudist bivalve Torreites sanchezi, which lived during the Late Cretaceous period, approximately 70 million years ago. This mollusk grew daily rings, much like a tree, allowing scientists to use cyclostratigraphy to count these layers precisely.
The analysis revealed that 70 million years ago, a year contained 372 days, compared to our current 365. Since the length of Earth’s orbit around the Sun has remained virtually unchanged, this finding indicates that the Earth was spinning faster. Calculations based on this faster rotation show that a day during the Late Cretaceous lasted approximately 23.5 hours.
Applying the principles of orbital mechanics to this 23.5-hour day allows researchers to estimate the Moon’s distance. The Moon was roughly 1% closer than its present average distance of 384,400 kilometers during the time the largest dinosaurs walked the Earth. This places the Moon’s orbit at about 380,556 kilometers from Earth.
Ancient Earth Effects of a Closer Moon
The primary consequence of the closer lunar proximity was the faster rotation of the Earth, resulting in a day that was half an hour shorter. This meant that the daily cycle of light and dark was compressed for every organism living in the Mesozoic.
A closer Moon also meant more powerful ocean tides due to the relationship between distance and tidal force. The force responsible for generating tides increases exponentially, following an inverse cube law relative to the distance between the bodies. A Moon that was 1% closer would have exerted a tidal force over 3% stronger than it does today.
While a 3% increase may seem small, this baseline force would have been amplified in coastal areas. The shape of ancient coastlines and shallow seas would have funneled this stronger force, leading to higher and more powerful tides in many regions. These extreme tidal ranges scoured coastlines, influenced the deposition of sediments, and created dynamic intertidal zones.