The Distance Between Earth and Sun 100 Million Years Ago: An In-depth Exploration

Throughout geological time scales, the average distance between the Earth and the Sun, known as an astronomical unit (AU), has remained relatively stable. Specifically, during the Late Cretaceous period, around 100 million years ago, this distance was approximately 93 million miles or 150 million kilometers.

Stability of Earth-Sun Distance

The distance between the Earth and the Sun, which is currently defined as 93 million miles (149 million kilometers), has remained nearly constant for billions of years. This remarkable stability is attributed to the gravitational interactions and orbital dynamics, which, over such a long timescale, yield minimal changes.

One may wonder if this distance has changed over the 4.54 billion-year history of the Earth. The answer is no, it has remained consistently at 93 million miles or 149 million kilometers. In fact, the Earth-Sun distance is an unchanging average, a fundamental constant in our solar system.

Inertial Frames of Reference and the Principle of Relativity

The concept of an inertial frame of reference is crucial for understanding the dynamics of the solar system. Rotational motion introduces forces, and determining a true inertial frame involves navigating through multiple layers of gravitational influence.

First, let's consider the Earth. While sitting on its surface might seem intuitive, it is not an inertial frame due to the rotation. The Earth's center of mass, or barycenter, is also not an inertial frame as it orbits the Earth-Moon barycenter. This barycenter itself orbits the Earth-Sun barycenter, which in turn moves around the center of the Milky Way. Each of these positions is an inertial frame, yet they are all in motion.

Earth's Motion in Inertial Frames

The Earth's rotation provides a speed of approximately 0.34 km/s relative to the Earth's surface. This speed varies slightly depending on the time of day. Considering Christchurch, New Zealand, located at 43.53 degrees South, the Earth's rotation speed can be calculated as follows:

Earth's circumference 40,075.017 kilometers

Sidereal day 23.93446959 hours

V (40,075.017 km / cos(43.53°)) / 23.93446959 hours 1,215.9 km/h ≈ 0.33776 km/s

This rotational speed is relatively insignificant when compared to other velocities in the solar system, such as the Earth's orbit around the Sun and the Sun's motion within the Milky Way.

Earth's Orbit around the Sun

The Earth orbits the Sun at an average speed of approximately 29.78 km/s. This speed varies over the course of the year, but its impact is still minimal in the grand scheme of things. The Earth's orbit brings it around the Sun in a little over 87,660 hours, covering a distance of about 939.9 million kilometers. Thus, the yearly orbital speed is calculated as follows:

V 939,900,000 km / 87,660 hours 107,221 km/h ≈ 29.78 km/s

This orbital speed is consistent, year-round.

Sun's Orbit within the Milky Way

The Sun's motion within the Milky Way is even more significant. The Sun is located approximately 28,000 light years from the center of the Milky Way and takes about 230 million years to complete its orbit. This results in a characteristic motion of:

V 28,000 light years / (2π / 230 million years) 826,096 km/s ≈ 229.47 km/s

This speed is nearly 42 times the escape velocity from the Milky Way, pointing it towards the Virgo Cluster.

Understanding these inertial frames and their motions is essential to comprehending the stability of the Earth-Sun distance and the broader dynamics of our solar system and galaxy.