Our Rare Moment Under the Sun

Avi Loeb
4 min readApr 8, 2024
The moment of 93.1% eclipse of the Sun by the Moon at the roof of the Harvard College Observatory on April 8, 2024.

Experiences that inspire awe connect people to one another and to entities that are larger than them. A burning bush that was never consumed by its flames, revealed God to Moses and motivated him to lead the Israelites out of slavery. This theme extends well beyond religious experiences.

Consider the total solar eclipse across North America today, April 8, 2024. My students and my daughters were thrilled to watch together the 93.1% solar eclipse (which coincided with the timing my class) from the roof of the Harvard College Observatory. The awe they felt stems from the rare circumstances that led to this precious moment. Let us recap these circumstances.

First, the Moon was born as a result of the collision of a Mars-size planet, named Theia, with Earth, about 4.466 billion years ago, merely 105 million years after the birth of the Solar system.

Life started on Earth about a hundred million years later. Back then, the Earth was spinning fast and the day-night cycle was rapid. Through its tide, the Moon robbed Earth’s spin and its orbit expanded to larger distances.

Evolution of the duration of a day on Earth (in hours on the vertical axis) as a function of lookback time in billions of years. (Credit: M. Farhat et al. 2022)

The spin of the Earth blocks the Sun every night but the Moon does it rarely for a good reason. If the Moon’s orbital plane around the Earth would have been aligned with that of Earth around the Sun, solar eclipse would have occurred once per month every new Moon. But because of the 5 degrees tilt between the two planes, the Moon’s shadow often misses Earth and solar eclipse occurs only 2–5 each year.

The Moon started closer to Earth and appeared much larger on the sky than the Sun. Hundreds of millions of years ago, every solar eclipse was total or partial with no annular eclipses witnessed by the dinosaurs. In a billion years, the Moon will be too far away and the Sun will get bigger, so no total eclipses will be possible.

Evolution of the Moon-Earth separation in units of the Earth radius as a function of lookback time in billions of years. (Credit: M. Farhat et al. 2022)

Putting this all together, we should be grateful for the coincidences that enable total solar eclipses in our lifetime at this time. We live at a special time when locally — the Moon and the Sun occupy similar sizes on our sky, and cosmically — dark energy started dominating the mass budget of the Universe. Inhabitants of other planets throughout cosmic history may have not been as lucky.

For the first time since the Moon formed, NASA’s Artemis Program is contemplating establishing a sustainable human base on its surface. Last week, a memo from the Office of Science and Technology Policy at the White House had tasked NASA to create a new time zone for the Moon by the end of 2026. The new lunar standard will be called “Coordinated Lunar Time (LTC).”

Because the lunar gravitational potential is lower than Earth’s, lunar time progresses slightly faster according to Albert Einstein’s General Relativity. In other words, people who settle on the Moon will age faster than their terrestrial counterparts. The gravitational time dilation on the Earth’s surface amounts to time ticking slower relative to the Moon by 2.14 seconds per century or 58.7 microseconds every terrestrial day. This does not constitute a significant change in human lifespan, but the small difference could be important for communication between future missions and mission control as it attempts to accurately track satellite and crew positions.

Session 5 of the TED2024 conference on April 17, 2024.

Gladly, today’s solar eclipse did not obscure my morning jog at sunrise, nor did it dim the light shed on my research by a Business Insider profile earlier in the day. Also today, I was notified that my “Session 5” at the TED2024 Annual Conference on April 17, 2024 in Vancouver, Canada will include Bill Ackman in conversation with Alison Taylor, Bari Weiss, Scott Galloway, Andrew Yang and the artist XIUHTEZCATL. The eclipse experience with my students and two daughters, the Business Insider profile, and the upcoming TED2024 Session, intensified my gratitude for the precious time that we all spend together through the rare circumstances that this amazing Universe enables around us.


Image credit: Chris Michel (October 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".