In Our Time: Eclipses

BBC Radio 4
Host: Melvyn Bragg
Guests: Lucy Green (Professor of Physics, UCL/Mullard Space Science Laboratory), Frank Close (Emeritus Professor of Physics, Oxford), Carolyn Crawford (Public Astronomer, Institute of Astronomy; Fellow, Emmanuel College, Cambridge)
First Aired: December 18, 2025 (Archive from December 2020)

Episode Overview

This episode of "In Our Time" explores the phenomenon of eclipses—particularly solar and lunar eclipses—through the lenses of science, history, culture, and personal experience. Melvyn Bragg and his panel of expert physicists and astronomers dive into what causes eclipses, their impact on scientific discovery, and how witnessing them has inspired generations of both scientists and poets.
Listeners are treated to memorable firsthand eclipse accounts, discussion of their role in historic events, and the scientific frontiers still being explored through these celestial events.

Key Discussion Points & Insights

1. What Are Eclipses? (03:44)

• Solar Eclipse: Occurs when the Moon moves directly between the Sun and Earth, casting its shadow on Earth. Requires precise alignment; path of totality covers less than 1% of Earth.

  • Quote: “If you're a lucky observer standing within that shadow, the moon appears to move directly in front of the Sun. … That’s when you get the total solar eclipse.”
    —Carolyn Crawford (03:44)

• Lunar Eclipse: Happens when the Earth casts a shadow on the Moon during a full moon. More widely visible and lasts longer.

  • “Everybody on the nighttime side of the Earth gets to see the eclipse in progress.”
    —Carolyn Crawford (06:17)

• Frequency & Alignment:

  • Despite the monthly new moon, the different orbital tilts mean total solar eclipses only occur about once every 1.5 years somewhere on Earth, but rarely in any one spot.
  • The Moon’s shadow races across Earth at over 1,000 mph.

2. The Coincidence of Size and Distance (07:15)

• Unique Alignment: The Sun and Moon appear the same size from Earth because the Moon is 400 times smaller yet 400 times closer than the Sun.

  • “We happen to live at a very fortunate time in history that this coincidence happens. … There will come a time somewhere in the future that there will be no more total solar eclipses.”
    —Frank Close (07:54)

• Future Changes: The Moon is moving away from Earth (rate of fingernail growth). One day, total eclipses will cease to exist.

3. The Awe of Experiencing a Total Eclipse (08:43)

• Firsthand Reactions: All three guests recount the profound, almost mystical experience of witnessing a total solar eclipse.

  • “The difference between a 99.9% partial eclipse and a total eclipse... it’s just like being outside Covent Garden when the opera is taking place and being in a box seat at the front. They’re totally different experiences.”
    —Frank Close (08:53)

  • “It's more than just the spectacle … it really is this full body experience. … The temperature drops, a wind will pick up, you get the eerie sounds, it goes quiet and the birds and the animals are confused...”
    —Carolyn Crawford (12:15)

  • “The eclipse experience … looked to me like a gun had shot a bullet through the sky. And it was this small black dot where the moon was…”
    —Lucy Green (13:00)

• The Diamond Ring Effect & Shadow Bands:

  • Descriptions of Bailey’s Beads and the diamond ring effect—last bursts of sunlight through lunar valleys.
  • “We saw shadow bands, sort of shimmering effects on the ground…” —Lucy Green (13:40)

4. Historical Perceptions & Early Records (16:48)

• Ancient China & Babylon: Earliest eclipse records date back to 2000 BCE in China; often seen as omens of doom or divine messages.

  • “There are tales from different cultures of it being eaten by a dragon… The people's duty [was] to bang on their pots… to scare the beast away.”
    —Carolyn Crawford (16:56)

• Greece and the Move Toward Science:

  • Greeks started interpreting eclipses scientifically—Aristotle used lunar eclipse shadows to deduce Earth’s spherical shape; Aristarchus used eclipse timing to estimate the size and distance of the moon.

5. Eclipses and Historical Events (19:25)

• Dating Ancient Events: Eclipse records used to date events like Abraham's 'great darkness' (Genesis) and the darkness at the crucifixion.

  • “The crucifixion took place at Passover… at full moon, you can have a lunar eclipse, but not a solar eclipse. … There was a total lunar eclipse on Friday 3rd April, 33 AD…”
    —Frank Close (19:31)

6. The Renaissance, Enlightenment, and Citizen Science (21:21)

• Advances via Kepler, Newton, and Halley: New understanding of orbital motion via Kepler’s Laws and Newton’s gravity allowed prediction of eclipses.
• Halley’s Eclipse of 1715:
  • Halley created Britain’s first citizen science project, asking the public to record the path and timings of the eclipse, enabling calculation of the eclipse shadow’s speed.
  • “Halley made an accurate prediction… and gathered data from observers along the path of totality.”
    —Lucy Green (21:21)
  • “He’s entertainingly disparaging about the professor of Astronomy at Cambridge… who missed both the time of the beginning of the eclipse and that of total darkness!”
    —Carolyn Crawford (23:30)

7. Discovery of Helium via Eclipse (24:25)

• Spectroscopy Advances: During the 1868 total solar eclipse in India, French astronomer Jules Janssen and English astronomer Norman Lockyer independently discovered a spectral line corresponding to a new element—helium—first found in the Sun before it was found on Earth.
  • “It’s amazing that we only see it through a total solar eclipse. … It wasn’t observed in the lab until 1890.”
    —Carolyn Crawford (24:30)

8. Eclipses Confirming Einstein’s Relativity (28:43)

• Einstein’s 1919 Test:
  • During a total eclipse, Arthur Eddington’s team measured the bending of starlight by the Sun’s gravity, confirming general relativity’s predictions (double the effect expected from Newtonian gravity).
  • “New York Times had this amazing headline, ‘lights all askew in the heavens’. And Einstein was an overnight celebrity.”
    —Frank Close (28:48)

9. Eclipses Across the Solar System & in Binary Stars (30:29)

• Different Worlds, Different Moons:
  • Mercury and Venus lack moons; Mars’ small moons create mini-eclipses visible to rover cameras.
  • Jupiter’s moons can eclipse the S un—one of the best places for an alien to witness a total eclipse.
  • Eclipsing binary star systems reveal sizes, orbits, and masses thanks to periodic light dips.

10. Eclipses and Navigation (33:58)

• Longitude Measurement:
  • By comparing local eclipse times across continents (Greenwich and Virginia), early explorers could determine longitude, estimating ocean widths.
  • Miscalculations in antiquity led Columbus to believe Asia was closer than reality—one reason he risked sailing west.

11. Modern Eclipse Science: Satellites vs. Earth (36:41)

• Ground Observations Still Needed:
  • While coronagraphs and satellites provide near-constant solar monitoring, only real eclipses allow study of the lower corona, where solar wind and eruptions originate.
  • “When we have artificial solar eclipses, our disks have to be bigger… so we lose precious information about that lower corona.”
    —Lucy Green (36:41)
  • Earth-based observations offer more data flexibility, adaptability, and scientific richness.

12. Eclipse-Chasing, Community, and the Uniqueness of Each Event (39:26)

• Personal Reflections:

  • Eclipse chasing becomes a lifelong, sometimes expensive passion.
  • The exact experience is different each time—sky location, terrain, cloud, and light all change.
  • “What you can’t predict is whether the weather will allow you to see it.”
    —Frank Close (39:37)

• Still Scientific Mysteries:

  • Why is the Sun’s corona so hot? How do the Sun’s magnetic fields and plasma interact? Many questions remain best tackled during total eclipses.

• “These eclipses do make poets out of scientists. … I started as a scientist because of an eclipse a long time ago.”
  —Frank Close (43:55)

Memorable Quotes & Moments

• “Where the sun was, there's been a black hole. … It's utterly weird. … The ancient Chinese were convinced that a dragon was eating the sun. … It is totally awe inspiring.”
  —Frank Close (09:46)

• “My favourite [tool] is a colander… you can make a pinhole camera many times over and you can see the eclipsed sun.”
  —Lucy Green (14:31)

• “You have the sense of the moon's shadow rushing towards you at phenomenal speed. … The hairs on the back of your neck go up.”
  —Carolyn Crawford (12:15)

• “Somebody once said it makes poets of scientists and scientists of poets.”
  —Frank Close (12:47)

• “Halley really wanted the public to be informed about the eclipse as a scientifically understood and natural event, rather than an event to be taken by surprise and to be superstitious about.”
  —Lucy Green (23:56)

• “One day, total solar eclipses will no longer occur.”
  —Frank Close (07:54)

• “The decision that we took way back in Cornwall was: where's the next one? This must be a great way of visiting places on the earth that you would never otherwise go to, with the eclipse as the icing on the cake.”
  —Frank Close (50:46)

Notable Timestamps for Important Segments

• 03:44: Defining Solar & Lunar Eclipses
• 07:15: The Unique Coincidence of Sun & Moon Size/Distance
• 08:53: What It’s Like to Witness a Total Solar Eclipse
• 16:56: Ancient Records and Cultural Reactions
• 19:31: Eclipses in Historical Context (Crucifixion, Abraham, etc)
• 21:21: Kepler, Newton, Halley, and the Advancement of Prediction
• 24:30: The Discovery of Helium via Solar Eclipse
• 28:48: Eclipses and Einstein’s General Relativity
• 30:38: Eclipses Beyond Earth—Mars, Jupiter, Binary Stars
• 33:58: Using Eclipses for Navigation (Longitude)
• 36:41: Value of Ground v. Satellite-based Solar Observations
• 39:37: The Personal Allure and Scientific Promise of Eclipses
• 43:55: How an Eclipse Sparked a Career in Science (Frank Close)
• 45:51: Bonus: Mr. Laxton and Intergenerational Eclipse-Watching

Bonus Material Highlights (45:44+)

• Eclipse tourism & the “club”: Enthusiasts travel worldwide, sometimes to remote, exotic places, risking cloud cover and unpredictable weather for the fleeting magic of totality.
• Famous scientists’ adventures: Jules Janssen's dramatic escape by balloon from besieged Paris to see an eclipse.
• Shadow bands: Ephemeral light/shadow effects observed just before totality (“…the road was moving, the road was moving”—Frank’s grandson, 49:16).
• The special 2021 Antarctic eclipse: A rare east-to-west shadow track due to Earth’s tilt.

Final Thoughts

The discussion concludes with the panel reflecting on the unique beauty and continued scientific relevance of eclipses. Every eclipse is different, unpredictable, and awe-inspiring—reminding us of both the grandeur and the clockwork predictability of the cosmos.
Frank Close credits an early eclipse with sparking his lifelong love of science, and the panel highlights the mix of unpredictability, wonder, and discovery that continues to draw scientists and enthusiasts alike to chase the shadow around the globe.

For listeners: This episode is a journey across time, space, and human experience, illustrating why eclipses remain among the most stirring and informative of natural phenomena.