Summary
Overview
Michael Stevens and Hannah Fry explore why sundials don't match the time on our phones, diving deep into the equation of time, Earth's eccentric orbit, axial tilt, and how these factors create discrepancies of up to 16 minutes. They discuss solar versus civil time, the subsolar point, and fascinating phenomena like Lahaina Noon in Hawaii where shadows disappear when the sun is directly overhead.
The Sundial Problem: Why Time Doesn't Match
Sundials never quite agree with our phones or watches, sometimes running 16 minutes fast in November or 14 minutes slow in February. This discrepancy stems from the fundamental difference between solar time (measured by the sun's position) and civil time (our standardized clocks). The equation of time accounts for this variation, which is caused by Earth's eccentric orbit and axial tilt creating irregular solar days throughout the year.
- Sundials can be off by as much as 16 minutes ahead in November or 14 minutes behind in February compared to civil time
- Ancient peoples measured days from sunrise to sunset, making winter days much shorter than summer days
- Solar day (noon to noon) is less variable than sunrise to sunset, but still not constant throughout the year
- Earth moves faster around the sun in winter when it's closer, affecting the length of solar days
" What time is it really? "
" It's a wobbly rock in space, and it's not going around the sun in a perfect circle at the same speed all the time. "
Understanding Earth's Motion: Orbit and Rotation
The length of a solar day varies because Earth must rotate more than 360 degrees to point back at the sun—about 4 extra minutes on average. This happens because while Earth spins, it's also moving along its orbit around the sun. The variation in this extra rotation time is caused by two factors: Earth's elliptical orbit (moving faster when closer to the sun) and Earth's axial tilt affecting how the subsolar point moves across the planet's surface.
- Earth must rotate approximately 360 degrees plus 4 additional minutes for a complete solar day because it's also orbiting the sun
- A sidereal day (based on distant stars) is much more constant than a solar day
- Earth's eccentric elliptical orbit causes it to move faster when closer to the sun in January
- Earth's axial tilt causes the subsolar point to shift northeast or southeast rather than purely east during spring and fall
" In order for the day to end you are sort of like trying to move east to catch up with where you were the day before with this with your meridian pointing at the sun but if you have moved a little bit by the time you've turned 360 degrees the sun isn't right in front of you anymore "
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