Astronomy

• Day 7: A Day Is Not 24 Hours

  Last week we talked about what time is, what it might not be, and what your brain does to make you feel it. This week we measure.

  The obvious place to start is to look up at the sky. Humans did this for several thousand years before they realized how badly the sky was lying to them.

  The Sundial Problem

  The oldest clock is a literal stick in the figurative ground.

  Watch the shadow move. When the shadow is shortest, the sun is overhead. That’s noon.

  For a long time, this was good enough. If you wanted to coordinate a meeting in ancient Egypt, you and your buddy could both look at the sun and agree on roughly when to show up. Our Civilizations were built on this.

  Here’s the problem. If you mark where noon falls on a sundial every day for a year, and compare it to a clock that ticks steady seconds, the sundial drifts. Sometimes the sun is “late.” Sometimes it’s “early.” Over the course of a year, the gap swings by up to about sixteen and a half minutes one way (early November) and just over fourteen minutes the other (mid-February).

  This is called the equation of time, and it has two main causes.

  First, Earth’s orbit isn’t a circle. It’s an ellipse. We move faster when we’re closer to the sun (around January 3rd) and slower when we’re farther away (around July 4th). When we’re moving faster, the sun appears to drift across the sky faster, and noon comes sooner than the clock predicts.

  Second, Earth’s axis is tilted. The sun doesn’t ride along the equator, it rides along the ecliptic at a 23.5 degree angle. That tilt distorts the projection of the sun’s motion onto our daily rotation, which means the sun runs ahead of the average for parts of the year and behind it for others.

  If you graph the equation of time across a year, you get a wobbly figure-eight called the analemma. You’ve probably seen it on a globe somewhere and ignored it. It’s the actual shape of “noon” over a calendar year.

  So if you want a 24-hour clock that doesn’t drift around with the seasons, you can’t use a sundial directly. You have to average. The result is called mean solar time, the time you’d see if the sun behaved itself.

  Two Kinds of Day

  Hopefully following along so far, because now we need to talk about what a “day” is. There are two ways to define it and they disagree.

  The solar day is what you’d guess. Sun is straight overhead; rotate Earth until the sun is straight overhead again. That’s one day. About 24 hours.

  The sidereal day is what astronomers use. Pick any distant star; rotate Earth until that star is back in the same position in the sky. That’s one sidereal day.

  A sidereal day is 23 hours, 56 minutes, and 4.09 seconds. Almost exactly four minutes shorter than a solar day.

  Why? Because Earth is doing two things at once.

  While you spin on your axis, you’re also moving around the sun. By the time you finish one full rotation relative to the stars, you’ve also moved a tiny bit along your orbit. The sun has effectively shifted in the sky from your perspective. You have to rotate a tiny bit further to point at the sun again.

  That tiny bit further takes about four extra minutes. Add it up over 365 days and it equals exactly one full rotation. That’s why a year has one more sidereal day than solar days. The arithmetic comes out clean. The universe is just doing this weird double-counting thing where one of your rotations gets eaten by your orbit.

  If you’re an astronomer trying to point a telescope at a star, sidereal time is what you want. The star is in a fixed place in inertial space; your dome needs to compensate for Earth’s actual rotation, not for “where the sun appears to be.”

  If you’re a person trying to know when to eat lunch, solar time is what you want. The sun is the thing your body cares about.

  These two definitions don’t reconcile. They are answering different questions.

  Earth Doesn’t Tick Steadily

  Even after you average the equation of time and pick which kind of day you want, Earth still doesn’t make a great clock.

  Earth’s rotation is slowing down. Tidal friction with the Moon transfers angular momentum outward, the Moon drifts farther away (about 3.8 centimeters per year, measured by bouncing lasers off Apollo-era retroreflectors), and our days get longer by roughly 1.7 to 2.3 milliseconds per century. Slow, but cumulative. A really, really long time ago, a day was about 22 hours.

  So we know, Earth’s rotation is jittery in the short term. The atmosphere (air mass) sloshes around with weather. Ocean currents shift mass around because hot water weighs less than cold water. There is some coupling between the outer core and the mantle that yanks the rotation rate around. It’s very hard to predict all these factors in advance. All of these factors cause the the length of a day to fluctuate from one week to the next.

  For a long time none of this mattered. If a day was off by a few milliseconds, who cares? Sundials don’t have that resolution.

  The moment it started mattering was when we got better clocks than the those based on the Earths rotation.

  How We Measure Earth’s Rotation Today

  I hope you are ready to learn some astronomy.

  The most precise measurement of Earth’s rotation right now comes from watching distant quasars, supermassive black holes billions of light years away whose positions in the sky are effectively fixed. A technique called Very Long Baseline Interferometry, or VLBI, uses arrays of radio telescopes spread across continents to triangulate Earth’s exact orientation against these quasars.

  That’s worth reading again. The way we figure out what time it is on Earth is by triangulating against the cores of ancient galaxies billions of light years away.

  VLBI pins down Earth’s orientation to the level of microseconds and millimeters. It’s how we know, day by day, by exactly how many milliseconds the planet ran fast or slow. It’s how we know, to staggering precision, exactly how badly the planet underneath us is failing to be a steady clock.

  That measurement, and what we did about it, is going to matter in a bit, in future articles. This week on Time is all about how we measure time.

  Tomorrow: the second we use today isn’t measured by Earth at all. It’s measured by an atom that doesn’t care which planet you’re on.

  Sources

  • Equation of time — Wikipedia
  • Sidereal time — Wikipedia
  • Earth’s rotation — Wikipedia
  • Seeing the Light: lunar laser ranging — Eos
  • VLBI — NASA Earthdata

  I’d appreciate a follow. You can subscribe with your email below. The emails go out once a week, or you can find me on Mastodon at @[email protected].

  May 30, 2026 / Science / Time / 30daysoftime / Astronomy

  Enjoyed this?