Day

• 1. Introduction

  ›

  A day is the time it takes the Earth to make one full rotation with respect to the Sun, and on average that comes to 24 hours, or 86,400 seconds. As that span passes at any given spot on the planet, the place moves through morning, afternoon, evening, and night. That same rolling cycle drives circadian rhythms in many organisms, rhythms that turn out to be vital to many life processes. The word itself is old. It descends from the Old English daeg, with cousins like dagur in Icelandic, Tag in German, and dag across Norwegian, Danish, Swedish, and Dutch, all tracing back to a Proto-Germanic root. But the closer you look at this everyday unit, the stranger it becomes. Why is a day measured against a distant star nearly four minutes shorter than one measured against the Sun? Why did the Earth's day once last only six hours? And why does humanity keep arguing over the exact moment a new day should begin?

• 2. The Many Solar Days

  ›

  The solar day is the time it takes for the Sun to return to its culmination point, its highest position in the sky. That sounds fixed, yet it is not. Earth follows an eccentric orbit, with the Sun sitting in one of the orbit's foci rather than its middle. Because of Kepler's second law, the planet moves at different speeds at different points along that orbit. The result is that a solar day is not the same length throughout the orbital year. As the Earth travels its eccentric path while spinning on an inclined axis, a single such day can run up to 7.9 seconds longer or shorter than 24 hours. In recent decades the average solar day has measured about 86,400.002 seconds, which works out to 24.0000006 hours. There are currently about 365.2421875 solar days in one mean tropical year. To smooth out the wobble, astronomers invented a fictitious mean Sun that glides at constant speed along the celestial equator, matching the real Sun's average pace but erasing the yearly variation. In terms of rotation, the average day spans about 360.9856 degrees, slightly more than a full turn, because the Earth's daily progress around the Sun is slightly less than one degree.

• 3. The Star-Measured Day

  ›

  A sidereal day, also called a stellar day, is the time the Earth takes to complete one full rotation relative to a distant star treated as fixed against the celestial background. Astronomers rely on this measure, and it runs about four minutes shorter than a 24-hour solar day. The precise figure is 23 hours, 56 minutes, and 4.09 seconds, or 0.99726968 of a 24-hour solar day. Across one mean tropical year there are about 366.2422 stellar days, exactly one more than the count of solar days. Earth is hardly alone in having such a rotation. Mars turns in about 24.7 hours, close to our own. Jupiter races around in roughly 9.9 hours, and Saturn in about 10.7. Venus is the outlier, taking around 2,802 hours, while Mercury needs about 4,222.6. The Earth's Moon rotates in roughly 708.7 hours, and distant Pluto in about 153.3.

• 4. Defining The Second

  ›

  In the International System of Units, the day is not an official unit, yet it is accepted for use alongside SI. It carries the symbol d and is defined as exactly 86,400 seconds, with the second serving as the base unit of time. That second was given a precise meaning in 1967 to 1968, during the 13th CGPM, when the International Bureau of Weights and Measures redefined it. The new definition fixed the second as the duration of 9,192,631,770 periods of the radiation from the transition between two hyperfine levels of the ground state of the caesium-133 atom. Run that out across a full day and the SI day lasts exactly 794,243,384,928,000 of those periods. Reformers have repeatedly tried to bend time into tidier shapes without touching the day itself. Metric time keeps the day as its base unit and slices it with metric prefixes, so a metric hour is a tenth of a day and a metric minute a thousandth. Decimal time splits the day into 10 hours and bundles 10 days into a decade, the rough equivalent of a week, with three decades making a month. Henri de Sarrauton's proposal kept the day but cut hours into 100 minutes. Mendizabal y Tamborel built his scheme on the sidereal day, while Rey-Pailhade divided the day into 100 ces.

• 5. A Slowing Spin

  ›

  Tidal deceleration is reshaping the day across deep time. The Moon's gravitational pull drags on the Earth, slowing its rotation, so the mean length of a solar day now sits at about 86,400.002 seconds and grows by roughly 2 milliseconds per century. As the planet slows, the rotation-based second drifted out of step with the atomic second, and that mismatch created the need for leap seconds. These extra seconds are inserted into Coordinated Universal Time, making a civil day occasionally 86,401 or even 86,399 SI seconds long. In the half-century from 1972 through 2022-27 leap seconds were added, roughly one every other year. The placement is irregular because factors beyond tidal drag nudge the day's length, so the International Earth Rotation and Reference Systems Service measures the spin and announces each leap second in advance. The deep past tells the more dramatic version. At the Earth's formation the day is estimated to have lasted only 6 hours. Paleontologist John W. Wells found he could read ancient day lengths in the sedimentation rings of coral fossils, since some biological systems respond to the tide. By his method, the Cambrian held about 425 days in a year, each lasting 20 hours and 40 minutes. The Devonian had about 410 days of 21 hours and 20 minutes, and the Cretaceous about 380 days of 23 hours and 20 minutes. Arbab I. Arbab later plotted these lengths and found a curved line, which he attributed to changes in the volume of water affecting the planet's rotation.

• 6. Where The Day Begins

  ›

  Genesis 1:5 fixes the day between evening and morning before the Sun is even created: "And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day." That ordering still governs the Jewish day, which begins at sunset or at nightfall, when three second-magnitude stars appear. Medieval Europe followed the same instinct through Florentine reckoning, where a phrase like two hours into the day meant two hours after sunset. Christmas Eve, Halloween as All Hallows' Eve, and the Eve of Saint Agnes are surviving traces of that older pattern, when a holiday opened during the previous evening. Other peoples drew the line elsewhere. In ancient Egypt the day ran from sunrise to sunrise, while the ancient Romans and ancient Chinese began the civil day at midnight, the convention common today. Italian reckoning counted 24 hours from sunset, and as late as before 1926 Turkey kept two systems at once, a Turkish one counting from sunset and a French one counting from midnight. Astronomers added their own twist by starting a day at noon, so a single night's observations all fall on the same date. The International Meridian Conference of 1884 expressed the hope that astronomical and nautical days would everywhere be arranged to begin at midnight.

• 7. Carving Up The Hours

  ›

  Daytime is the stretch when sunlight directly reaches the ground, and it averages slightly more than half of the 24-hour day. Two effects tip the balance. The Sun is not a point but spans about 32 minutes of arc, and the atmosphere bends sunlight so that some reaches the ground even when the Sun sits about 34 minutes of arc below the horizon. First light therefore arrives when the Sun's center is still about 50 minutes of arc down, leaving daytime roughly 7 minutes longer than 12 hours. Within daytime, morning runs from sunrise to noon, and afternoon from noon to sunset or the start of evening. The afternoon brings the human body's highest temperature, a rise in traffic collisions, and a dip in productivity. Twilight fills the edges, the lit but sunless periods before sunrise and after sunset, each split into civil twilight up to 6 degrees below the horizon, nautical up to 12 degrees, and astronomical up to 18 degrees. Cultures cut the whole differently. In Brazil the day falls into four six-hour blocks: madrugada from midnight to 6 a.m., manha until noon, tarde until 6 p.m., and noite back to midnight. Night itself, the dark span between dusk and dawn, carries its own hazards, since light pollution can disrupt the sleep of humans and animals alike.