our Twitter (X) feedView our Youtube channelView our Instagram feedView our Substack feedView our Spotify feed As the Earth spins and wobbles on its axis and revolves elliptically around the Sun, each day differs from the last. This iconic image of Earth, known as the Blue Marble, was taken on December 7, 1972, by the crew of Apollo 17. Although one typically thinks of a day as 24 hours, this isn't precisely true, nor does 24 hours describe the time needed for one Earth rotation.
Science and TechHistory and SocietyAstrophysicsAstronomyUniverse Expansion To mark the passage of time, we assign 24 hours to each day, universally. Most of us are very used to 24 hours being in a day but aren’t used to the fact that this doesn’t correspond to a 360 degree rotation of the Earth. Most days, however, are either longer or shorter than 24 hours; only 4 days a year have precisely 24 hours to them.
While that’s accurate for a day’s average length, most days aren’t 24 hours long. This diagram shows an analemma, constructed by photographing the Sun at the same time throughout the year. The fact that the Sun isn’t in the same place is the combined effects of our axial tilt/obliquity and our orbital eccentricity and speed variations as we revolve around the Sun.
The Sun is not in the same position every day because each day is not exactly 24 hours. Counterintuitively, a day isn’t the time required for Earth to rotate 360°. The Earth, moving in its orbit around the Sun and spinning on its axis, always defines “noon” and “midnight” in the same fashion: where the Sun’s height above or below the horizon is maximized.
This moment in time doesn’t correspond to when Earth has rotated 360 degrees from the prior day, but rather closer to 360.9856 degrees, owing to the added effects of Earth’s motion around the Sun. We rotate 360° in 23 hours, 56 minutes and 4.09 seconds, leaving us 00:03:55.91 short. These star trails appear in the sky due to long-exposure photography of the north pole, combined with the physical phenomenon of the rotating Earth.
Although no one has ever successfully captured a full 360 degree star trail, it surprises people to learn that you wouldn’t need a 24 hour photograph to complete the circle, merely a 23 hour, 56 minute and 4.09 second exposure, as it relies on a sidereal, rather than a solar day. A full rotation, astronomically, is a sidereal day: different from a solar (calendar) day. When the Earth spins a full 360 degrees about its axis, it hasn’t yet aged by one full day, because it has also shifted in its orbit around the Sun.
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