From: veritasium
When observing the Earth’s motion, different methods of tracking time emerge based on the perspective of the observer. This difference leads to the concepts of solar time and sidereal time, which account for the Earth’s rotation and its orbit around the Sun [11:32:00]. This phenomenon is analogous to the coin rotation paradox, where a rolling object completes an unexpected additional rotation when moving around a circular path [04:05:00].
Solar Time
A normal solar day is defined as the time it takes for the Sun to appear directly overhead again from a specific location on Earth [12:22:00]. This duration is approximately 24 hours [12:31:00]. Because the Earth is simultaneously rotating on its axis and orbiting the Sun, it must rotate slightly more than 360 degrees to bring the Sun back to the same overhead position [12:27:00].
A solar year is the common measurement of time, counting approximately 365.24 days [11:40:00]. This count is from the perspective of an observer on Earth [11:50:00]. Using solar time on Earth is practical because it aligns with the daily cycle of day and night [13:54:00].
Sidereal Time
From the perspective of an external observer, such as one looking from distant stars, the Earth completes one extra rotation in its orbit around the Sun compared to an Earth-bound observer [11:54:00]. This is known as a Sidereal year, which has 366.24 days [12:05:00]. “Sidereal” refers to “with respect to the stars,” which is where an external observer would effectively be [12:12:00].
A sidereal day is the time it takes for Earth to rotate exactly 360 degrees, allowing a distant star to appear directly overhead again [12:44:00]. This is shorter than a solar day, lasting approximately 23 hours, 56 minutes, and four seconds [13:00:00]. This difference accounts for the “extra day” in the sidereal year [13:09:00].
Divergence of Solar and Sidereal Time
If a solar day and a sidereal day began at the same moment, they would gradually diverge over the course of a year [13:14:00]. After six months, the sidereal day would be 12 hours ahead of the solar day, meaning solar noon would correspond to sidereal midnight [13:21:00]. This divergence continues until the sidereal day is a full day ahead of the solar day, coinciding with the completion of a new year and orbit [13:29:00].
The equivalence can be expressed as: 365.24 solar days (each 24 hours long) are equal to 366.24 sidereal days (each 23 hours, 56 minutes, and four seconds long) [13:39:00].
Applications
- Solar Time: This is the standard timekeeping method for daily life on Earth, as it keeps day and night cycles consistent [13:54:00].
- Sidereal Time: This is crucial for astronomy [14:12:00]. Astronomers use sidereal time for their telescopes to ensure they are consistently observing the same region of space each night [14:14:00]. Geostationary satellites, used for communication and navigation, also employ sidereal time to maintain their orbits locked with Earth’s rotation [14:20:00].