4.3 Keeping Time
4 min read•june 12, 2024
Earth's rotation shapes our perception of time, influencing how we measure days and organize our lives. Solar and sidereal days differ slightly, reflecting Earth's movement around the Sun. This difference impacts timekeeping systems and calendar development.
and the help standardize time globally, facilitating communication and travel. The evolution of timekeeping, from to , has improved accuracy and coordination across regions, shaping our modern understanding of time.
Timekeeping and Earth's Rotation
Solar vs sidereal days
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- represents the time for the Sun to return to the same position in the sky from Earth's perspective
- Lasts approximately 24 hours
- Varies slightly throughout the year due to Earth's elliptical orbit () and axial tilt ()
- represents the time for Earth to complete one rotation relative to the distant stars
- Lasts approximately 23 hours, 56 minutes, and 4 seconds
- About 4 minutes shorter than a
- Difference between solar and sidereal days arises because Earth orbits the Sun while rotating
- Earth must rotate slightly more than 360° for the Sun to return to the same position in the sky ()
- This extra rotation accounts for the difference between solar and sidereal days (about 1°/day)
- This rotation is observed relative to the , an imaginary sphere surrounding Earth
Time zones and date line
- divide Earth into 24 regions, each approximately 15° of longitude wide
- Offset from (UTC) by a whole number of hours (-12 to +12)
- Helps standardize local time within regions and facilitate communication and transportation
- (DST) is observed in some regions, advancing clocks by one hour during summer months
- Aims to make better use of daylight hours and reduce energy consumption
- Not all countries or regions observe DST (Arizona, Hawaii)
- International Date Line (IDL) is an imaginary line running from the North Pole to the South Pole, roughly along the 180° meridian
- Demarcates the change of calendar date
- Crossing the IDL from west to east, the date moves back by one day (lose a day)
- Crossing the IDL from east to west, the date moves forward by one day (gain a day)
Evolution of timekeeping systems
- is based on the Sun's actual position in the sky, varying with location and time of year
- Sundials used to measure , but accuracy limited by factors like Earth's obliquity
- is the average of apparent solar time over a year, accounting for variations in the length of a solar day
- Mechanical clocks developed to measure more consistently than sundials
- is the mean solar time for a specific longitude, differing between locations due to Earth's rotation
- Cities and towns set their own local time based on their longitude, leading to many different local times
- established in the late 19th century to facilitate communication and transportation
- Each time zone follows the mean solar time of a central meridian (usually a multiple of 15°)
- Reduced the number of local times and improved coordination between regions
- Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time
- Based on atomic clocks, which are more precise and stable than Earth's rotation
- UTC is the basis for civil time and time zones worldwide, with local times defined as offsets from UTC
- (GMT) was the predecessor to UTC, based on the mean solar time at the Royal Observatory in Greenwich, London
Calendar systems and astronomical events
- Various have been developed throughout history to track longer periods of time
- Many calendars are based on astronomical events, such as the cycles of the Moon or the Sun
- are added to calendar systems to account for the fact that Earth's orbital period is not exactly 365 days
- In the Gregorian calendar, a leap year occurs every 4 years, except for years divisible by 100 but not by 400
- are points in Earth's orbit when the Sun appears to cross the celestial equator
- Mark the beginning of spring and autumn in each hemisphere
- is the gradual change in the orientation of Earth's rotational axis
- Causes the positions of equinoxes to slowly shift over long periods of time
- Affects the timing of seasons and the visibility of certain stars from Earth
Key Terms to Review (29)
Apparent solar time: Apparent solar time is a method of measuring time based on the actual position of the Sun in the sky, as observed from a specific location on Earth. It reflects the true solar noon when the Sun reaches its highest point in the sky.
Apparent Solar Time: Apparent solar time is a measure of time based on the position of the sun in the sky. It is the time indicated by a sundial, which tracks the apparent daily motion of the sun across the celestial sphere. Apparent solar time is closely related to the concept of keeping time, as it provides a natural and intuitive way to measure the passage of a day.
Atomic Clocks: Atomic clocks are highly precise timepieces that use the natural vibrations of atoms as a reference to measure time. They are the most accurate time and frequency standards known, capable of keeping time to within a few nanoseconds per day, making them essential for applications that require extremely precise timekeeping, such as navigation, telecommunications, and scientific research.
Calendar Systems: Calendar systems are methods of organizing and keeping track of time, typically by dividing it into units such as days, weeks, months, and years. These systems play a crucial role in various aspects of human life, including religious, cultural, and social activities, as well as the coordination of events and the measurement of time intervals.
Celestial sphere: The celestial sphere is an imaginary sphere that extends infinitely into space with Earth at its center, used to map the positions of stars and planets. Astronomers use it to simplify the complex motions of celestial bodies for observation and navigation.
Celestial Sphere: The celestial sphere is an imaginary, three-dimensional sphere that surrounds the Earth, upon which all celestial objects and events appear to be projected. It serves as a conceptual framework for understanding the motions and positions of objects in the sky, and is a fundamental concept in the study of astronomy.
Coordinated Universal Time: Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time. It is the basis for civil time and time zones worldwide, and serves as the reference for time-keeping systems.
Daylight Saving Time: Daylight Saving Time (DST) is the practice of setting clocks forward by one hour during the warmer months of the year to shift daylight hours to better match typical waking hours. This allows for more daylight in the evenings and less in the mornings, providing more usable daylight for daily activities.
Eccentricity: Eccentricity is a measure of how much an orbit deviates from being a perfect circle. It ranges from 0 (a perfect circle) to 1 (a parabolic trajectory).
Eccentricity: Eccentricity is a measure of how elliptical or elongated the orbit of a celestial body, such as a planet or comet, is around its parent body. It describes the degree to which an orbit deviates from a perfect circle, with a value ranging from 0 for a perfect circle to 1 for a parabolic orbit.
Equinoxes: Equinoxes are the two points in the year when the Sun's position in the sky results in equal lengths of day and night across the globe. This occurs when the Sun's rays strike the Earth's surface at a perpendicular angle, marking the transition between the seasons.
Greenwich Mean Time: Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, which serves as the international standard of civil time. It is the primary time zone to which the world's time zones are referenced.
International Date Line: The International Date Line is an imaginary line that runs from the North Pole to the South Pole, roughly following the 180th meridian of longitude. It is the line that separates one calendar day from the next, allowing for the tracking of time and date across the globe.
Leap Years: Leap years are special years in the calendar system that have 366 days instead of the usual 365 days. This extra day, February 29th, is added to the calendar to keep it aligned with the astronomical year and the seasons.
Local Mean Time: Local Mean Time (LMT) is a time standard that is based on the mean solar time of a specific location or meridian. It represents the average time of day as determined by the position of the sun in the sky, rather than the exact time of solar noon or midnight.
Mean solar time: Mean solar time is a measure of the average passage of time based on the position of the mean sun, an imaginary point that moves uniformly along the celestial equator. This concept helps standardize timekeeping by averaging out the variations caused by Earth's elliptical orbit and axial tilt.
Mean Solar Time: Mean solar time is a timekeeping system that measures the passage of time based on the average or 'mean' motion of the Sun across the sky, rather than its actual position. It serves as the standard for civil timekeeping and is the basis for the 24-hour clock used in everyday life.
Obliquity: Obliquity refers to the tilt of a planet's rotational axis relative to its orbital plane around the sun. This angular difference between the planet's axis and the plane of its orbit is a crucial factor in determining the planet's climate and seasons.
Precession: Precession is the slow, conical motion of Earth's rotation axis, which causes the celestial poles to trace out circles in the sky over a period of approximately 26,000 years. This phenomenon affects the position of stars and constellations over long time periods.
Precession: Precession is the gradual shift in the orientation of an object's rotational axis in relation to a fixed reference frame. This phenomenon occurs in various celestial bodies, including Earth, and has significant implications for the study of astronomy, timekeeping, and the calendar.
Sidereal day: A sidereal day is the time it takes for Earth to rotate once relative to distant stars, approximately 23 hours, 56 minutes, and 4 seconds. It is different from a solar day, which is based on the position of the Sun in the sky.
Sidereal Day: A sidereal day is the time it takes for the Earth to complete one rotation on its axis, as measured by the position of distant stars in the sky. It is slightly shorter than a solar day, which is the time it takes for the Earth to complete one rotation relative to the Sun.
Solar day: A solar day is the time it takes for Earth to rotate once on its axis relative to the Sun, resulting in a full cycle from one noon to the next. It averages about 24 hours but can vary slightly due to Earth's elliptical orbit and axial tilt.
Solar Day: A solar day is the length of time it takes for the Earth to complete one full rotation on its axis, as measured by the position of the sun in the sky. It is the fundamental unit of time used to measure the duration of a day on Earth.
Solar Noon: Solar noon, also known as midday, is the time of day when the sun reaches its highest point in the sky, directly overhead at the observer's location. This event marks the middle of the day and is an important concept in understanding the daily motion of the sun and keeping track of time.
Standard Time Zones: Standard time zones are a system of dividing the world into geographic regions, each with a designated standard time that is used for civil purposes. This system allows for the coordination of timekeeping and the synchronization of activities across different locations, particularly in the context of transportation, communication, and international commerce.
Sundials: Sundials are ancient timekeeping devices that use the position of the sun to indicate the time of day. They rely on the consistent and predictable movement of the sun across the sky to cast a shadow that can be used to determine the current time.
Time zones: Time zones are regions of the Earth that have the same standard time. They are used to coordinate activities across different areas.
Time Zones: Time zones are geographical regions that have adopted the same standard time, usually defined by their longitudinal position relative to the prime meridian. They are an essential concept in keeping track of time and coordinating activities across different locations around the world.