What is a second (s or sec)?
The second (s or sec) is the International System of Units (SI) unit of time measurement.
One second is the time that elapses during 9,192,631,770 (or 9.192631770 x 109 in decimal form) cycles of the radiation produced by the transition between two levels of the cesium-133 atom.
How is a second expressed?
There are other expressions for the second. It is the time required for an electromagnetic (EM) field to propagate 299,792,458 meters (2.99792458 x 108 m) through a vacuum.
This figure is sometimes rounded to 3 x 108 m, or 300,000 kilometers (3 x 105 km). One second is equal to 1/86,400 of a mean solar day. This is easy to derive from the fact that there are 60 seconds in a minute, 60 minutes in an hour and 24 hours in a mean solar day.
This definition is, however, subject to limited accuracy because of irregularities in the earth's orbit around the sun.
Are their units of time smaller than seconds?
Engineers and scientists often use smaller units of measurement than the second by attaching power-of-10 prefix multipliers. One millisecond is 10-3 seconds; 1 microsecond is 10-6 seconds; 1 nanosecond is 10-9 seconds; and 1 picosecond is 10-12 seconds.
During these spans of time, respectively, an EM field propagates through a vacuum over distances of approximately 300 km, 300 m, 300 millimeters and 300 micrometers.
Use cases for seconds
A second is typically the base unit of time measurement in timing for events that occur in everyday life, such as the following:
- a person's heart rate (beats per minute);
- a car's speed (miles per hour or kilometers per hour); and
- cooking time frames (minutes and seconds).
The smaller subdivisions are usually used for events that occur rapidly, such as the following:
- electron impulses in nerves and muscles;
- sound waves;
- light waves; and
- gamma ray bursts.
The second is also a common base unit for expressing extremely long spans of time, such as geologic time units, like the millennium, mega-annum, or giga-annum.
The second is sometimes specified as a unit of angular measure, especially in astronomy and global positioning.
In these contexts, it is also known as an arc second or a second of arc and is equal to exactly 1/3,600 of an angular degree or 1/1,296,000 of a circle.
Sixty arc seconds constitute an arc minute; 60 arc minutes constitute an angular degree. One arcsecond of latitude at the earth's surface corresponds to a north-south distance of only about 31 m.
At the equator, 1 second of longitude corresponds to an east-west distance of about 26 m.
See also: table of physical constants, atomic clock, radiant energy, pascal and newton.
This was last updated in June 2022
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As a seasoned expert in the field of time measurement and units, I bring forth a comprehensive understanding of the concepts outlined in the provided article by Katie Terrell Hanna. My expertise is rooted in a deep knowledge of the International System of Units (SI) and its application in time measurement.
The second (s or sec), as discussed in the article, is the SI unit of time, defined by the specific number of cycles of radiation produced by the transition between two levels of the cesium-133 atom. This standard, set at 9,192,631,770 cycles, establishes the precise duration of one second.
Furthermore, the article delves into alternative expressions for the second, such as the time it takes for an electromagnetic field to propagate a defined distance through a vacuum. This distance is approximately 299,792,458 meters or 300,000 kilometers. Additionally, the connection between a second and the mean solar day is explained, considering the subdivisions of time: 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day.
The article also explores units of time smaller than seconds, employing power-of-10 prefix multipliers. It elucidates that one millisecond is 10^(-3) seconds, one microsecond is 10^(-6) seconds, one nanosecond is 10^(-9) seconds, and one picosecond is 10^(-12) seconds. The corresponding spans of time and distances an electromagnetic field travels during these intervals are also provided.
Practical applications of seconds in daily life are highlighted, emphasizing its role as the base unit for measuring events such as heart rate, car speed, and cooking time. Smaller time subdivisions find relevance in rapidly occurring events like electron impulses, sound waves, light waves, and gamma ray bursts.
Moreover, the article touches upon the broader scope of the second, being a unit for expressing extremely long time spans, such as geological time units (millennium, mega-annum, or giga-annum). In astronomy and global positioning, the second is utilized as an arc second, defining angular measures.
To enhance your understanding, it's crucial to recognize that the second is intricately linked to various scientific disciplines, making it a fundamental and versatile unit in the realm of measurement and timekeeping.
