Light-second
{{short description|Unit of length}}
{{Infobox unit
| quantity = length
| units1 = SI units
| inunits1 = {{val|299792458|ul=m}}
| units2 = astronomical units
| inunits2 = {{convert|299792458|m|AU|disp=out|lk=on|sigfig=5|comma=gaps}}
{{convert|299792458|m|ly|disp=out|lk=on|sigfig=5}}
{{convert|299792458|m|pc|disp=out|lk=on|sigfig=5}}
| inunits3 = {{convert|299792458|m|mi|disp=out|lk=on|0|comma=gaps}}
}}
The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly {{val|299792458|ul=m}} (approximately {{Val|983571055|u=ft}} or {{val|186282 |ul=miles}}).
Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond ({{val|299.8|u=mm}} or just under one international foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly used light-year is also currently defined to be equal to precisely {{val|31557600|u=light-seconds}}, since the definition of a year is based on a Julian year (not the Gregorian year) of exactly {{Val|365.25|u=days}}, each of exactly {{val|86400|u=SI seconds}}.[http://www.iau.org/Units.234.0.html IAU Recommendations concerning Units] {{webarchive|url=https://web.archive.org/web/20070216041250/http://www.iau.org/Units.234.0.html |date=2007-02-16 }}
Use in telecommunications
Communications signals on Earth rarely travel at precisely the speed of light in free space.{{Citation needed|date=September 2017}} Distances in fractions of a light-second are useful for planning telecommunications networks.
- One light-nanosecond is almost 300 millimetres (299.8 mm, 5 mm less than one footDavid Mermin suggested one light-nanosecond might be called a phoot at page 22 of It's About Time (2005), Princeton University Press), which limits the speed of data transfer between different parts of a computer.
- One light-microsecond is about 300 metres.
- The mean distance, over land, between opposite sides of the Earth is 66.8 light-milliseconds.
- Communications satellites are typically 1.337 light-milliseconds{{Citation needed|date=August 2017}} (low Earth orbit) to 119.4 light-milliseconds (geostationary orbit) from the surface of the Earth. Hence there will always be a delay of at least a quarter of a second in a communication via geostationary satellite (119.4 ms times 2); this delay is just perceptible in a transoceanic telephone conversation routed by satellite. The answer will also be delayed with a quarter of a second and this is clearly noticeable during interviews or discussions on TV when sent over satellite.
Use in astronomy
File:1e13m comparison Hale Bopp and smaller - HQ no transparency.png and Pioneer 10 (red and green arrows respectively). It is larger than the heliosphere's termination shock (blue shell) but smaller than Comet Hale-Bopp's orbit (faint orange ellipse below). Click on the image for a larger view and links to other scales.]]
File:1e10m comparison Rigel, Aldebaran, and smaller - antialiased no transparency.png (the blue star in the top left) and Aldebaran (the red star in the top right) are shown to scale. The large yellow ellipse represents Mercury's orbit.]]
The light-second is a convenient unit for measuring distances in the inner Solar System, since it corresponds very closely to the radiometric data used to determine them. (The match is not exact for an Earth-based observer because of a very small correction for the effects of relativity.) The value of the astronomical unit (roughly the distance between Earth and the Sun) in light-seconds is a fundamental measurement for the calculation of modern ephemerides (tables of planetary positions). It is usually quoted as "light-time for unit distance" in tables of astronomical constants, and its currently accepted value is {{gaps|499.004|786|385(20)}} s.{{cite web |last=Standish |first=E. M. |date=1998 |title=JPL Planetary and Lunar Ephemerides, DE405/LE405 |url=http://iau-comm4.jpl.nasa.gov/de405iom/de405iom.pdf |id=JPL IOM 312.F-98-048 |url-status=dead |archiveurl=https://web.archive.org/web/20120220062549/http://iau-comm4.jpl.nasa.gov/de405iom/de405iom.pdf |archivedate=2012-02-20 }}.{{cite book |editor1=McCarthy, Dennis D. |editor2=Petit, Gérard |date=2004 |contribution=IERS Conventions (2003) |url=http://www.iers.org/MainDisp.csl?pid=46-25776 |title=IERS Technical Note No. 32 |location=Frankfurt |publisher=Bundesamts für Kartographie und Geodäsie |isbn=3-89888-884-3}}
- The mean diameter of Earth is about 0.0425 light-seconds.
- The average distance between Earth and the Moon (the lunar distance) is about 1.282 light-seconds.
- The diameter of the Sun is about 4.643 light-seconds.
- The average distance between Earth and the Sun (the astronomical unit) is 499.0 light-seconds.
Multiples of the light-second can be defined, although apart from the light-year, they are more used in popular science publications than in research works. For example:
- A light-minute is 60 light-seconds, and so the average distance between Earth and the Sun is 8.317 light-minutes.
- The average distance between Pluto and the Sun (34.72 AU{{Cite web |title=Pluto distance from sun - Wolfram{{!}}Alpha |url=https://www.wolframalpha.com/ |access-date=2023-03-07 |website=www.wolframalpha.com |language=en}}) is 4.81 light-hours.{{Cite web |title=Pluto distance from sun in light hours - Wolfram{{!}}Alpha |url=https://www.wolframalpha.com/ |access-date=2023-03-07 |website=www.wolframalpha.com |language=en}}
- Humanity's most distant artificial object, Voyager 1, has an interstellar velocity of 3.57 AU per year,{{Cite web |title=Voyager - Fast Facts |url=https://voyager.jpl.nasa.gov/frequently-asked-questions/fast-facts/ |access-date=2023-03-07 |website=voyager.jpl.nasa.gov |language=en}} or 29.7 light-minutes per year.{{Cite web |title=3.57 au/year in light-minutes/year - Wolfram{{!}}Alpha |url=https://www.wolframalpha.com/ |access-date=2023-03-07 |website=www.wolframalpha.com |language=en}} As of 2023 the probe, launched in 1977, is over 22 light-hours from Earth and the Sun, and is expected to reach a distance of one light-day around November 2026 – February 2027.{{cn|date=April 2024}}
| class="wikitable"
!rowspan=2|Unit ! rowspan=2| Definition ! colspan="3" | Equivalent distance in ! rowspan=2 | Example |
| Meters
!Kilometers !Miles |
|---|
| light-second
| 1 light-second | style="text-align:right" | {{val|299792458|u=m}} || style="text-align:right" |{{val|2.998|e=5|u=km}} || style="text-align:right" |{{val|1.863|e=5|u=miles}} | Average distance from the Earth to the Moon is about 1.282 light-seconds |
| light-minute
| 60 light-seconds | style="text-align:right" | {{val|17987547480|u=m}} || style="text-align:right" |{{val|1.799|e=7|u=km}} || style="text-align:right" |{{val|1.118|e=7|u=miles}} | Average distance from the Earth to the Sun is 8.317 light-minutes |
| light-hour
| 60 light-minutes | style="text-align:right" | {{val|1079252848800|u=m}} || style="text-align:right" |{{val|1.079|e=9|u=km}} || style="text-align:right" |{{val|6.706|e=8|u=miles}} | The perihelion of Saturn's orbit is about 1.25 light-hours |
| light-day
| 24 light-hours |style="text-align:right" |{{val|25902068371200|u=m}} || style="text-align:right" |{{val|2.590|e=10|u=km}} || style="text-align:right" |{{val|1.609|e=10|u=miles}} | Voyager 1 is about 0.96 light-days from the Sun (as of March 2025) |
| light-week
| 7 light-days | style="text-align:right" |{{val|181314478598400|u=m}} || style="text-align:right" |{{val|1.813|e=11|u=km}} || style="text-align:right" |{{val|1.127|e=11|u=miles}} | The Oort cloud is thought to extend between 41 and 82 light-weeks out from the Sun |
| light-year
| 365.25 light-days |style="text-align:right" | {{val|9460730472580800|u=m}} || style="text-align:right" |{{val|9.461|e=12|u=km}} || style="text-align:right" |{{val|5.879|e=12|u=miles}} | Proxima Centauri is the nearest star to the Sun, about 4.24 light years away |
See also
References
{{reflist}}
{{Units of length used in Astronomy}}