Editing Speed of light (section)

=== Spaceflight and astronomy ===
[[File:Earth and Moon speed of light by James O'Donoghue.gif|thumb|upright=1.8|alt=The diameter of the moon is about one quarter of that of Earth, and their distance is about thirty times the diameter of Earth. A beam of light starts from the Earth and reaches the Moon in about a second and a quarter.|A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.]]
Similarly, communications between the Earth and spacecraft are not instantaneous. There is a brief delay from the source to the receiver, which becomes more noticeable as distances increase. This delay was significant for communications between [[Mission Control Center|ground control]] and [[Apollo 8]] when it became the first crewed spacecraft to orbit the [[Moon]]: for every question, the ground control station had to wait at least three&nbsp;seconds for the answer to arrive.<ref>
{{Cite web
 |url         = https://history.nasa.gov/ap08fj/15day4_orbits789.htm
 |title       = Day 4: Lunar Orbits 7, 8 and 9
 |work        = The Apollo 8 Flight Journal
 |publisher   = NASA
 |access-date = 16 December 2010
 |url-status  = dead
 |archive-url  = https://web.archive.org/web/20110104032114/http://history.nasa.gov/ap08fj/15day4_orbits789.htm
 |archive-date = 4 January 2011
}}</ref>

The communications delay between Earth and [[Mars]] can vary between five and twenty minutes depending upon the relative positions of the two planets. As a consequence of this, if a robot on the surface of Mars were to encounter a problem, its human controllers would not be aware of it until approximately {{nowrap|4–24 minutes}} later. It would then take a further {{nowrap|4–24 minutes}} for commands to travel from Earth to Mars.<ref>{{Cite web |last=Ormston |first=Thomas |date=2012-05-08 |title=Time delay between Mars and Earth – Mars Express |url=https://blogs.esa.int/mex/2012/08/05/time-delay-between-mars-and-earth/ |access-date=2024-07-16 |website=MARS EXPRESS ESA’s mission to the Red Planet |language=en-US}}</ref><ref>{{Cite journal |last1=Parisi |first1=Megan |last2=Panontin |first2=Tina |last3=Wu |first3=Shu-Chieh |last4=Mctigue |first4=Kaitlin |last5=Vera |first5=Alonso |date=2023 |title=Effects of Communication Delay on Human Spaceflight Missions |url=https://openaccess.cms-conferences.org/publications/book/978-1-958651-74-2/article/978-1-958651-74-2_6 |journal=Human-Centered Aerospace Systems and Sustainability Applications |publisher=AHFE Open Acces |volume=98 |doi=10.54941/ahfe1003920 |isbn=978-1-958651-74-2|url-access=subscription }}</ref>

Receiving light and other signals from distant astronomical sources takes much longer. For example, it takes 13&nbsp;billion (13{{e|9}}) years for light to travel to Earth from the faraway galaxies viewed in the [[Hubble Ultra-Deep Field]] images.<ref name=Hubble>
{{Cite press release
 |date=5 January 2010
 |title=Hubble Reaches the "Undiscovered Country" of Primeval Galaxies
 |url=https://www.nasa.gov/mission_pages/hubble/science/undiscovered-country.html
 |publisher=[[Space Telescope Science Institute]]
}}</ref><ref>
{{Cite web
 |title=The Hubble Ultra Deep Field Lithograph
 |url=http://www.nasa.gov/pdf/283957main_Hubble_Deep_Field_Lithograph.pdf
 |publisher=NASA
 |access-date=4 February 2010
}}</ref> Those photographs, taken today, capture images of the galaxies as they appeared 13&nbsp;billion years ago, when the universe was less than a billion years old.<ref name=Hubble/> The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the [[evolution of stars]], [[Galaxy formation and evolution|of galaxies]], and [[history of the universe|of the universe]] itself.<ref>{{Cite book|last=Mack|first=Katie|url=https://www.worldcat.org/oclc/1180972461|title=The End of Everything (Astrophysically Speaking)|date=2021|publisher=Penguin Books|isbn=978-0-141-98958-7|location=London|pages=18–19|oclc=1180972461|author-link=Katie Mack (astrophysicist)}}</ref>

Astronomical distances are sometimes expressed in [[light-year]]s, especially in [[popular science]] publications and media.<ref>{{Cite web
 |title=The IAU and astronomical units
 |url=http://www.iau.org/public/measuring/
 |publisher=[[International Astronomical Union]]
 |access-date=11 October 2010
 |archive-date=5 June 2013
 |archive-url=https://web.archive.org/web/20130605024231/http://www.iau.org/public/measuring/
 |url-status=dead
 }}</ref> A light-year is the distance light travels in one [[Julian year (astronomy)|Julian year]], around 9461&nbsp;billion kilometres, 5879&nbsp;billion miles, or 0.3066 [[parsec]]s. In round figures, a light year is nearly 10&nbsp;trillion kilometres or nearly 6&nbsp;trillion miles. [[Proxima Centauri]], the closest star to Earth after the Sun, is around 4.2 light-years away.<ref name=starchild>Further discussion can be found at 
{{Cite web
 |year=2000
 |title=StarChild Question of the Month for March 2000
 |url=http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question19.html
 |work=StarChild
 |publisher=NASA
 |access-date=22 August 2009
}}</ref>