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<h2 id="definitions">Definitions</h2> <p>As defined by the <a href="/facts/International_Astronomical_Union/GMFPImXo">International Astronomical Union</a> (IAU), the light-year is the product of the <a href="/facts/Julian_year_(astronomy)/EVdRNUe3">Julian year</a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> (365.25 days, as opposed to the 365.2425-day <a href="/facts/Gregorian_calendar/axJfTXV1">Gregorian year</a> or the 365.24219-day <a href="/facts/Tropical_year/ASBLWNMY">Tropical year</a> that both approximate) and the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a> (299792458 m/s).<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> Both of these values are included in the <a href="/facts/Astronomical_constant/zkD0guxA">IAU (1976) System of Astronomical Constants</a>, used since 1984.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> From this, the following conversions can be derived: </p> <table><tbody><tr><td rowspan="6">1 light-year </td><td>= 9460730472580800 metres (exactly)</td></tr><tr><td>c. 9.461 <a href="/facts/Petametre/YzL1bHGX">petametres</a></td></tr><tr><td>c. 9.461 trillion (<a href="/facts/Long_and_short_scales/qkxEuNw2">short scale</a>) kilometres (5.879 trillion miles)</td></tr><tr><td>c. 63241.077 <a href="/facts/Astronomical_unit/gjsbu5xU">astronomical units</a></td></tr><tr><td>c. 0.306601 <a href="/facts/Parsec/SwSsk57J">parsec</a></td></tr></tbody></table> <p>The abbreviation used by the IAU for light-year is "ly",<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> International standards like <a href="/facts/ISO_80000/k1nrIjMW">ISO 80000:2006</a> (now superseded) have used "l.y."<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> and localized abbreviations are frequent, such as "al" in French, Spanish, and Italian (from <i>année-lumière</i>, <i>año luz</i> and <i>anno luce</i>, respectively), "Lj" in German (from <i>Lichtjahr</i>), etc. </p><p>Before 1984, the <a href="/facts/Tropical_year/ASBLWNMY">tropical year</a> (not the Julian year) and a measured (not defined) speed of light were included in the IAU (1964) System of Astronomical Constants, used from 1968 to 1983.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> The product of <a href="/facts/Simon_Newcomb/KXaRfpwC">Simon Newcomb</a>'s <a href="/facts/J1900.0/RDdj6SmA">J1900.0</a> mean tropical year of 31556925.9747 <a href="/facts/Ephemeris_second/bwgzySNE">ephemeris seconds</a> and a speed of light of 299792.5 km/s produced a light-year of 9.460530×1015 m (rounded to the seven <a href="/facts/Significant_digits/RUfYtpgo">significant digits</a> in the speed of light) found in several modern sources<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a><a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a><a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> was probably derived from an old source such as <a href="/facts/Clabon_Allen/a2tJFge8">C. W. Allen</a>'s 1973 <i>Astrophysical Quantities</i> reference work,<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> which was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits).<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p><p>Other high-precision values are not derived from a <a href="/facts/Coherent_units/bg3o7Msc">coherent</a> IAU system. A value of 9.460536207×1015 m found in some modern sources<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> is the product of a mean Gregorian year (365.2425 days or 31556952 s) and the defined speed of light (299792458 m/s). Another value, 9.460528405×1015 m,<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> is the product of the J1900.0 mean tropical year and the defined speed of light. </p><p>Abbreviations used for light-years and multiples of light-years are: </p> <ul><li>"ly" for one light-year<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a></li> <li>"kly" <a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> or "klyr"<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> for a kilolight-year (1,000 light-years)</li> <li>"Mly" for a megalight-year (1,000,000 light-years)<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a></li> <li>"Gly"<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> or "Glyr"<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> for a gigalight-year (1000000000 light-years)</li></ul> <h2 id="history">History</h2> <p>The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, by <a href="/facts/Friedrich_Bessel/82K1lJ7t">Friedrich Bessel</a> in 1838. The star was <a href="/facts/61_Cygni/ISchaxhy">61 Cygni</a>, and he used a 160-millimetre (6.2 in) <a href="/facts/Heliometer/BDXGVUJn">heliometre</a> designed by <a href="/facts/Joseph_von_Fraunhofer/1dWvIByM">Joseph von Fraunhofer</a>. The largest unit for expressing distances across space at that time was the <a href="/facts/Astronomical_unit/gjsbu5xU">astronomical unit</a>, equal to the radius of the Earth's orbit at 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni's <a href="/facts/Parallax/Gu8W21LP">parallax</a> of 0.314 arcseconds, showed the distance to the star to be 660000 astronomical units (9.9×1013 km; 6.1×1013 mi). Bessel added that light takes 10.3 years to traverse this distance.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> He recognized that his readers would enjoy the mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resisted expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light. </p><p>The speed of light was not yet precisely known in 1838; the estimate of its value changed in 1849 (<a href="/facts/Hippolyte_Fizeau/Y3QrvXGf">Fizeau</a>) and 1862 (<a href="/facts/L%C3%A9on_Foucault/xG7tEDjk">Foucault</a>). It was not yet considered to be a fundamental constant of nature, and the propagation of light through the <a href="/facts/Luminiferous_aether/UzRf78a2">aether</a> or space was still enigmatic. </p><p>The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (<i>Wegstunde</i>). </p><p>A contemporary German popular astronomical book also noticed that light-year is an odd name.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> In 1868 an English journal labelled the light-year as a unit used by the Germans.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> <a href="/facts/Arthur_Eddington/b0jPBaSA">Eddington</a> called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> </p><p>Although modern astronomers often prefer to use the <a href="/facts/Parsec/SwSsk57J">parsec</a>, light-years are also popularly used to gauge the expanses of interstellar and intergalactic space. </p> <h2 id="usage-of-term">Usage of term</h2> <p>Distances expressed in light-years include those between <a href="/facts/Star/PFbd6ICx">stars</a> in the same general area, such as those belonging to the same <a href="/facts/Spiral_arm/hRT2dtxJ">spiral arm</a> or <a href="/facts/Globular_cluster/FL0EERJU">globular cluster</a>. Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and <a href="/facts/Galaxy_groups_and_clusters/saKzQDap">galaxy clusters</a> by millions of light-years. Distances to objects such as <a href="/facts/Quasar/HS96328l">quasars</a> and the <a href="/facts/Sloan_Great_Wall/ISF27bp0">Sloan Great Wall</a> run up into the billions of light-years. </p> List of orders of magnitude for <a href="/facts/Length/EwFyAqGH">length</a><table><tbody><tr><th>Scale (ly)</th><th>Value</th><th>Item</th></tr><tr><td>10−9</td><td>4.04×10−8 ly</td><td>Reflected sunlight from the <a href="/facts/Moon/zKHqEMPb">Moon</a>'s surface takes 1.2–1.3 seconds to travel the distance to the Earth's surface (travelling roughly 350000 to 400000 kilometres).</td></tr><tr><td rowspan="3">10−6</td><td>1.58×10−5 ly</td><td>One <a href="/facts/Astronomical_unit/gjsbu5xU">astronomical unit</a> (the distance from the <a href="/facts/Sun/CadPCVHh">Sun</a> to the Earth). It takes approximately 499 seconds (8.32 minutes) for light to travel this distance.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a></td></tr><tr><td>1.27×10−4 ly</td><td>The <a href="/facts/Huygens_probe/a4CxSs7N"><i>Huygens</i> probe</a> lands on <a href="/facts/Titan_(moon)/lg0l9PCH">Titan</a> off <a href="/facts/Saturn/D54LcGJ9">Saturn</a> and transmits images from its surface, 1.2 billion kilometres from Earth.</td></tr><tr><td>5.04×10−4 ly</td><td><i><a href="/facts/New_Horizons/G5XTU5DK">New Horizons</a></i> encounters <a href="/facts/Pluto/egq7MBeR">Pluto</a> at a distance of 4.7 billion kilometres, and the communication takes 4 hours 25 minutes to reach Earth.</td></tr><tr><td rowspan="2">10−3</td><td>2.04×10−3 ly</td><td>The most distant <a href="/facts/Space_probe/AeWRBITP">space probe</a>, <i><a href="/facts/Voyager_1/39jCxR9I">Voyager 1</a></i>, was about 18 light-hours (130 au,19.4 billion km, 12.1 billion mi) away from the Earth as of October 2014<a href="https://en.wikipedia.org/w/index.php?title=Light-year&action=edit">[update]</a>.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> It will take about 17500 years to reach one light-year at its current speed of about 17 km/s (38000 mph, 61 200 km/h) relative to the Sun. On 12 September 2013, NASA scientists announced that <i>Voyager 1</i> had entered the <a href="/facts/Interstellar_medium/mokU0WEz">interstellar medium</a> of space on 25 August 2012, becoming the first manmade object to leave the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>.<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a></td></tr><tr><td>2.28×10−3 ly</td><td><i>Voyager 1</i> as of October 2018, nearly 20 light-hours (144 au, 21.6 billion km, 13.4 billion mi) from the Earth.</td></tr><tr><td rowspan="7">100</td><td>1.6×100 ly</td><td>The <a href="/facts/Oort_cloud/wdP3VNxd">Oort cloud</a> is approximately two light-years in diameter. Its inner boundary is speculated to be at 50000 au ≈ 0.8 ly, with its outer edge at 100000 au ≈ 1.6 ly.</td></tr><tr><td>2.0×100 ly</td><td>Approximate maximum distance at which an object can orbit the <a href="/facts/Sun/CadPCVHh">Sun</a> (<a href="/facts/Hill_sphere/wutNvPyw">Hill sphere</a>/<a href="/facts/Roche_sphere/wutNvPyw">Roche sphere</a>, 125000 au). Beyond this is the deep ex-solar gravitational <a href="/facts/Interstellar_medium/mokU0WEz">interstellar medium</a>.</td></tr><tr><td>4.24×100 ly</td><td>The nearest known <a href="/facts/Star/PFbd6ICx">star</a> (other than the Sun), <a href="/facts/Proxima_Centauri/6vSeytbA">Proxima Centauri</a>, is about 4.24 light-years away.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a></td></tr><tr><td>8.6×100 ly</td><td><a href="/facts/Sirius/sHCkClse">Sirius</a>, the brightest star of the night sky. Twice as massive and 25 times more <a href="/facts/Luminosity/1tdhpk9l">luminous</a> than the Sun, it outshines more luminous stars due to its relative proximity.</td></tr><tr><td>1.19×101 ly</td><td><a href="/facts/Tau_Ceti_e/eHDy3PB7">Tau Ceti e</a>, an extrasolar candidate for a habitable planet. 6.6 times as massive as the earth, it is in the middle of the habitable zone of star <a href="/facts/Tau_Ceti/1tzg8cmQ">Tau Ceti</a>.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a><a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a></td></tr><tr><td>2.05×101 ly</td><td><a href="/facts/Gliese_581/BNGbGeGn">Gliese 581</a>, a red-dwarf star with several detectable exoplanets.</td></tr><tr><td>3.1×102 ly</td><td><a href="/facts/Canopus/zD5WcDy5">Canopus</a>, second in brightness in the terrestrial sky only to Sirius, a type A9 <a href="/facts/Bright_giant/qH3X0fuw">bright giant</a> 10700 times more luminous than the Sun.</td></tr><tr><td rowspan="4">103</td><td>3×103 ly</td><td><a href="/facts/A0620-00/R7aEtngp">A0620-00</a>, the second-nearest known <a href="/facts/Black_hole/1r2lmkcF">black hole</a>, is about 3000 light-years away.</td></tr><tr><td>2.6×104 ly</td><td>The <a href="/facts/Galactic_Center/TI4MwWwM">centre</a> of the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a> is about 26000 light-years away.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a></td></tr><tr><td>1×105 ly</td><td>The <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a> is about 100000 light-years across.</td></tr><tr><td>1.65×105 ly</td><td><a href="/facts/R136a1/4kMEN0aX">R136a1</a>, in the <a href="/facts/Large_Magellanic_Cloud/7VYAehNa">Large Magellanic Cloud</a>, the most luminous star known at 8.7 million times the <a href="/facts/Luminosity/1tdhpk9l">luminosity</a> of the Sun, has an apparent magnitude 12.77, just brighter than <a href="/facts/3C_273/P11TI6Ez">3C 273</a>.</td></tr><tr><td rowspan="4">106</td><td>2.5×106 ly</td><td>The <a href="/facts/Andromeda_Galaxy/MjmSLV6m">Andromeda Galaxy</a> is approximately 2.5 million light-years away.</td></tr><tr><td>3×106 ly</td><td>The <a href="/facts/Triangulum_Galaxy/etLCCGhP">Triangulum Galaxy</a> (<a href="/facts/Messier_object/YKz2MxzQ">M33</a>), at about 3 million light-years away, is the most distant object visible to the naked eye.</td></tr><tr><td>5.9×107 ly</td><td>The nearest large <a href="/facts/Galaxy_cluster/BhIEgbx8">galaxy cluster</a>, the <a href="/facts/Virgo_Cluster/dBsDKTzT">Virgo Cluster</a>, is about 59 million light-years away.</td></tr><tr><td>1.5×108 – 2.5×108 ly</td><td>The <a href="/facts/Great_Attractor/gFXDuMRg">Great Attractor</a> lies at a distance of somewhere between 150 and 250 million light-years (the latter being the most recent estimate).</td></tr><tr><td rowspan="3">109</td><td>1.2×109 ly</td><td>The <a href="/facts/Sloan_Great_Wall/ISF27bp0">Sloan Great Wall</a> (not to be confused with <a href="/facts/Great_Wall_(astronomy)/r8XpATsh">Great Wall</a> and <a href="/facts/Hercules%E2%80%93Corona_Borealis_Great_Wall/kszfcMlc">Her–CrB GW</a>) has been measured to be approximately one billion light-years distant.</td></tr><tr><td>2.4×109 ly</td><td><a href="/facts/3C_273/P11TI6Ez">3C 273</a>, optically the brightest <a href="/facts/Quasar/HS96328l">quasar</a>, of apparent magnitude 12.9, just dimmer than <a href="/facts/R136a1/4kMEN0aX">R136a1</a>. 3C 273 is about 2.4 billion light-years away.</td></tr><tr><td>4.57×1010 ly</td><td>The <a href="/facts/Comoving_distance/55cYbfbY">comoving distance</a> from the Earth to the edge of the visible universe is about 45.7 billion light-years in any direction; this is the comoving <a href="/facts/Radius/9Gr9deYD">radius</a> of the <a href="/facts/Observable_universe/CdTGMdsp">observable universe</a>. This is larger than the <a href="/facts/Age_of_the_universe/BaByputq">age of the universe</a> dictated by the <a href="/facts/Cosmic_background_radiation/NxByCI8f">cosmic background radiation</a>; see <a href="/facts/Observable_universe/CdTGMdsp">here</a> for why this is possible.</td></tr></tbody></table> <h2 id="related-units">Related units</h2> <p>Distances between objects within a <a href="/facts/Star_system/1vX99BMV">star system</a> tend to be small fractions of a light-year, and are usually expressed in <a href="/facts/Astronomical_unit/gjsbu5xU">astronomical units</a>. However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, the <a href="/facts/Light-second/0H0qncnG">light-second</a>, useful in astronomy, telecommunications and relativistic physics, is exactly 299792458 metres or 1/31557600 of a light-year. Units such as the light-minute, light-hour and light-day are sometimes used in <a href="/facts/Popular_science/cnhLw9rI">popular science</a> publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a><a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a> The <a href="/facts/Hayden_Planetarium/xvlAuKMC">Hayden Planetarium</a> specifies the light month more precisely as 30 days of light travel time.<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> </p><p>Light travels approximately one foot in a <a href="/facts/Nanosecond/gARLTp4c">nanosecond</a>; the term "light-foot" is sometimes used as an informal measure of time.<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Orders_of_magnitude_(length)/YzL1bHGX">1 petametre</a> (examples of distances on the order of one light-year)</li> <li><a href="/facts/Einstein_protocol/RGDtkvJD">Einstein protocol</a></li> <li><a href="/facts/Hubble%27s_law/FJYfELCP">Hubble length</a></li> <li><a href="/facts/Orders_of_magnitude_(length)/YzL1bHGX">Orders of magnitude (length)</a></li></ul> <h2 id="notes">Notes</h2> <h2 id="external-links">External links</h2> <ul><li> The dictionary definition of light-year at Wiktionary</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Mutel, R. L.; Aller, H. D.; Phillips, R. B. (1981). "Milliarcsecond structure of BL Lac during outburst". Nature. 294 (5838): 236–238. Bibcode:1981Natur.294..236M. doi:10.1038/294236a0. hdl:2027.42/62626. <a href="https://www.nature.com/articles/294236a0" target="_blank">https://www.nature.com/articles/294236a0</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units, retrieved 10 November 2013 <a href="https://www.iau.org/public/themes/measuring/" target="_blank">https://www.iau.org/public/themes/measuring/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Bruce McClure (31 July 2018). "How far is a light-year?". EarthSky. Retrieved 15 October 2019. <a href="https://earthsky.org/astronomy-essentials/how-far-is-a-light-year" target="_blank">https://earthsky.org/astronomy-essentials/how-far-is-a-light-year</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Bruce McClure (31 July 2018). "How far is a light-year?". EarthSky. Retrieved 15 October 2019. <a href="https://earthsky.org/astronomy-essentials/how-far-is-a-light-year" target="_blank">https://earthsky.org/astronomy-essentials/how-far-is-a-light-year</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units, retrieved 10 November 2013 <a href="https://www.iau.org/public/themes/measuring/" target="_blank">https://www.iau.org/public/themes/measuring/</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>One Julian year is exactly 365.25 days (or 31557600 s based on a day of exactly 86400 SI seconds)[5] <a href="/wiki/International_System_of_Units" target="_blank">/wiki/International_System_of_Units</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>The speed of light is precisely 299792458 m/s by definition of the metre. <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"Selected Astronomical Constants Archived 2014-07-26 at the Wayback Machine" in Astronomical Almanac, p. 6. <a href="http://asa.usno.navy.mil/static/files/2009/Astronomical_Constants_2009.pdf" target="_blank">http://asa.usno.navy.mil/static/files/2009/Astronomical_Constants_2009.pdf</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>International Astronomical Union, Measuring the Universe: The IAU and Astronomical Units, retrieved 10 November 2013 <a href="https://www.iau.org/public/themes/measuring/" target="_blank">https://www.iau.org/public/themes/measuring/</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>ISO 80000-3:2006 Quantities and Units – Space and Time <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>IEEE/ASTM SI 10-2010, American National Standard for Metric Practice <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>P. Kenneth Seidelmann, ed. (1992), Explanatory Supplement to the Astronomical Almanac, Mill Valley, California: University Science Books, p. 656, ISBN 978-0-935702-68-2 <a href="978-0-935702-68-2" target="_blank">978-0-935702-68-2</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Basic Constants, Sierra College <a href="http://astronomy.sierracollege.edu/Resources/Reference/basic%20Contants%20app2.htm" target="_blank">http://astronomy.sierracollege.edu/Resources/Reference/basic%20Contants%20app2.htm</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Marc Sauvage, Table of astronomical constants, archived from the original on 11 December 2008 <a href="https://web.archive.org/web/20081211001523/http://marc.sauvage.free.fr/astro_book/Cts_pages/astro.html" target="_blank">https://web.archive.org/web/20081211001523/http://marc.sauvage.free.fr/astro_book/Cts_pages/astro.html</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Robert A. 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