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Opposition (astronomy)
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<h2 id="average-interval-between-oppositions">Average interval between oppositions</h2> <p>When two planets are on the same side of the Sun then from the point of view of the interior planet the other planet is in opposition with the Sun. When two planets are on opposite sides of the Sun, there is an opposition from the point of view of the Sun. In either case, the interval between two such occurrences involving the same two planets is not constant because the orbits are not circular and because the planets perturb one another. But the average interval between them can be calculated from the periods of the two planets. The "speed" at which a planet goes around the Sun, in terms of revolutions per time, is given by the inverse of its period, and the speed difference between two planets is the difference between these. Since the time interval between two oppositions is the time it takes for 360° to be covered by that speed difference, the average interval is: </p> 1 | 1 / p 1 − 1 / p 2 | {\displaystyle {\frac {1}{|1/p_{1}-1/p_{2}|}}} <p>The following table gives these average intervals, in <a href="/facts/Julian_year_(astronomy)/EVdRNUe3">Julian years</a> (of 365.25 days), for combinations of the nine traditional planets. Since <a href="/facts/Pluto/egq7MBeR">Pluto</a> is in resonance with <a href="/facts/Neptune/1eucwHIC">Neptune</a> the period used is 1.5 times that of Neptune, slightly different from the current value. The interval is then exactly thrice the period of Neptune. </p> Average interval between oppositions<table><tbody><tr><td>Planet</td><td></td><td>Mercury</td><td>Venus</td><td>Earth</td><td>Mars</td><td>Jupiter</td><td>Saturn</td><td>Uranus</td><td>Neptune</td><td>Pluto</td></tr><tr><td></td><td>Period</td><td>0.241</td><td>0.615</td><td>1.000</td><td>1.881</td><td>11.863</td><td>29.447</td><td>84.017</td><td>164.791</td><td>247.187</td></tr><tr><td>Mercury</td><td>0.241</td><td></td><td>0.396</td><td>0.317</td><td>0.276</td><td>0.246</td><td>0.243</td><td>0.242</td><td>0.241</td><td>0.241</td></tr><tr><td>Venus</td><td>0.615</td><td>0.396</td><td></td><td>1.599</td><td>0.914</td><td>0.649</td><td>0.628</td><td>0.620</td><td>0.618</td><td>0.617</td></tr><tr><td>Earth</td><td>1.000</td><td>0.317</td><td>1.599</td><td></td><td>2.135</td><td>1.092</td><td>1.035</td><td>1.012</td><td>1.006</td><td>1.004</td></tr><tr><td>Mars</td><td>1.881</td><td>0.276</td><td>0.914</td><td>2.135</td><td></td><td>2.235</td><td>2.009</td><td>1.924</td><td>1.903</td><td>1.895</td></tr><tr><td>Jupiter</td><td>11.863</td><td>0.246</td><td>0.649</td><td>1.092</td><td>2.235</td><td></td><td>19.865</td><td>13.813</td><td>12.783</td><td>12.461</td></tr><tr><td>Saturn</td><td>29.447</td><td>0.243</td><td>0.628</td><td>1.035</td><td>2.009</td><td>19.865</td><td></td><td>45.338</td><td>35.855</td><td>33.430</td></tr><tr><td>Uranus</td><td>84.017</td><td>0.242</td><td>0.620</td><td>1.012</td><td>1.924</td><td>13.813</td><td>45.338</td><td></td><td>171.406</td><td>127.277</td></tr><tr><td>Neptune</td><td>164.791</td><td>0.241</td><td>0.618</td><td>1.006</td><td>1.903</td><td>12.763</td><td>35.855</td><td>171.406</td><td></td><td>494.374</td></tr><tr><td>Pluto</td><td>247.187</td><td>0.241</td><td>0.617</td><td>1.004</td><td>1.895</td><td>12.461</td><td>33.420</td><td>127.277</td><td>494.374</td><td></td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Astronomical_transit/4s6Mll1a">Astronomical transit</a></li> <li><a href="/facts/Conjunction_(astronomy)/TgujumHI">Conjunction</a></li> <li><a href="/facts/Phase_angle_(astronomy)/rPhG45gN">Phase angle</a></li> <li><a href="/facts/Spherical_astronomy/MIs7fBeu">Spherical astronomy</a></li> <li><a href="/facts/Syzygy_(astronomy)/77KkThLa">Syzygy</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://web.archive.org/web/20181005171356/http://britastro.org/computing/charts_asteroid.html">Asteroids around opposition</a> – British Astronomical Association – Computing Section.</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>U.S. Naval Observatory Nautical Almanac Office (1992). P. Kenneth Seidelmann (ed.). Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. p. 733. ISBN 0-935702-68-7. <a href="0-935702-68-7" target="_blank">0-935702-68-7</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Newcomb and Holden (1890), p. 115 <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Newcomb, Simon; Holden, Edward S. (1890). Astronomy. pp. 115, 273. <a href="https://books.google.com/books?id=gek3AAAAMAAJ" target="_blank">https://books.google.com/books?id=gek3AAAAMAAJ</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Moulton, Forest Ray (1918). An Introduction to Astronomy. pp. 255, 256. <a href="https://books.google.com/books?id=PJoUAQAAMAAJ&q=astronomy+moulton" target="_blank">https://books.google.com/books?id=PJoUAQAAMAAJ&q=astronomy+moulton</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Newcomb and Holden (1890), p. 334 <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>see references at opposition surge. <a href="/wiki/Opposition_surge" target="_blank">/wiki/Opposition_surge</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Moulton (1918), p. 191 <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> </ol>