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Q: Timing of Moon Phases ( Answered ,   1 Comment )
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 Subject: Timing of Moon Phases Category: Science > Astronomy Asked by: tombo06-ga List Price: \$25.00 Posted: 15 May 2006 05:48 PDT Expires: 14 Jun 2006 05:48 PDT Question ID: 728949
 ```Why is the time between moon phases irregular? For example, between Full Moon on 2006 Jan 14 09:48 UTC and Last Quarter on 2006 Jan 22 15:14 UTC there are 8 days, 5 hours and 26 minutes. However, between Full Moon on 2006 June 11 18:03 UTC and Last quarter on 2006 June 18 14:08 UTC there are only 6 days, 20 hours and 5 minutes. That's over a day of difference! However, isn't the moon phase cycle supposed to be 29.53 days? I know what libration is - but wouldn't libration affect only the angle at which we see the moon, not the light/shadow on the moon itself (and hence its phase)?```
 ```Hi tombo06-ga, Because of the eccentricity of the moon's orbit, its distance from the surface of the earth varies throughout the cycle of its orbit. The closest approach ("perigee") is 363,104 km and the furthest retreat ("apogee") is 405,696 km. Around perigee (when the gravitational force between earth and moon is strongest), the moon moves faster in its orbit, having a maximum orbital speed of 1082 m/s. Around apogee (when the gravitational force is weakest), the moon moves slower, having a minimum orbital speed of 968 m/s. Therefore, at perigee the moon will move more rapidly from the angle at which a given phase occurs to the angle at which the next phase occurs (a sun-moon angle of 180 degrees for full moon, 90 degrees for quarter moon and 0 degrees for new moon). At apogee the moon will move more slowly from phase to phase. You can view the dates of perigee and apogee for 2006 at the following page: "The Sun, The Moon, Earth and Eclipses" http://www.delscope.demon.co.uk/news/skywatch1SMEE.htm (scroll to the section titled "APSIDES OF THE MOON") There you will see that an apogee occurred on January 17 2006 (during your "slow" phase change) and a perigee occurred on June 16 2006 (during your "fast" phase change). The variation in the orbital motion also manifests itself as longitudinal libration. Because the moon's rotation is constant but its orbital motion is not, one of them can "get ahead of" the other or behind it, enabling us to see a few degrees around the edges of the closest lunar hemisphere. Libration does affect the perceived phase, because although the terminator (line between dark and light) is not changed by the angle at which we view it, the terminator will occur in a different part of the perceived disk if we are looking from a different angle. A great animation of this can be found by clicking the "animation" link on the following page: "The Moon's Orbit" http://home.insightbb.com/~lasweb/lessons/moonorbit.htm I trust you find this answer satisfying, otherwise please request clarification. Regards, eiffel-ga Additional links: "Moon - Wikipedia" http://en.wikipedia.org/wiki/Moon "Libration of the Moon" http://www-spof.gsfc.nasa.gov/stargaze/Smoon4.htm Google Search Strategy: "moon phases" "varies because" ://www.google.com/search?q=%22moon+phases%22+%22varies+because%22 "lunar orbit" ://www.google.com/search?hl=en&q=%22lunar+orbit%22 "lunar orbit" libration ://www.google.com/search?hl=en&q=%22lunar+orbit%22+libration``` Request for Answer Clarification by tombo06-ga on 15 May 2006 10:06 PDT ```You know that an answer is good when it seems obvious in retrospect. Great job! I'd like one clarification though: I asked "isn't the moon phase cycle supposed to be 29.53 days?" Obviously, the differences due to speed at perigee/apogee would cancel themselves out at the end of each full cycle, and it makes sense that its duration would be a regular 29.53 days. But, when calculating times from full to full moon during my "fast" and "slow" phase changes, there is still a difference: From and including: Saturday, January 14, 2006 at 9:48:00 AM To, but not including : Monday, February 13, 2006 at 4:44:00 AM The duration is 29 days, 18 hours, 56 minutes and 0 seconds From and including: Sunday, June 11, 2006 at 6:03:00 PM To, but not including : Tuesday, July 11, 2006 at 3:02:00 AM The duration is 29 days, 8 hours, 59 minutes and 0 seconds There's still a ~10 hour difference. Why? And the strict answer to "is the moon phase cycle 29.53 days?" would be NO, right? I'd like to know what's its variation, if that's still within the scope of my question. (an answer like "from 29.49 to 29.54 days" would be great) I'm asking this because I'm working on a lunar calendar by the way. Great answer otherwise, thanks!``` Clarification of Answer by eiffel-ga on 15 May 2006 14:18 PDT ```Hi tombo06-ga, I agree with rracecarr-ga's comment that the variation in time from one full moon to the next is primarily due to the variation in the earth's orbital speed around the sun due to its non-circular orbit. A standard figure for the mean lunar month is 29.5305888531 days, or 12.369 cycles per year. The time from one new moon to the next is called a lunation, or synodic month. A web page at Eric Weisstein's World of Astronomy states that "any particular phase cycle may vary from the mean by up to seven hours": http://scienceworld.wolfram.com/astronomy/SynodicMonth.html On that same page is a formula for calculating the length of any specific lunar month, given a Julian date. The following web page explains how to find the Julian date corresponding to a specific lunation: Lunation http://scienceworld.wolfram.com/astronomy/Lunation.html That page also refers to a table by Meeus of every lunation from 1900 to 2100, during which time the shortest lunation is 29 days 06 hours 35 minutes and the longest is 29 days 19 hours 55 minutes. I trust you find this additional information useful. Regards, eiffel-ga```
 tombo06-ga rated this answer: `Great answer, clarification, and comment. Thanks!`
 ```Full moon comes when the sun-earth-moon are lined up as close to a straight line as they get during a partular lunar cycle. The angular speed of the earth around the sun is important, as well as the angular speed of the moon around the earth. Just like the moon around the earth, the earth moves in an elliptical orbit around the sun, sometimes closer and faster, and sometimes farther and slower. In January, the earth is closest to the sun, so moves the fastest, and so the extra amount the mooon has to move in its orbit to regain a straight line configuration is maximum. So the time from one full moon to the next is longest in January.```