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I recently read this article: http://www.cbsnews.com/news/supermoon-lunar-eclipse-coming-this-month/ I'm having some trouble understanding how a "supermoon" lunar eclipse is possible. enter image description here

Judging from this diagram, it would seem that the moon always reaches its apsides above or below the orbital plane of the earth around the sun, making any kind of eclipse at these points impossible. Can someone clarify for me?

Thank you.

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  • $\begingroup$ The moon reaches apsides about twice a month (one apogee and one perigee), and the diagram above appears to be for an entire year? The moon reaches perigee every 26 days or so [this is a very rough number], and is full every 29.5 days or so, so they can occur at the same time. When they both occur AND we the moon is at ascending/descending node, we have a supermoon eclipse. $\endgroup$ – barrycarter Sep 5 '15 at 1:23
  • $\begingroup$ @barrycarter Thanks for this comment. I guess what I'm confused about is that the diagram above shows the ascending and descending nodes along the minor axis of the lunar ellipses. Doesn't the moon's perigee and apogee occur at the vertices of the major axis of the ellipse? If so, it seems from this figure that a moon's perigee/apogee could never coincide with a lunar node. $\endgroup$ – Steve Sep 5 '15 at 3:14
  • $\begingroup$ The diagram is in some form of perspective and does not represent the shape of the Earth's or the Moon's orbits (nor is it to scale) $\endgroup$ – Conrad Turner Sep 5 '15 at 4:42
  • $\begingroup$ This image is highly misleading and is not meant to represent Earth's orbit around the Sun. It's actually part of a larger image: pages.uoregon.edu/jimbrau/BrauImNew/Chap01/FG01_27.jpg which might be slightly more helpful. $\endgroup$ – barrycarter Sep 5 '15 at 13:11
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Lunar Eclipses occur when a full moon occurs when the the moon crosses the ecliptic. The ecliptic is the plane in which the Earth orbits the sun, and so also the track which the sun appears to trace in the sky relative to the stars.

It appears from the diagram that there are two nodes each month (ascending and descending), but this is only approximately correct. The influence of the sun causes the nodes to move.It takes 29.5 days between two full moons, but only 27.2 days between two ascending and descending nodes). This means that a node can occur on any day of the year, and any day of the month.

A "supermoon" occurs when the moon when it is closest to the earth, the perigee. This point of perigee also moves, so the perigee can also occur at any time during the month, and the time of perigee is not related to the time of the node. Unlike terms like "node" which astro(nom|log)ers have used for many years, Supermoon is a neologism.

Thus is is possible, but rare for a full moon, a lunar node, and a perigee to occur at about the same time, and if it does, a supermoon eclipse will occur.

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  • $\begingroup$ Thank you for this response. When you say that the nodes move, are you referring to lunar precession? Is that the process that allows the perigee/apogee to sometimes coincide with the line of nodes, resulting in a supermoon eclipse? $\endgroup$ – Steve Sep 6 '15 at 13:46
  • $\begingroup$ Yes. Nodal precession. $\endgroup$ – James K Sep 6 '15 at 17:40

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