Tomorrow morning you can see the Moon slicing through invisible planes in space.
You don’t really need to be out before dawn: the Moon will shine on into the daytime sky, keeping 90 degrees away from the Sun. But a night-time view lets us show these planes (the ecliptic and the celestial equator), along with some stars that can serve as guides to them (Fomalhaut and the Great Square of Pegasus).
What happens is that the Moon reaches right ascension zero at June 10 11:08 Universal Time, then reaches descending node on the same day, June 10 23:31 UT. These times are about 7 AM and PM in the eastern US, so they’re in the daytime anyway, later than the exact situation shown in the picture, but the situation won’t have changed much.)
What the events mean is that the Moon crosses from west to east of the zero line of right ascension (from which all positions in the sky are measured) and, only half a day later, crosses from north to south of the ecliptic (the plane of Earth’s orbit).
The ecliptic and the celestial equator cross each other at a spot called the Vernal Equinox Point. (Or, more quaintly, the First Point of Aries, because, thousands of years ago, before precession moved everything along, it was indeed situated at the western gates of that constellation; it’s now in Pisces.) This important and invisible point is the zero-point or origin for mapping everything in the sky. It is where the two planes cut, the equatorial plane in which Earth rotates and the ecliptic plane in which Earth travels. It’s the descending node of the equator through the ecliptic; or, equivalently, the ascending node of the ecliptic through the equatorial plane.
Node means “knot” (Latin nodus, which may or may not cognate with the English word – etymologists are unsure). It’s used for various sorts of point where string-like things cross or have lumps in them, such as your lymph nodes.
And, at this time, a third plane passes through, or almost exactly through, this same point: the plane in which the Moon is traveling. This point becomes a third node: the descending node of the Moon across the ecliptic. Thus the Moon descends through the ecliptic close to where the equator does. A three-way node, like some of the crossroads in FLorence where six streets meet!
Though these planes and their nodes are invisible, the stars try to help us see them. The eastern side of the Great Square of Pegasus points south through the First Point of Aries to the great southern star Fomalhaut. In other words the two Pegasus stars (whose names are Sirrah and Algenib) and Fomalhaut lie – roughly – on the zero line of right ascension. On June 10. the Moon joins them on that line. Of course it does so every time it comes around the sky, but not always does the line read: Sirrah, Algenib, Moon-very-close-to-First-Point-of-Aries, Fomalhaut.
The Moon’s orbital plane changes, and vastly more rapidly than the Earth’s does; so its descending node isn’t always here. It shifts back (westward) at a rate of about 19 degrees a year. As explained more fully on page 35 of Astronomical Calendar 2015, “This determines how far north and south the Moon ranges in the sky…” There are “hilly” years like 2006, when the Moon’s orbit ascends at about this point, therefore climbs quite far north above the northern ecliptic, and “flat” years, like 2015, when it stays nearer to the celestial equator.
Hey, I just hit “Control-I” when I meant to hit “Alt-I” – an easy slip to make in Photoshop – and accidentally “inverted” the colors! But it looks rather good that way too. This is how the diagram was meant to look:
You can see all these lines crossing each other: the Moon’s steep path of a year like 2006 and its smoother path of a year like 2015 crossing each other just about where the equator crosses the ecliptic.
(In case you wonder, such a diagram could be plotted with any one of the lines straight and the others curved, because they are all “great circles” around the celestial sphere.)
Actually, each Moon path in the diagram is for just one month in the given year. Every month it shifts a little way westward. As another diagram in the Astronomical Calendar (the Moon chart) shows, the track in January passed slightly north of the First Point of Aries; in June it is still doing so, more closely; and by December it will be passing on the other side.
As if all this weren’t enough coincidence, the Moon is, at almost the same time, at two other “knots” in its journey. It is at Last Quarter phase (meaning it is just 90 degrees west of the Sun) on June 9 at 15:42 UT. And it is at perigee, its nearest to Earth in its orbit, on June 10 at 4:44 UT.
Not a very near perigee, like those of February and September. But it’s worth looking out at this large Last Quarter Moon, shaped like a blade, as it slices through the main node of the sky, like Alexander’s sword slicing the Gordian Knot.