On this Good Friday, the Moon is Full and serves as a tape-measure of precession. But. after describing that, I got to worrying whether I had made the idea clear, or even whether I was getting it right. So I tried to simplify it into this diagram.
The ecliptic and the celestial equator are great circles around the celestial sphere, so either could be projected as a straight line; we are more accustomed to maps with the equator straight and the ecliptic curving over and under it, but for this purpose I make the ecliptic straight. It is the more fundamental line – the plane of Earth’s orbit. It stays fixed, and so do the starry constellations.
But, because of precession, the celestial equator slews gradually around, like a wobbling hula-hoop, so that the points where it intersects the ecliptic shift westward, at a rate of (roughly) 30° in 2,000 years.
The rightmost Sun in the picture is at the spring equinox point. It was there on March 20, entering the northern half of the ecliptic.
The Sun leftward of it is the Sun a month later – now. It actually does enter the constellation Aries, as now drawn, today. But it is in the position where the spring equinox point was about 2,000 years ago. That is why the equinox point is still called the First Point of Aries.
Reading on leftward, the first Moon symbol is at the other intersection, where the Sun is at our autumn equinox, September 23. The Moon is there when Full if this happens to coincide with the date of the spring equinox. Which is what it did, this year, to within less than 4 hours.
Finally, the leftmost Moon is our Full Moon a month later – now – happening so pleasantly to coincide not only with Good Friday but with the moment when the Sun enters the constellation Aries. Two millennia ago, this was the other equinox point, the “First Point of Libra”.
The 30° distance between the two Suns is the distance the spring equinox point has slid in two thousand years – but you can’t look at it. You can look at the Full Moon, and, with a bit of imagination, look at the distance between it and the September equinox point.
Perhaps a better way of “seeing” that imaginary point, the September equinox, is that it is just short of half way from Spica to Regulus.
This evening’s scene drawn again, at smaller scale so as to include Regulus at the top.
These diagrams are like slices out of the large sphere picture in the “Precession” section of the Astronomical Companion. We’d like to do without precession, perhaps, it makes us a little dizzy even if we understand it, but it has some deepness to contribute to space and time. It’s hard to grasp without seeing the Earth as a top, spinning and – what word do we have for the other part of the action? Toppling, staggering, wambling – not quite – reeling, maybe closer. Word needed. Imagine a daffodil in a breeze, tracing out the shape of a cone.
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DIAGRAMS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor. One way: right-click, and choose “View image”, then enlarge. Or choose “Copy image”, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers). I am grateful to know of what methods work for you.
(I’m catching up on a week’s worth of posts.)
I’ve gotten used to “seeing” the celestial equator, in my mind’s eye, as a symmetrical curve that arches across the sky from due east to due west. It’s always in the same place in the sky regardless of the season of the year or the time of day or night. If I were to move to a different latitude I would need to learn to see it lower or higher in the sky. Give or take a degree or two, the equator is marked by delta Orionis, gamma Virginis, and alpha Aquarii, and more broadly by several other reasonably bright stars.
The ecliptic curves above and below the equator, differently during the different seasons of the year. It is marked, of course, by the Sun and planets, and the Moon dances above and below the ecliptic over the course of a lunar nodical month. Regulus is right on the ecliptic, Spica and Aldebaran are close enough to sometimes be occulted by the Moon.
Precession doesn’t make me dizzy. A 26,000 year wobble is so slow as to be viscerally unnoticeable. Intellectually precession serves as a reminder that everything is interacting with everything else, the Sun, Moon, and planets (hello Jupiter) all tugging on the Earth. There is no fixed point of reference, no center to the universe. Within all this ebb and flow, we Earthlings are lucky to have such a large Moon orbiting roughly in the ecliptic plane, serving as an outrigger on our cosmic canoe, keeping our seasons much more consistent across the eons than poor Mars’.
Moon as “outrigger on our cosmic canoe”…
I so love your diagrams — they make so much sense to me.
I also love that you taught me a new word! Wambling!
My spell checker did not know “wambling” either.
It wanted to replace it with gambling.
Wobbling and wambling,
weaving and rolling,
wandering and toppling,
our Earth staggers though the universe.
Lovely!
I think I may have got “wambling” from “The Nine Tailors” (Dorothy Sayers). It isn’t in the dictionary, except with a difference sense – “feeling queasy in the stomach”.
Very nice description, thanks. I’ll try to look in the direction of the September equinox (between Regulus & Spica as you pointed out).
How about “circular wobble”?
I tried to visualize precession by observing a spinning coin but it didn’t work. I could see the axis of the spinning coin wobble as the coin slowed down, but the wobble was in the same direction as the rotation, whereas the Earth’s axis traces a clockwise wobble when viewed from above while the Earth is rotating counterclockwise.
I suppose that if the Earth’s wamble were counterclockwise it would be called postcession or regression.
From the Astronomical Companion:
“An analogy to this is that if you waggle a round stick counterclockwise in a round hole – for example a post you are trying to pull out of a post-hole – you will find it turning clockwise in your hands.”
This doesn’t get far into the science. I’ve had more experience of post hole digging than of physics.