The coming Full Moon will be one of the nearest and largest-looking. The Full moment is April 27 at about 3:30 Universal Time, which is 4 or more hours earlier by American clocks, so the evening on which to gaze at it is that of Monday April 26.
This is one of the few times in the year when the Moon is at a climax of nearness to us, because its perigee (nearest-to-Earth moment in its elliptical orbit) almost coincides with a “syzygy,” that is, a New or Full moment. For at either of these syzygies (voluptuous word!) the Sun is lined up with Moon and Earth and adds its lesser tidal pull; so the perigee is an extreme perigee.
To compare these occasions, here are the Universal Times of perigee and of the Full or New moment, with the difference in hours, and the approximate minimum distance of the Moon:
. perigee diff. distance
Apr 27 15:20 Full 3:32 -11.8hr 357,400 km
May 26 1:46 1Full 1:14 9.5hr 357,300 km
Dec 4 10:12 New 7:43 -2.5hr 356,800 km
This large graph of the Moon’s distance throughout the year is like those I used to print in the Astronomical Calendar, on the pages of which there was plenty of space. I’ve experimented with another route to get the graphic into a screen form. I hope you can zoom in on it to see detail. (See the end note about enlarging illustrations.)
The zigzagging curve showing the Moon’s distance is black when the Moon is north of the ecliptic, blue when south of it; and thicker when farther north or south. The scale is in Earth-radii (units of the Earth’s equatorial radius of 6,378 km). The Moon is drawn, to scale, at the moments of its cardinal phases – New (dark side toward us), First Quarter (sunlit side to west), Full (sunlit side toward us), Last Quarter (sunlit side to east).
The two large smooth curves connect New Moons, and Full Moons, showing the overall rhythm.
You can see that the April 27 and May 26 Full Moons constitute a sort of twin peak of nearness; the second will be slightly nearer. Similarly the Dec. 4 New Moon will be followed by an almost as near New Moon in January 2022.
The wave-like rhythm happens every year, displaced in timing. In 2020, the near-coincidences of perigee and syzygy fell on April 7 (Full) and October 16 (New), also with almost-as near moments at adjacent perigees.
Well, here is where the Full Moon looms as she stares down at her big-sister planet in the night between April 26 and 27.
Sublunary Department
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ILLUSTRATIONS 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). Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.
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This weblog maintains its right to be about astronomy or anything under the sun.
I understand why the Full Moon at perigee is closer to the Earth than when the Moon is at perigee during other phases — both the Earth and the Sun are pulling the Moon in the same direction. It’s harder to understand why the New Moon at perigee would be closer than usual, since the Earth and the Sun are pulling the Moon in opposite directions. If anybody could explain this I would be grateful.
I don’t think Vladimir Putin cares what I think about Alexei Navalny, but one does what one can.
It’s not really a “pulling”: it isn’t that the center of Earth moves toward Moon or Sun. The tidal effect of the Moon is the *difference* between the gravitational force it exerts on Earth’s nearer and farther sides. This difference is greater when the Moon is nearer. Earth is slightly stretched along that axis. So there are high tides on *both* sides of Earth. The stretching happens mostly in the fluid sphere. The Sun does a similar stretching. So when Moon and Sun are on the same axis, the stretchings are added.
The small things we do add up and sometimes tip a balance.
I understand high tides on both sides of the Earth, toward and away from the Moon. I can fit that into my “pulling” paradigm. The Moon pulls on the water on the close side of the Earth more than it pulls on the Earth’s center. The Moon pulls on the center of the Earth more than it pulls on the water on the far side of the Earth.
I still don’t understand why a new Moon at perigee would be closer than an average perigee. Maybe because the Moon is in line with an exceptionally high spring tide? And thanks to the inverse square law, that closeness exerts a stronger pull on the Moon than the Sun’s pull in the opposite direction?
I’m not trying to be obstinate, I just understand gravity as two objects pulling on one another, the strength of the pull being proportional to their masses and to the inverse square of the distance between them. If it was good enough for Newton, it’s good enough for me. I’ve seen the rubber sheet diagram illustrating general relativity and that makes sense too, in a general way (no pun intended), but I don’t think we’re talking about relativistic effects here.
Gravitational attraction falls off in proportion to the square of the distance. So when the Moon-Earth distance is greater, the difference between the attraction on the near and far sides of the Earth is less.
Yes, I think relativity doesn’t enter into it. But, as I’ve admitted before, I’m self-taught and still self-teaching in mathematics, and I’m nowhere near ready to explain anything relativistic.
In an interesting coincidence, here’s an article posted today on the Sky and Telescope website.
https://skyandtelescope.org/astronomy-news/dark-matter-wake-in-the-milky-ways-outer-reaches/
The Large Magellanic Cloud dwarf galaxy’s orbit around the Milky Way Galaxy creates a bimodal tidal distribution of stars and, presumably, dark matter, in the Milky Way’s halo. One blob is on the same side of the Milky Way’s disk as the LMC, and a large more diffuse blob is on the opposite side.
As above, so below.
Anthony, I thought the reason for the lunar perigee being closer than average during new Moon was that instead of thinking that the Earth is pulling the Moon away from the Sun, think that the Sun and Moon are united in pulling the Earth toward them, hence a particularly close perigee.
Your diagrams are outstanding as usual Guy! Unfortunately the December total solar eclipse is only going to be visible in the Antarctic region, because with the coincidence of lunar perigee and syzygy, the duration of that eclipse would have to make it one of the best ever if visible from a temperate or equatorial region. For us here in the U.S., we’ll have to wait for the annular in 2023 and the total one in 2024. You won’t be too far from the 2026 total eclipse, followed in 2027 by a saros series successor to the 1991 eclipse in Mexico, and then a good annular one in 2028. Spain gets to see all of them!
https://eclipse.gsfc.nasa.gov/SEatlas/SEatlas3/SEatlas2021.GIF
Guy, you have been a hero and huge resource for me and my school over the years….but—-I hope you will soon see what a scam this whole Covid narrative is for what it is—-a total scam.
Giamario, your url (thegiamarioapproach.com) seems not to work.
It works for me: http://www.thegiamarioapproach.com