Daniel Cummings, famous for his Moon Hat and other neat products, set me a puzzle, which my brain was too weary to tackle.
Suppose you want to be as close to the Moon as possible. You want to stand at a point on Earth when that point is as near to any point on the surface of the Moon as is possible. To find those points, on Earth and Moon, and the time when you need to be on that Earth-point, there are 10 factors you’d have to consider.
For instance, one of them is that the top of Mount Chimborazo in Ecuador is the farthest point on Earth’s surface from Earth’s center. (Being nearer to the equator than Everest is, it is farther from the center.)
When you’ve had a stab at listing the other factors, Dan will tell us them all.
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Daniel, thanks to you and Guy for the thought-inspiring topic! In every astronomical calendar, Guy includes a sine-wave like diagram of the Moon’s distance each year, overlaid with the new Moon and full Moon dates. From those diagrams, it is clear that the lunar perigees that are the closest to Earth are the ones that coincide with new and full Moon, so clearly to achieve your goal, the Moon would have to be at one of those points. Because you stated that Mount Chimborazo, almost on the equator, is the closest point on Earth to the Moon, the Moon would have to be overhead at that point, which means that the new or full Moon would have to occur almost exactly on the celestial equator, which means it could only occur on or about Mar 21 or Sept 21. I also think it is true that the Moon would be closer to Earth if it was at either ascending or descending node in its orbit than if it were north or south of the ecliptic, hence my thought the line of nodes should pass through the equinox points as well.
Hi Eric,
Thanks for clarifying! That all makes sense to me now. The Moon’s distance chart you mention is on page 93 of the 2023 Astronomical Calendar. Guy also includes a table of the Moon’s perigee and syzygy, calling that out as a correspondence as you did.
When Earth equinox dates coincide with the Moon’s node crossings and full or new Moon, these would naturally be eclipse dates…
Here is a page I reference for those crossings that indicates eclipse dates: http://www.astropixels.com/ephemeris/moon/moonnodes2001.html It indicates that there will be an ascending node total solar eclipse on March 20th, 2034 https://en.wikipedia.org/wiki/Solar_eclipse_of_March_20,_2034
That date 2034-03-20 is not the closest lunar perigee (it is near a perigee on 2034-03-21). Interestingly, that 2034 eclipse begins at the equator. :-)
Guy gives these data points for the upcoming Jan 21, 2023 lunar perigee – and I see that 356,596km is the closest lunar perigee for at least 10 years:
Jan 21 20:55 Jan 21 20:54 new -0.0 356,596
Since the orbit is the biggest contributor to the Moon’s distance, Jan 21, 2023 would be a good time to travel to Ecuador and climb a volcano!
FYI – I referenced this page to see 10 years of future perigee distance estimates:
For 2023: https://www.timeanddate.com/astronomy/moon/distance.html?year=2023&n=179
For 2034: https://www.timeanddate.com/astronomy/moon/distance.html?year=2034&n=179
Cheers!
D
Hi Everyone! Daniel Cummings here. Thanks to Guy for sharing my Moon puzzle – and highlighting my Moon Hat invention.
This Moon riddle came out of a series of short Astronomy-oriented phrases that I use to encode specific facts in a poetic way – I call them “Astronomy Koans” and this is one of them: “Close to the Moon but not as close as I can be.” It’s the shortest way I could express this puzzle!
I have put together an article that has all of the elements that I came up with. I did a fair amount of research to understand this geometric relationship (references at the bottom of the article), but I am not sure that I captured everything. For instance, I see suggestions in the comments here that did not occur to me… specifically a notion about Earth equinox dates affecting the distance between Earth and Moon. I am not sure of the import of that specific geometry – maybe someone here could elaborate?
Thank you all for discussing this with such enthusiasm!
https://starinastar.com/get-closer-to-the-moon-without-leaving-the-earth/
Allen hit on one key point that I missed, which is that:
7. The date must also be March 21 or Sept 21 (to within a day I suppose)
Also I probably am not correct in saying that Mons Huygens has to be pointed at Earth. Wikipedia says that is the highest point on the Moon, but its latitude is 19 deg, so I am guessing that libration will never bring it to the point that it is the closest point on the Moon to us, so maybe better would be:
6. Whichever point on the Moon is the highest within the band of latitudes and longitudes that could be pointed directly at Earth due to libration must be oriented that way at the moment we are looking at it.
I was about a foot from the moon in the 1970’s. I saw a traveling display of moon rocks put on by NASA.
I’m certainly a neophyte at examining questions of geometry of the geoid, but I’m not entirely convinced that it has to be the summit of Mt. Chimborazo. Mt. Huascaran is 500 m taller and not very far south of the Equator. Is the earth that far out of round? OK, now I’ve read Wikipedia and am convinced. To be closest to the sun one would have to be standing on Chimborazo at solar noon a few days before the spring equinox or a few days after the autumn equinox on the date when the sun at its zenith is 1 degree south of the celestial equator (Chimborazo is 1 degree south of the terrestrial equator). But the plane of the moon’s orbit is 5 degrees off of the plane of earth’s orbit. Maybe that is the point of Eric’s item #3, which I can’t quite parse. Apologies for thinking while writing, and not getting very far.
Just a reminder, you are over 6,000 km closer to the Moon when it crosses the meridian than when it is on the horizon.
1. Must be on Mount Chimborazo
2. Moon must be at perigee
3. Nodes of Moon’s orbit must lie along the March 21 – Sept 21 axis of Earth’s orbit
4. Moon must be new or full
5. It must be either noon (new Moon) or midnight (full Moon)
6. Libration state must be such that Mons Huygens is pointed at us
7. You must have a taller step ladder than anyone else on the mountain
8. You must be taller than anyone else on the mountain
Can’t wait to read the real list!