How close can I get to the Moon while still staying on the Earth’s surface?
I frame this as a sort of astronomy poem I call an Astronomy Koan. These are short sayings that contain astronomy puzzles. The answers to these puzzles carry insights into physical astronomy concepts.
Guy Ottewell has posted an article at UniversalWorkshop.com about getting close to the Moon. https://www.universalworkshop.com/2022/12/11/how-to-touch-the-moon-almost/
“I am close the the Moon but not as close as I can be.” This Astronomy Koan invites the reader to consider all of the ways which an Earth-bound observer might get as close to the Moon as humanly possible – without leaving the surface of the Earth.
The Earth and Moon are moving, I am moving
The Moon and the Earth are two oblate spheroids interacting in complex ways.
The Earth-bound observer is able to move around on the surface of the Earth while we calculate the “closest” point of the Moon to be the surface point that is closest to a surface point on Earth.
The goal of the Earth-bound observer is to find the location and time where they will be physically closest to the Moon.
Motions of the Moon that bring the Moon closer and further to your “closest personal point”
- Daily apparent revolution – every 24h 50m wherever we are on the Earth surface, the Moon transits (crosses the meridian) and reaches the “closest personal point” at that time.
- The apsides of the Moon’s orbit – the Moon’s orbit is slightly elliptical and has an apogee (furthest point from Earth) and a perigee (closest point to Earth). If a perigee coincides with a Moon transit, this brings the Moon even closer to the surface of the Earth. Approximately 357000km (perigee) and 406000km (apogee)
- The inclined orbital track of the Moon. Close to the ecliptic, but an additional 5 degrees offset, this gives the Moon an opportunity to be closest to people located on the surface of the Earth who are slightly north (up to 5.14 degrees) of the Tropic of Cancer and south of the Tropic of Capricorn.
- The Moon’s orbit is slowly moving away from the Earth (by 3.8cm per year, about 38km in 1million years) – so the longer you wait, the further away it will be!
- Surface of the Moon orientation toward my location – East-west libration moves the “orientation” of the Moon as it relates to the Earth
- The surface of the Moon has crater walls and valleys. The closest point would be at the height of a crater wall or central peak. Hipparchus crater or Triesnecker crater seem like likely candidates because they are “central”, but this is beyond my understanding of the topology of the Moon surface and how it might interact with the orientation changes caused by libration, nutation, and the inclined orbit.
My motions on the Earth that can get me closer to the Moon
- The transit can be made closer by moving closer on the surface of the Earth to the current declination of the Moon. Generally speaking, that involves going toward the equator, but it get complicated by the fact that the orbit is inclined to the equator and ecliptic.
- The transit can be made closer by going to a higher elevation
- The transit can be made closer by going to the top of a mountain close to the equator (Mt. Chimborazo as opposed to a tall mountain like Everest that is not near the Equator)
The “ideal” situation that would bring a human on the surface of the Earth as close as can be to the Moon would be
Stand at the top of Mt. Chimborazo, at the moment of Moon transit, at the exact orbital perigee, with Hipparchus crater wall oriented toward Mt. Chimborazo, as soon as possible!
We looked at all of the ways that the Earth and Moon approach and recede from each other. The goal is to understand more directly when I am close to the Moon and observe the motions that change that distance.
Please let me know if I have missed anything here! Add in comments below.
NASA Moon page: https://svs.gsfc.nasa.gov/5048 – has “an animated diagram of the subsolar and sub-Earth points for 2023” Indicates the general sweep of the “closest Moon-Earth point” as it traces out irregular shapes on the surface of the Moon.
NOAA answers a question about the “tallest” mountain, Mt. Chimborazo: https://oceanservice.noaa.gov/facts/highestpoint.html
ASC page with information about the various motions of the Moon: https://www.rasc.ca/motions-moon
Wikipedia Orbit of the Moon article: https://en.wikipedia.org/wiki/Orbit_of_the_Moon
Wikipedia Lunar Distance (measure the distance from the center of the Earth to the center of the Moon): https://en.wikipedia.org/wiki/Lunar_distance_(astronomy)
Moon orbit distance growing: https://astronomy.com/magazine/ask-astro/2022/08/ask-astro-how-quickly-is-the-moon-moving-away-from-earth
Quora Answer that mentions Mt. Chimborazo: https://www.quora.com/Where-on-Earth-is-the-closest-point-to-the-Moon
2 Replies to “Get closer to the Moon without leaving the Earth”
Interesting but a bit too esoteric for me
Thanks for the note! Basically, you are as close as you can be to the Moon once per day when it is directly overhead (as we rotate underneath it) and then you can get even closer when the Moon is at perigee (closest point in its orbit around the Earth). The other motions are interesting but have less of an effect on how close you are to the Moon at any given time.