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"Selene and Endymion" by Sebastiano Ricci (1659-1734 CE; Italian), from 1713. Selene was the Greek mythological personification of the Moon, known as Luna in the Roman mythology. She is typically depicted in artwork with the tell-tale crescent Moon on her forehead.
The familiar Moon and...
The familiar Moon and...
The near side of the Moon. This is the view we always see from Earth, because the Moon is tidally locked in its orbit - gravitational interactions between Earth and the Moon have caused the rotational period (day/night) of the Moon to be the same length as its orbital period around Earth.
Credit: Gregory H. Revera, CC BY SA 3.0 [link]...the unfamiliar Moon
...the unfamiliar Moon
The far side of the Moon is distinguished by extensive cratering and the almost complete absence of dark maria. The latter characteristic is probably linked to the fact that the far side crust is somewhat thicker than on the near side. This could have prevented the seepage of magma from the molten interior of the Moon into large craters left by giant impacts during the Late Heavy Bombardment period of the early Solar System (which happened 3.8-4.1 Gyr ago, when Earth was only about 500 million years old).
Credit: NASA/GSFC/Arizona State University [linkWalking on the Moon
Walking on the Moon
The Moon has the distinction of being the only place in our Solar System other than Earth to be visited by humans. NASA's Apollo program landed astronauts on the Moon six times during 1969-1972 with the Apollo 11 through Apollo 17 missions.
Apollo 13, launched in April 1970, would have been the third Moon landing. However, the landing was aborted after an oxygen tank in the spacecraft Service Module exploded two days into the mission, disabling its electrical and life support systems. Supported by backup systems on the Lunar Module, the crew looped around the Moon and were able to return safely to Earth.
NASA's Artemis program plans to return humans to the Moon in 2028, for the first time since 1972. The first crewed mission of this program, Artemis II, launched in April 2026 on a lunar flyby mission. The astronauts of Artemis II broke the record of most distant travel from Earth previously set by the astronauts of Apollo 13.
Map of Apollo landing sites on the Moon. The Artemis missions, starting with Artemis III, will land near the Moon's south pole.
Credit: NASA/Goddard Space Flight Center Scientifiic Visualization Studio [link]
This frame from a video camera mounted on the Apollo 11 Lunar Module "Eagle" captures the last moment in history before humans had set foot on another world. Neil Armstrong (1930-2012), Commander of Apollo 11, became the first human to step onto the surface of the Moon on 20 July 1969.
Credit: NASA [link]
This is the expression on your face after you become the first human to set foot on another world. Photo of Neil Armstrong back inside the Lunar Module "Eagle" taken by the second human to walk on the Moon, astronaut Edwin "Buzz" Aldrin.
Credit: NASA [link]
This is not a photograph of the first footprint on the Moon. It's not even any of Neil Armstrong's footprints. Buzz Aldrin made this footprint on a pristine surface so that he could then photograph it for study by soil mechanics experts back on Earth, to learn about the properties of the lunar surface.
Credit: NASA [link]
Apollo astronauts made many footprints on the Moon. In this photo, astronaut James B. Irwin, Lunar Module pilot, gives a military salute while standing beside the deployed U.S. flag during the Apollo 15 lunar surface extravehicular activity (EVA) at the Hadley-Apennine landing site. The Lunar Module "Falcon" is visible on the right, as is the first Lunar Rover vehicle. Hadley Delta in the background rises approximately 4,000 meters (about 13,124 feet) above the lunar plain. The base of the mountain is approximately 5 kilometers (about 3 statute miles) away. This photograph was taken by Astronaut David R. Scott, Commander of Apollo 15 on 1 August 1971.
Credit (image and some text): NASA [link]Water on the Moon
Water on the Moon
Distribution of water ice on the lunar surface at the Moon's south pole (left) and north pole (right), detected by NASA's Moon Mineralogy Mapper instrument. Blue represents the ice locations, plotted over an image of the lunar surface, where the gray scale corresponds to surface temperature (darker representing colder areas and lighter shades indicating warmer zones). The ice is concentrated at the darkest and coldest locations, in the shadows of craters.
Credit (image and some text): NASA [link]Prominent lunar surface features
Prominent lunar surface features
Topographic map of the Moon (left: near side, right: far side) from the laser altimeter on NASA's Clementine satellite (red = high, purple = low). The large (2500 km diameter) South Pole-Aitken Basin can be seen on the far side map. It is the site of a gigantic meteor impact about 4.3 billion years ago and the lowest area on the Moon.
Credit (image and some text): NASA; image processing by Brian Fessler and Paul Spudis (LPI) [link]
Comparison of the size of the South Pole-Aitken Basin on the Moon (diameter 2500 km, depth 13 km) with the United States.
Credit: Lunar and Planetary Institute [link]
Central peak "mountain range" of Tycho Crater at sunrise, from the Lunar Reconnaissance Orbiter in 2011. The summit of the central peak is 2 km (6562 ft) above the crater floor, and the crater floor is about 4.7 km (15,420 ft) below the rim. Tycho's features are so steep and sharp because the crater is young by lunar standards, only about 108 million years old. Over time, micrometeorites, and not so micro meteorites, will grind and erode these steep slopes into smooth mountains.
Credit (image and some text): NASA/GSFC/Arizona State University [link]Formation of the Moon
Formation of the Moon
Although exact details are not conclusively known, there is general agreement that the Moon formed about 4.5 billion years ago (less than 100 Myr after Earth formed) from debris produced by the collision of a Mars-sized body with the young Earth.
This theorized impactor is named Theia, after the mythological Greek titan who gave birth to Selene, the Moon goddess.
Illustration of a Mars-sized object colliding with the early Earth and splashing out a large amount of debris that later coalesced to form the Moon.
Credit: attribution unknownPhases of the Moon
Phases of the Moon
Diagram showing the origin of the phases of the Moon. In this view, we are looking down on the north pole of Earth. For both Earth and the Moon, the hemisphere facing the Sun is always illuminated (light), while the opposite hemisphere is in shadow (dark). Earth rotates counter-clockwise around the north pole, causing the day/night cycle as a given location on our planet's surface is successively illuminated by the Sun then in shadow. As the Moon orbits Earth in a counter-clockwise direction over the course of 29.5 days, we see different amounts of the illuminated and shadowed halves of the Moon, causing the observed cycle of lunar phases.
Credit: Andonee/Wikimedia Commons [link]Phases of the Moon during a complete lunar cycle
Phases of the Moon during a complete lunar cycle
The change in size is due to the slightly elliptical shape of the Moon’s orbit around Earth – sometimes it is a little closer, sometimes a little more distant.
The “wobble” is due to libration, which is caused by the elliptical orbit of Moon, a slight tilt to the Moon’s axis of rotation relative to its orbit, and observing the Moon from the surface of Earth rather than the center of Earth.
Even though the Moon always faces the same side toward Earth, because of libration we see about 58% of the Moon’s surface over the course of a lunar cycle.
Credit: Tomruen (Wikimedia Commons) [link]
Remember This!
Remember This!
If you remember one thing about the phases of the Moon, make it this:
The phases of the Moon are NOT caused by the shadow of Earth!
The phases of the Moon are caused by different views of the side of the Moon illuminated by the Sun as the Moon orbits around Earth.
Viewed from the Moon, Earth shows phases for the same reason.
References
References
Brandon, A. 2007, Nature, 450, 1169 - "Planetary science: A younger moon" (doi:10.1038/4501169)
Cameron, A. G. W. 2000, Origin of the Earth and Moon, R.M. Canup and K. Righter (eds) (Tucson: University of Arizona Press), 133 - "Higher-resolution Simulations of the Giant Impact" (2000orem.book..133C)
Canup, R. M. 2004, Icarus, 168, 433 - "Simulations of a late lunar-forming impact" (2004Icar..168..433C)
Canup, R. M., & Asphaug, E. 2001, Nature, 412, 708 - "Origin of the Moon in a giant impact near the end of the Earth's formation" (2001Natur.412..708C)
Choi, C. Q., 2019 (April 29), Space.com - “The Moon May Have Formed When Earth's Magma Was Blasted into Space” [link]
Hosono, N., Karato, S., Makino, J,. & Saitoh, T. R., 2019 (April 29), Nature Geoscience - “Terrestrial magma ocean origin of the Moon“ [link] [link]
Lock, S. J., & Stewart, S. T. 2017, Journal of Geophysical Research (Planets), 122, 950 - "The structure of terrestrial bodies: Impact heating, corotation limits, and synestias" (2017JGRE..122..950L)
Lock, S. J., Stewart, S. T., Petaev, M. I., et al. 2018, Journal of Geophysical Research, 123, 910 - "The origin of the Moon within a terrestrial synestia" (2018JGRE..123..910L)
Mackenzie, D. 2003, The Big Splat, or How The Moon Came To Be (Hoboken, New Jersey: John Wiley & Sons) (ISBN 978-0-471-15057-2)
Pahlevan, K., & Stevenson, D. 2007, Earth and Planetary Science Letters, 262, 438 - "Equilibration in the Aftermath of the Lunar-forming Giant Impact" (2007E&PSL.262..438P)
Wiechert, U., Halliday, A. N., Lee, D.-C., et al. 2001, Science, 294, 345 - "Oxygen Isotopes and the Moon-Forming Giant Impact" (2001Sci...294..345W)
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