The Moon, like most other Solar System moons, is a synchronous rotator; that is, the period of time it needs to rotate on its axis once is equal to the period of time it needs to orbit its primary once. For the Moon, the time required for both one rotation about its axis and one revolution about the Earth is about 28 days.
That is why humans on Earth see only the Moon's nearside hemisphere. The farside hemisphere, always turned away from Earth, remained mysterious until 1959, when humans glimpsed it for the first time courtesy of the Soviet Union's Luna III spacecraft.
Occasionally the Moon crosses over the Sun; new Moon is the time of partial, annular, and total solar eclipses. An eclipse does not occur every time the Moon is new because the Moon's orbit about the Earth is inclined slightly (about 5.1°) relative to Earth's orbit about the Sun.
The disc of the Moon is about 0.5° wide as viewed from Earth. The Sun, though 400 times farther away than the Moon (about 149,600,000 kilometers versus 363,100 kilometers) is also 400 times bigger than the Moon (about 1,392,000 kilometers versus 3475 kilometers), so it appears to be the same size as the Moon (0.5° wide) in Earth's sky. This means that, during total eclipses, the Sun's ghostly corona becomes visible as the Moon blocks the bright disk of the Sun.
The Moon is, however, moving away from Earth at a rate of about four meters per century. In just a few million years, it will no longer be wide enough in Earth's sky to cause total solar eclipses. All eclipses after that will be either partial or annular.
About two days past new Moon, people on Earth can look west in evening twilight and glimpse a slender crescent Moon (2 on the lunar phase diagram). It is called a waxing (growing) crescent. The horns of the waxing crescent point toward the east, away from the setting Sun. If one looks carefully, one might observe that the part of the nearside that is not yet lit by the Sun is visible.
This is probably a good place to mention that the Earth seems to change shape as viewed from the nearside hemisphere. When the Moon is new, the Earth is full. In fact, Earth phases are always the opposite of nearside phases.
As seen from the nearside, the full Earth is about four times larger than the full Moon (2° wide in the black lunar sky) and reflects about 75 times as much light. When the Moon is a waxing crescent, Earth is mostly full. This means that sunlight reflected off Earth can light the part of the nearside that direct sunlight does not yet reach.
As on Earth, the Sun rises in the east on the Moon. The line between light and darkness — the terminator — advances westward a little faster than a typical human can comfortably jog. High mountains and crater rims catch the morning Sun's bright rays first. Viewed through even a modest-size telescope, they appear as isolated islands of light. As the Sun climbs higher, light fills in the plains, lowlands, and crater floors.
Seven days past new, the nearside is half lit for observers on Earth (3 on the lunar phase diagram). This phase is called first quarter. The first-quarter Moon rises in the east as the Sun stands at noon, reaches its highest point at sunset, and sets in the west at midnight.
About 10 days past new, the nearside is halfway between first quarter and full (4 on the lunar phase diagram). We call this phase waxing gibbous ("gibbous" means convex — the term refers to the shape of the advancing dawn terminator).
Fourteen days past new, the nearside is fully lit by the Sun as viewed from Earth (5 on the lunar phase diagram). The full Moon is visible all night; it rises in the east as the Sun sets in the west, stands highest at midnight, and sets in the west as the Sun rises in the east. When the nearside is full, the Earth is new as viewed from the Moon.
When the Moon is full, the Earth stands between it and the Sun. For this reason, full Moon is when partial and total lunar eclipses — during which the shadow of the Earth falls on the moon — can occur. As with solar eclipses, lunar eclipses do not occur at every full Moon because the Moon's orbit is tilted relative to Earth's orbit about the Sun.
Newcomers to the pleasures of amateur astronomy often turn their first telescope toward the Moon for the first time at full Moon. If one can stand the bright glare from the fully lit nearside, one can examine many contrasting light and dark areas through a small telescope; many such albedo features (as they are known) are, in fact, best seen when the nearside is fully lit. Of particular interest are the nearside-spanning whitish-gray rays of the large impact crater Tycho.
All things considered, however, the fully lit nearside appears bland; crater rims and mountains cast no shadows, so all sense of surface relief is absent. The Moon might as well be a painted billiard ball. Viewing the Moon when it is less than full — and focusing on the line between light and dark — is, in my opinion, much more rewarding.
About 18 days past new, the Moon has reached waning (shrinking) gibbous phase (6 on the lunar phase diagram). It rises between dusk and midnight and is visible in clear skies in the west until mid-morning the next day. The terminator line, formerly the line of lunar dawn, has become the line of lunar dusk. Darkness advances from east to west as light advanced two weeks before.
Twenty-one days past new, night has reclaimed the nearside's eastern half. This phase is called last quarter (7 on the lunar phase diagram). For people on Earth, the Moon rises at midnight, stands highest at dawn, and sets at noon.
About 25 days past new, the crescent Moon — called the waning (shrinking) crescent — rises in the east just before the Sun (8 on the lunar phase diagram). Its horns point westward, away from the Sun. The dark part of the nearside is again lit by sunlight reflected off a nearly full Earth. A relatively small telescope reveals the advance of the sunset terminator; crater bottoms, lowlands, and plains grow dark, then mountains and crater rims slowly shrink and finally vanish in darkness.
If you look through a telescope at the crescent Moon before dawn, take care not to look at the Sun when it peeks above the horizon; eye damage will result. Instead, attempt to keep sight of the crescent Moon as fades into the blue sky of earthly day.
The end of Day 28 sees a new lunar day-night cycle begin (1 on the lunar phase diagram). The Moon stands between the Sun and Earth, lost in the Sun's glare, and it is again midnight at the center of the nearside hemisphere.
Changes in orbital geometry and lighting angles in the Earth-Moon system are today mainly of interest to stargazers amateur and professional, but a half-century ago it was different. Apollo missions were leaving Cape Kennedy, Florida, every few months bound for the Moon, and lighting conditions were a critical part of landing site selection and mission timing.
Conservative Apollo mission rules dictated that the LM should land only between 12 and 48 hours after sunrise at its target landing site, when the Sun would stand between 5° and 20° above the eastern horizon. At the appointed time, the Apollo mission Commander (CDR) and Lunar Module Pilot (LMP) would ignite the descent engine of their spindly-legged spacecraft over the farside to slow it so that its orbit would intersect the lunar surface at its nearside landing site.
As it approached its pre-planned landing site from the east, the LM would pitch up to point its descent engine and four round foot pads at the lunar surface. As the landing site became visible outside the twin triangular LM windows, the Sun would shine from behind the spacecraft. This would prevent it from shining into the astronauts' eyes. The shadow of the LM would then become visible on the surface, enabling the astronauts to gauge the size of lunar surface features to help them pick out a spot for a safe landing.
Because of limited supplies of avionics cooling water, battery power, and breathing oxygen, the longest an Apollo lunar surface mission could last was about 72 hours. The period during which Apollo explorers could gain experience working in lunar lighting conditions thus only spanned from 12 hours (the earliest permitted landing time) to five days (the latest permitted landing time of two days plus the maximum stay-time of three days) after dawn at the landing site.
In 1991, Dean Eppler, a geologist in the NASA Johnson Space Center (JSC) Lunar & Mars Exploration Program Office (LMEPO) with an interest in lunar geologic fieldwork, conducted a study of the effects on lunar surface operations of the whole range of lunar lighting conditions in support of Space Exploration Initiative (SEI) planning. SEI, launched amid great fanfare by President George H. W. Bush on 20 July 1989, aimed to complete Space Station Freedom, return American astronauts to the Moon to stay, and then launch humans to Mars. "To stay" implied that astronauts would need to land, drive, walk, and work on the Moon throughout its day-night cycle at multiple locations all over the Moon.
Eppler had help from a spaceflight legend. John Young (1930-2018) joined NASA in 1962 as a member of the second Astronaut Class ("the New Nine") and was a veteran of six space missions (Gemini III, Gemini X, Apollo 10, Apollo 16, STS-1, and STS-9), four of which he commanded. He was Chief of the Astronaut Office at JSC from 1974 until 5 May 1987, when he was made JSC Director Aaron Cohen's Special Assistant for Engineering, Operations, and Safety.
|Apollo 16 Commander John Young (left) with Command Module Pilot Kenneth Mattingly (center) and Lunar Module Pilot Charles Duke (right). Image credit: NASA|
Young first had an opportunity to observe the Moon's surface from lunar orbit when he served as Apollo 10 Command Module Pilot (CMP) in May 1969. He told Eppler that, viewed from a spacecraft in lunar orbit, the transition from the sunlit part of the Moon to the earth-lit part was sudden and that the eye adjusted almost immediately to the reduced light level. Features on the lunar surface remained almost as visible as they had been under direct sunlight, and it was even possible to pick out features within shadows in earth-lit areas.
Young reported that the change from the earth-lit part of the Moon to unlit portions of the farside, out of reach of light from both Sun and Earth, was "dramatic." Nothing could be seen of the Moon's surface even at an orbital altitude of only a few tens of kilometers. The horizon was discernible only because stars were visible above it but not below it.
As Apollo 16 CDR in April 1972, Young piloted the LM Orion to a landing at Descartes, the only Apollo site entirely within the lunar highlands. The highlands, which cover about 80% of the Moon's surface, are lighter in hue than basaltic maria plains like Sinus Medii.
Young told Eppler that, in his opinion, landing a spacecraft equivalent to the Apollo LM would be possible at a site lit only by light reflected off the Earth. Landing in earthlight at a prepared site — that is, one with flashing strobes and electronic landing aids — would be easier than landing a helicopter at night on Earth, Young added.
Young experienced the challenges of getting about on the lunar surface under low-angle sunlight soon after climbing down Orion's ladder at Descartes. Moving toward the Sun (eastward) was difficult because of its fierce glare, and moving away from the Sun (westward) was treacherous because shadows disappeared behind the rocks and crater rims that cast them. This created a washed-out landscape where obstacles were hard to see and avoid.
Moving north or south meant reduced glare and visible shadows. This is one reason why the first two Apollo flights that included a Lunar Roving Vehicle (LRV), Apollo 15 and Apollo 16, had pre-planned lunar traverses that were oriented generally toward north and south.
|Image credit: NASA|
|Image credit: NASA|
|Image credit: NASA|
The top image shows the view toward the glaring low-angle Sun. The middle image shows John Young at work near the Apollo 16 LRV. He is facing north. As the orientation of the shadows indicates, the Sun is located to the right of the field of view, so surface feature visibility is near optimum. Rocks, footprints, and LRV tracks are obvious.
The bottom image, taken facing west directly away from the low Sun, looks very different, but in reality displays a rocky landscape similar to that shown in the top and middle images. Apart from rocks close to Duke (and Duke's own helmet), however, surface features obscure their own shadows and thus are almost invisible.
Based on Young's observations and his own calculations, Eppler proposed schedules for operations at various lunar surface locations. He determined that in Sinus Medii the period from local dawn until 5.5 days after local sunrise would be optimal for walking, driving, and landing.
From 5.5 days to nine days after sunrise at Sinus Medii the Sun would hang within 20° of local vertical, with noon taking place on day seven. The near-vertical lighting angle would mean that terrain features would cast no shadows, making walking and driving difficult. A descending lander would cast a shadow, but only directly beneath the lander, where it would most likely not be visible to the pilot. Eppler advised that only "restricted surface operations" should occur during the near-noon period. Landings should take place only at prepared sites.
The period from nine to 28 days after sunrise at Sinus Medii would be optimal for surface activity, Eppler found, though lighting conditions would vary greatly over that span of time. Between nine and 14 days after sunrise, the Sun would lower toward the west and would again cast visible shadows (except toward the east, away from the Sun). A lunar lander approaching an outpost landing field from the east would have to contend with both direct solar glare and absence of a handy lander shadow. Sunset would occur on day 14, with a half-lit Earth shining high in the sky.
On day 21 — midnight at Sinus Medii — full Earth would light the landscape. Seven days later, with a half-Earth high in the sky, the Sun would rise again in the east. Surface activity could thus take place at Sinus Medii without break or restrictions for 24.5 days of the 28-day lunar day/night cycle; that is, from day nine after sunrise to day 5.5 after sunrise.
At the center of the farside, the lighting situation would be very different. Starting 14 days after local dawn, the Sun would set and — with no Earth in the sky — the landscape would become lost in darkness. Only by using artificial lighting could astronauts find their way. Landings would be prohibited throughout farside night except at prepared sites.
Eppler also examined lighting on the east and west lunar limbs (that is, on the edges of the nearside hemisphere near the equator) and at the Moon's poles. The western limb would see the Sun set in the west 14 days after local sunrise with a full Earth on the eastern horizon. The lighted fraction of the Earth would decrease as night progressed.
Between day 23 and day 28 after sunrise at the western limb site, Earth would provide too little light for surface operations without artificial lights. It would become completely invisible at western limb sunrise — which would, of course, occur in the east.
The eastern limb would experience sunset while Earth was new, so would become very dark immediately. Eppler expected that a fat crescent Earth, located just above the western horizon, would provide adequate lighting for surface operations starting on day 19 after local sunrise. On day 21, Earth would be half lit, and it would be full on day 28, when the Sun would once again rise in the east.
The lunar poles would see Earth phases like those at Sinus Medii. Earth would hover, bobbing and tilting slightly, near the nearside southern horizon for north pole sites and near the nearside northern horizon for south pole sites.
The Sun would circle the horizon at a polar site, never setting. Astronauts would need to note its position on the horizon and take care not to turn directly toward it without adequate eye protection. In addition, local mountains and crater rims would occasionally block the Sun or Earth and some areas — mainly deep crater bottoms — would forever lie in cold shadow.
Lighting Constraints on Lunar Surface Operations, NASA Technical Memorandum 4271, Dean B. Eppler, NASA Johnson Space Center, May 1991
Forever Young: A Life of Adventure in Air and Space, John W. Young with James R. Hansen, University Press of Florida, 2012
The Eighth Continent
What If an Apollo Lunar Module Ran Low on Fuel and Aborted its Landing? (1966)
Keep My Memory Green: Skill Retention During Long-Duration Spaceflight (1968)
Log of a Moon Expedition (1969)