Apollo 16 LM Pilot Charles Duke/NASA
One of the most common questions members of the public ask space educators is, “why does the moon change shape?” The answer is, of course, that our planet’s natural satellite does not change shape; it is always a sphere. What changes is the way light from the Sun strikes the side of the moon we can see.
Earth’s moon, like most other Solar System moons, is a synchronous rotator; that is, the period of time it needs to revolve about its axis once (about 28 days) is equal to the period of time it needs to orbit the Earth once. This is why humans on Earth see only the moon’s Nearside hemisphere. The Farside hemisphere, turned always away from Earth, remained mysterious until 1959, when the Soviet Union’s Luna III spacecraft imaged it for the first time.
For Earthlings, the day/night cycle for the Nearside begins with new moon. The moon is in fact not visible when it is new; that is because it is at the point in its orbit when it is between the Earth and the Sun. This means that the Nearside is not lit by the Sun and the moon is lost in the Sun’s glare. Occasionally it means that the moon crosses over the Sun; new moon is when solar eclipses occur.
As the moon orbits the Earth, the areas the Sun’s light can reach change. Three or four days after new moon, people on Earth who look west in evening twilight will glimpse a slender crescent moon. The horns of the crescent point away from the setting Sun, toward the east. If one looks carefully, one will see that the part of the Nearside not yet lit by the Sun is just barely visible.
This is probably a good place to note that the Earth changes shape as viewed from the Nearside. When the moon is new, the Earth is full. Full Earth is about four times larger and reflects about 75 times as much light as full moon. When the moon is a crescent, the Earth is mostly full. This means that sunlight reflected off the Earth can light the part of the Nearside that direct sunlight has not yet reached.
As on Earth, the Sun on the moon rises in the east. The line of dawn – the dawn 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 Earth-based telescope, they appear as isolated islands of light amid dark lowlands. As the Sun climbs higher at any given location, light fills in the lowlands and crater floors.
Seven days past new, the Nearside is half-lit. This shape, or “phase,” is called first quarter. For Earthlings, the moon rises at noon, stands in the south at sunset, and sets at midnight.
Fourteen days past new, the Nearside is fully lit by the Sun. 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. The full Nearside looks up at a new Earth. The moon is at the point in its orbit where Earth stands between it and the Sun; full moon is when lunar eclipses can occur.
Many people make the mistake of looking at the moon through a small telescope for the first time when it is full. When the Nearside is fully lit by the Sun, all sense of surface relief disappears because all features are lit evenly from directly above. The moon might as well be a billiard ball. If one can stand the bright glare of the full Nearside, then one can examine many high-contrast light and dark areas; many are best seen when the Nearside is fully lit. Of particular interest are the Nearside-spanning whitish-gray rays of the large southern-hemisphere impact crater Tycho.
Twenty-one days past new, the sunset terminator is advancing and night now covers the Nearside’s eastern half. This phase is called last quarter. The moon rises at midnight, stands highest at earthly dawn, and sets at noon.
About 24 days past new, the crescent moon rises just before the Sun. The dark part of the Nearside is again lit by sunlight reflected off the nearly full Earth. A telescope will reveal the advance of the sunset terminator; lowlands will grow dark, then mountains and high crater limbs will slowly shrink and then wink out. If you look at the crescent moon before dawn, take care not to point your telescope at the Sun as it peeks above the horizon and makes the crescent moon fade from view.
Day 28 begins the moon’s eternal day-night cycle all over again. The moon stands between the Sun and Earth, lost in the Sun’s glare. and it is midnight at the center of the Nearside hemisphere.
The crater-pocked small basaltic plain Sinus Medii – Latin for “Central Bay” – marks the center of the Nearside. When the clock strikes midnight in Sinus Medii, it is high noon at the center of the rugged Farside hemisphere. If you stood at the center of Farside – on the lunar equator north of the impact crater Daedalus – the Sun would glare down on you from directly overhead.
Conversely, when it is high noon on Sinus Medii, it is midnight at the center of the Farside hemisphere; that is, exactly the opposite of the situation 14 days earlier. Midnight on Farside is different from Nearside midnight. No full Earth shines down on its rugged landscapes, so they are utterly dark. Only the faint light of stars, planets, and the silvery, powdery Milky Way relieves the blackness.
A careful reader will have noted that the Nearside phases are the opposite of those visible on Earth as viewed from the Nearside. The Farside phases, on the other hand, match those of Earth as viewed from the Nearside.
Changes in the orbital geometry and lighting angles in the Earth-moon system are today mainly of interest to stargazers amateur and professional, but nearly a half-century ago it was different. Apollo missions were blasting off Earth every few months bound for the moon, and lighting conditions were a critical part of landing site planning and mission timing.
Conservative Apollo mission rules dictated that the bug-like Apollo Lunar Module (LM) spacecraft should land only between 12 and 48 hours after sunrise at its target landing site, when the Sun stood between 5° and 20° of the eastern horizon. Landing sites were restricted to the Nearside within about 20° of the equator.
The Apollo mission Commander (CDR) and Lunar Module Pilot (LMP) would ignite their spindly-legged spacecraft’s descent engine over the Farside to slow it so that it would intersect the lunar surface on the Nearside at its planned landing site. The LM would then gradually descend with its twin triangular windows pointed toward the sky.
As it neared its planned landing place, it would pitch up to point its descent engine and foot pads at the lunar surface. When their landing site became visible outside the LM windows, the Sun would shine behind the spacecraft so that it would not glare into the astronauts’ eyes. The low Sun angle would cast westward-pointing shadows on the lunar surface. Shadows would make crater floors and boulders stand out, making it easier for the CDR and LMP to dodge them during final descent and touchdown. The shadow of the LM would also become visible, thus enabling the astronauts to gauge the size of features on the lunar surface.
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 with working in lunar lighting conditions thus only spanned from 12 hours – the earliest permitted landing time – to 120 hours – the latest permitted landing time plus the maximum stay-time of 72 hours.
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 preliminary study of the effects on 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 them to Mars. “To stay” implied that astronauts would need to land, drive, walk, and work on the moon throughout its day-night cycle at many different locations.
Eppler had help from a spaceflight legend. Captain John Watts Young had 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.
Though his new job was widely seen as punishment for the views he expressed in the aftermath of the 28 January 1986 Challenger accident, Young tackled it with gusto. He delved into a wide range of technical and safety issues and distributed hundreds of memoranda offering advice. Young also made himself available to people such as Eppler (and, incidentally, to this author); that is, to individuals eager to learn from and commit to record Young’s unique body of experience and knowledge.
As Apollo 10 Command Module Pilot (CMP) in May 1969, Young had opportunity to observe the lunar surface from orbit under a range of lighting conditions. He told Eppler that transition from the sunlit part of the moon to the Earthlit part was quick and that the eye adjusted almost immediately. Features on the lunar surface remained almost as visible to the human eye as they were under sunlight, and it was even possible to pick out features inside shadows in Earthlit areas.
The change from the Earthlit part of the moon to the Farside, lit by neither Earth nor Sun, was “dramatic.” Nothing could be seen of the moon’s surface; it betrayed its presence even at a distance of a few tens of kilometers only because it blocked out the stars.
As Apollo 16 CDR, Young piloted the LM Orion to a landing at Descartes, the only Lunar Highland site Apollo visited. Young told Eppler that landing a spacecraft equivalent to the Apollo LM would be possible at an Earthlit site. Landing at a prepared site – that is, one with lights and electronic landing aids – would be easier than landing a helicopter at night.
Young was not the first astronaut to describe the problems of traveling toward the Sun while on the lunar surface. Shadows disappear and with them most craters and boulders. The two images at the top of this post, taken from the same location within moments of each other by Apollo 16 LMP Charles Duke, show this phenomenon clearly.
The top image shows John Young at work near the Apollo 16 Lunar Roving Vehicle (LRV). As shown by the orientation of shadows, the Sun is located to the upper right. Rocks, footprints, and LRV tracks are made obvious by the shadows they cast. The lower image, taken facing directly toward the Sun, looks very different, but in reality displays a landscape very similar to that in the top image. Rocks and other features cast no shadows and are almost invisible.
Based on Young’s observations and his own calculations, Eppler proposed schedules for operations at various lunar sites. He determined that in Sinus Medii the 5.5 days after lunar sunrise would be optimum for walking and driving. This would also be a good time for safe landings. The Apollo landing period spanned only 1.5 Earth days, but Young told Eppler that the landing period could be safely lengthened.
From 5.5 to nine days after sunrise at Sinus Medii, the Sun would hang within 20° of local vertical, with noon occurring on day seven. The near-vertical lighting angle would mean that terrain features would cast no shadows, making walking, driving and landing difficult. Eppler advised that only “restricted surface operations” occur during the near-noon period. Landings could occur only at prepared sites such as a lunar base landing field with electronic homing aids and bright flashing strobes.
The period from nine to 28 days after sunrise at Sinus Medii would be optimum for surface activity, Eppler found, though lighting conditions would vary greatly. Between nine and 14 days after sunrise, the Sun would lower toward the west and would again would cast shadows useful for astronauts traveling any direction (except west toward the Sun of course). Landers approaching at landing site from the east would have to contend with direct solar glare and loss of shadows. Sunset would occur on day 14, with a half-lit Earth shining high in the sky.
On day 21 – midnight at Sinus Medii – the full Earth would light the landscape as described above. Seven days later, with a half Earth high in the sky, the Sun would rise again. Surface activity could thus take place at Sinus Medii for 24.5 days of the 28-day lunar day/night cycle.
At the center of the Farside, the situation would be very different. Starting 14 days after dawn, the Sun would set and the landscape would be lost in darkness. Only by using artificial lighting could astronauts find their way. Landings would be prohibited except at prepared sites.
Eppler also examined the lighting situations on the east and west equatorial lunar limbs (that is, on the edges of the Nearside at the equator) and at the lunar poles. These closely paralleled the situation at Sinus Medii except that the Earth would sit close to the horizon – in the west in the case of the eastern limb, east for the western limb, south for the north polar region, and north for the south polar region – and, in the case of the two limb sites, would have a different phase at sunset than at Sinus Medii.
The western limb would experience sunset on day 14 under a full Earth. The lighted fraction of the Earth would shrink as night progressed. Between day 23 and day 28 after sunrise, Earth would provide too little light for surface operations without artificial lights. It would be completely dark at sunrise.
The eastern limb would experience sunset under a new Earth that would provide too little light for surface operations without artificial lighting. Eppler expected that the Earth would begin to provide adequate lighting on day 19. On day 21, Earth would be half lit, and it would be full on day 28 as the Sun rose in the east.
The polar regions would experience Earth and Sun phases similar to those at Sinus Medii, except that the Earth and Sun would both lie close to the horizon. Earth would remain in the south for north pole sites and in the north for south pole sites, but the Sun would circle the horizon. Both bodies would cause long, deep shadows, possibly making travel difficult, and moonwalkers would need to take care not to turn toward the Sun without eye protection. In addition, some areas – for example, deep crater bottoms – would be permanently in shadow, and local surface relief – mountains and craters – would periodically cut off line-of-sight radio communications with Earth.
References:
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.
Related Beyond Apollo Posts:
One-Way Space Man (1962) http://ift.tt/1iJSDJX
The Quest to Explore the Moon from Lunar Orbit (1967) http://ift.tt/1iJSE0j
Ludek Pesek’s Ill-Starred Lunar Expedition (1964) http://ift.tt/1k2LY8h
