What Apollo Saw in Sunlight While in Orbit

The crews of future spaceflights to the Moon, beginning with Artemis II, may be the first to lay eyes on certain features of the Moon in daylight, features never sunlit during any of the Apollo flights.

Nine Apollo missions were sent to the Moon between 1968 and 1972. All but one went into orbit around the Moon, and six landed astronauts on the surface. All of these missions were launched so that they would arrive at times when the landing sites were in early morning sunlight, when temperatures on the surface were not too extreme and shadows eased lander navigation. Apollos 8 and 10, never meant to land, photographed landing sites in the illumination that future missions would encounter. Apollo 13, unable to enter lunar orbit after an explosion damaged their spacecraft, flew around the lunar far side on a free return trajectory.

Aside from Apollo 13, the missions included anywhere from 20 hours to 6 days in lunar orbit. Sunrise on the Moon advances westward at the rate of roughly half a degree per hour, slowly growing the parts of the lunar surface the astronauts in orbit could observe in daylight. Based on the timing of their arrival and departure, a map of the sunlit portions of the Moon during their stay can be made for each mission. Combining these maps reveals a substantial span of the Moon's western hemisphere that was never seen in sunlight by human eyes from orbit, a gap that arose because of the early morning timing of the landings. This area includes the Orientale basin, a spectacular multiringed impact feature with dark basalt in both a central mare and along its ring moutains.

What was visible to the astronauts from orbit was also limited by their low altitude, typically 60 nautical miles (110 kilometers) above the surface. At that height, their horizon is 600 kilometers away, or 20 degrees in latitude north and south of their orbit. Since all of the orbits were more or less equatorial, features at high latitudes, including Mare Frigoris to the north and the South Pole-Aitken Basin to the south, were beyond what the astronauts could see while they remained in orbit. (Apollo 13, never as close to the surface, is again the exception. Their visibility swath, the cone shape in the eastern hemisphere, is artificially truncated so that it doesn't swamp the map.)

Although the Moon has been mapped in detail by robotic spacecraft, including Lunar Reconnaissance Orbiter, no one has actually seen some parts of it with their own eyes. If the timing of their launch is favorable, the crew of Artemis II may be the first.

Ideally, the orbits used to produce these maps would be based on a precise ephemeris for each mission, but that data isn't readily available. Instead, osculating orbital elements were calculated from values listed in the Trajectory Parameters table within the Mission Report for each flight. The numbers in the table are explained in the report, but a couple of things to note: The coordinate system for the direction appears to be mean-of-date celestial for most missions, but switches silently to Moon Mean Earth inertial at epoch for 16 and 17. And at least one of the positions for Apollo 15 is obviously incorrect.

The elements were derived from the position, speed, and direction of the Command and Service Module (CSM) at the time of engine cutoff for the orbit circularization maneuver. Orbits were calculated from that time until the Transearth Injection burn. (For Apollo 13, elements are from Transearth Injection at ignition, and the trajectory covers the two hours around pericynthion.)

A = mission number, rp = pericynthion distance in kilometers, ecc = eccentricity, inc = inclination, lnode = longitude of the ascending node, argp = argument of pericynthion, m0 = mean anomaly at epoch, and t0 = epoch in Greenwich Mean Time. The gravitational parameter of the Moon is GM = 4902.8 km³/s². The coordinate system is Moon Mean Earth inertial at epoch, meaning body-fixed Moon coordinates at t0 that are stationary (not rotating). The longitude of an orbit position must be adjusted by the sidereal rotation of the Moon.

All the usual caveats about the limited accuracy of orbital elements apply, along with possible errors in the Mission Report values and in the visualizer's interpretation of them. As a visual check, the maps here should be consistent with the Apollo Lunar Photography Index Maps archived by the Lunar and Planetary Institute.