In case you haven’t heard, the Moon is trending again… and in a big way. Like in the glory days of the 1960s and 1970s, our big white space neighbor is enjoying the attention of lunar explorers. Only this time, they’re going back to the moon for good. The award-winning 24-minute Google Lunar XPRIZE fulldome planetarium show, Back To The Moon For Good, chronicles teams around the world competing for the largest international incentivized prize in history, by landing a robotic spacecraft on the Moon. To win the $30 million Google Lunar XPRIZE, a team must land a robotic spacecraft on the Moon, navigate 500 meters over the lunar surface, and send video, images and data back to Earth. This global competition is designed to spark imagination and inspire a renewed commitment to space exploration, not by governments or countries – but by the citizens of the world.
A lunar day is the period of time it takes for the Earth’s Moon to complete one full rotation on its axis with respect to the Sun. Equivalently, it is the time it takes the Moon to make one complete orbit around the Earth and come back to the same phase. It is marked from a new moon to the next new moon.
On average, this synodic period lasts 29 days, 12 hours, 44 minutes and 3 seconds. Which just so happens to be what we call a synodic month here on Earth.
This is an average figure, since the speed of the Earth-Moon system around the Sun varies slightly over a year, due to the eccentricity of the orbit. The Moon’s own orbit also undergoes a number of periodic variations about its mean value, due to the gravitational perturbations of the Sun.
Types of Months used in Astronomy:
The following types of months are mainly of significance in astronomy, most of them (but not the distinction between sidereal and tropical months) first recognized in Babylonian lunar astronomy.
The sidereal month is defined as the Moon’s orbital period in a non-rotating frame of reference (which on average is equal to its rotation period in the same frame). It is about 27.32166 days (27 days, 7 hours, 43 minutes, 11.6 seconds). The exact duration of the orbital period cannot be easily determined, because the ‘non-rotating frame of reference’ cannot be observed directly. However, it is approximately equal to the time it takes the Moon to pass twice a “fixed” star (different stars give different results because all have proper motions and are not really fixed in position).
A synodic month is the most familiar lunar cycle, defined as the time interval between two consecutive occurrences of a particular phase (such as new moon or full moon) as seen by an observer on Earth. The mean length of the synodic month is 29.53059 days (29 days, 12 hours, 44 minutes, 2.8 seconds). Due to the eccentric orbit of the lunar orbit around Earth (and to a lesser degree, the Earth’s elliptical orbit around the Sun), the length of a synodic month can vary by up to seven hours.
The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky. It is 27.32158 days, very slightly shorter than the sidereal month (27.32166) days, because of precession of the equinoxes. Unlike the sidereal month, it can be measured precisely.
An anomalistic month is the average time the Moon takes to go from perigee to perigee – the point in the Moon’s orbit when it is closest to Earth. An anomalistic month is about 27.55455 days on average.
The draconic month or nodal month is the period in which the Moon returns to the same node of its orbit; the nodes are the two points where the Moon’s orbit crosses the plane of the Earth’s orbit. Its duration is about 27.21222 days on average.
A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth. Therefore, the Sun appears to move with respect to the stars, and it takes about 2.2 days longer for the Moon to return to the same apparent position with respect to the Sun.
An anomalistic month is longer than a sidereal month because the perigee moves in the same direction as the Moon is orbiting the Earth, one revolution in nine years. Therefore, the Moon takes a little longer to return to perigee than to return to the same star.
A draconic month is shorter than a sidereal month because the nodes move in the opposite direction as the Moon is orbiting the Earth, one revolution in 18 years. Therefore, the Moon returns to the same node slightly earlier than it returns to the same star.
Sources: Wikipedia, NASA Lunar Reconnaissance Orbiter.
Impact Bombardment Throughout the Solar System
Impact events are an intimate part of the formation of planets, both during the initial accretional phase and late in the growth of a planet when giant impact events may dramatically alter the final outcome. Large collisions, for example, have been implicated in the formation of the Moon from the Earth (Figure 1), the stripping of Mercury’s mantle, the northern-southern hemisphere dichotomy of Mars, and the formation of Charon from Pluto. Periods of enhanced impact bombardment of post-accretion planetary surfaces have also been deduced from solar system exploration studies. Apollo, for example, demonstrated that the Moon was heavily cratered sometime during the first ~600 million years of its existence in what has been termed by some to be the period of Late Heavy Bombardment (LHB).
Dusty circumstellar disks around young stars outside our solar system indicate the collisional evolution of young planetary systems can be violent. Initial Spitzer Space Telescope data indicate that some systems show dust signatures well above the average at ages from 100 to 600 million years old. Observations support the idea that the ~350 million-year-old A star Vega and the ~2 billion-year-old G star HD 69830 recently experienced collisions between large planetesimals that have generated these elevated dust signatures.
The consequences of the collisional evolution of young planetary systems, including our own solar system, are profound. It now seems clear that:
Early bombardment of planets can completely resurface them.
These impacts can alter the physical and chemical state of (and/or blow-off) planetary atmospheres.
The bombardment can make surface conditions unpalatable for biogenic processes.
In contrast, the impacts can also create subsurface environments that are suitable crucibles for pre-biotic reactions and possible habitats for any life that develops.
The impacting objects and interplanetary dust that accompanies them can deliver important biogenic components like water, carbon, nitrogen, sulfur, and phosphorus).
The impacting objects may also be the source of important siderophile (iron-loving) element addition.
The Apollo Legacy
While it is generally recognized that the impact cratering rate was more intense early in solar system history, it is not clear how that rate evolved. Some investigators have suggested there was a smooth decline with time, while others have suggested there were one or more episodes of particularly intense activity superimposed on a background decline in the impact rate.
The Apollo and Luna missions provided the first opportunities to investigate this issue. Argon-argon isotopic analyses of Apollo and Luna samples suggested three to possibly six of the impact basins on the nearside of the Moon had been produced between 3.88 and 4.05 billion years ago. Additional analyses of Apollo samples indicated the U-Pb and Rb-Sr systems had been disturbed nearly uniformly at ~3.9 billion years ago, which was attributed to metamorphism of the entire lunar crust by a large number of asteroid and/or cometary collisions in a brief pulse of time, <200 million years long, in what was termed the lunar cataclysm. A growing number of ~3.9 billion-year-old impact melt ages from the Apollo and Luna collections seemed to confirm the pattern. It was suggested that the decline in the impact rate was not smooth, but punctuated by at least one large influx of material.
The hypothesis of an intense period of bombardment ~3.9-4.0 billion years ago is still controversial, however. There are currently several models under consideration. Some investigators have argued for a lunar cataclysm ~3.9-4.0 Ga and a relatively low impact rate between ~4.4 and 4.0 billion years ago (lower curve in Figure 2). They also argued that the duration of the cataclysm may have been as short as 10-20 million years long. Others have argued that the time span of the bombardment may have been longer and/or that the impact rate prior to ~3.9-4.0 billion years ago was relatively high (upper two curves in Figure 2). In all cases, it is generally agreed that there was a significant decrease in the lunar cratering rate after ~3.8 billion years ago when the last basin-forming impact occurred.
Some investigators have suggested that sampling issues, particularly on the Moon, cloud our ability to resolve the impact cratering record prior to ~3.9 billion years ago, and do not accept the notion of a cataclysm on the Moon, asteroids, or any other body in the solar system. There are also interesting discrepancies in existing data. While Apollo samples suggest a relatively abrupt decline in the impact-cratering rate ~3.85 billion years ago, lunar meteorite data and chondritic meteorite data suggest it may have been drawn out until 3.5 to 3.4 billion years ago. To test these ideas, the CLSE team will analyze samples from the Moon and asteroids to determine the timing and magnitude of impact events that occurred in the Solar System.
Although the lunar cataclysm hypothesis is one of Apollo’s highlights and remains the number-one science priority of NASA (NRC 2007), it is representative of a broader range of questions. We now understand that impact cratering is the dominant process affecting the lunar surface. There are hints that the Moon’s origin may be intimately tied to a collisional event (the giant impact hypothesis) when the accretion rate was much higher (Figure 2). We have also gleaned from Apollo that impact events have produced a unique lunar surface regolith, which itself is a record of meteoritic and heliophysical processes and the medium with which future lunar surface exploration will be immersed. We have designed an integrated interdisciplinary study of impact processing of the Moon that tackles the highest science priorities identified by the NRC for NASA.