The MOON

Our Moon is the nearest celestial object to the Earth and has always held an interest for Man since the beginning of recorded time. Its light shone down on the Earth, allowing the surrounding landscape to be seen at night. For the military, a full moon may significantly affect their operations. In some instances, it allows the farmer’s crops to be gathered in during the Full Moon or the common name of the ‘harvest moon’. Its daily appearance changed from night to night, sometimes as a thin crescent, other times being at Full Moon or perhaps during the Moon’s three-quarter phase. The Moon is seen to influence tides, controlling the spawning of fish in the seas, and directly controlling many of the biological mechanisms for all the creatures of the Earth.

Some of the early Greek mythologies saw the moon as Diana the Huntress who made serene and peaceful order to the heavens. The Greek word for the Moon is Selene, and the Latin word is Luna. Hence, the study of the Moon’s features and terrain is usually called selenography. The Latin name appears commonly as an adjective, so we still refer to a lunar eclipse or the lunar surface. (The word lunatic is also commonly used for people who are mentally disturbed, presumed at the time of full moon.) The Moon has always been seen as feminine and usually is related to birth and creating life. All cultures in the past have made the Moon to have similar stories in folklore. Today, the general public views it with some romance. Many a young man has tempted a young lass to a romantic dinner’ under the pale moonlight‘. Why this happenss is probably playing on the age-old effects our moon has on all humankind.

The Moon is about a sixth the diameter of the Earth and about one eighty-first by mass. As the Earth’s nearest neighbour, it is merely a stones throw away, averaging about 384 373 kilometres. To imagine this distance, consider a car travelling at an averaging one hundred kilometres per hour, this distance could be traversed in 160 days or about 5 ¼ months. The distance travelled is equal to about eight to ten years that the average car driver spends behind the wheel!

Our nearby world has been visited by many space missions. The first was in 1959, when a Soviet spacecraft took photographs of the lunar far-side. The exploration quickly followed when the American Surveyor spacecraft both crash landed and landed on the surface. These were important for the Apollo program to land a man on the Moon. This program landed twelve astronauts in various places on the nearside between 1969 and 1972. The first landing was by Apollo 11 on the 21st July 1969, with the first step on this alien world taken by Neil Armstrong. Three Soviet Luna spacecraft landed between 1970 and 1976. Both visits brought many kilograms of samples from the lunar surface to the Earth for investigation.

A period of time has passed where no exploration occurred. In the mid-1990’s both Japanese and American have begun to continue to explore the Moon. The Clementine spacecraft in 1994 took large numbers of photographs of the surface in high resolution, discovering new facts about the lunar surfaces. Lunar photographs were taken of the poles - regions little known until recent years. It is believed that some of the far northern craters never see the sunlight and it is possible the moon may contain ice within the polar crater walls. Interestingly the 21st Century may see people returning to the Moon, except the next time it maybe for colonisation and mineral exploration - by countering the Earth’s dwindling resources. I is likely that the Moon will become a stepping stone for Man to land on the planet Mars in the next few decades.

The Lunar Phases

Our Moon is the natural satellite of the Earth and orbits in an approximate period of 27 days 8 hours, or more exactly 27.321 66 days or 27d 07h 43m 11s, and is called the sidereal period. The Moon gives out no light of its own, but shines, as do all planetary bodies, by the reflected light from the Sun.

The phases that we see on the Moon are produced by the differing angles regarding the positions of the Moon in its orbit around the Earth orbit relative to the Sun. (Figure 1) The time between consecutive alignment (Ie. New Moon to New Moon) is slightly longer in duration than the 27 day period. This is primarily caused by the yearly motion of the Earth around the Sun and the extra time required to bring the Sun, Earth and Moon back into line. The whole period for consecutive phases is called the synodic period. For the Moon this is 29½ or more precisely 29.530 59 days or 29d 12m 44s, which is about 2 1/6 days longer than the sidereal period. The true length of the synodic period can vary by 0.512 days, as this also depends on the distance of the Earth and the Sun, in the Earth’s own elliptical orbit. Between the phases of New Moon, First Quarter, Full Moon and Last Quarter the period is 7¼ days.

The anomalistic month is another measure of the Moon’s position. It is the measure from perigee to perigee, that is the closest approach to the Earth in the orbit. The value is about ¼ of a day longer (0.232 89) than the sidereal period - or 27.554 55 days (See Figure xx.)

The draconic month is the period between one ascending node of the orbit to the next ascending node. The ascending node is the intersection, from south to north, between the ecliptic and the Moon’s path, as normally it will lie either above and below the observed ecliptic. One draconic period is given as 27.212 22 days, 0.109 44 days shorter than the Sidereal Period. This value means that the orbit regresses by some 193 per year, or 16 times per draconic month. The realignment of these nodes takes about nine years to complete. (See Figure 01.) Combining the movement of the draconic month with the time the Sun crosses the same lunar node, is the basis for the calculation of eclipses with the Saros cycle. For these two to align it takes 6585.78 or just over nineteen years eleven 11 days. A repetition of a solar eclipse within the Saros cycle, can only occur, when these two are again in alignment.

After New Moon, each succesive night sees the lunar phase increase in size or waxes. After 14½ days Full Moon is reached and then the phase starts to decrease or wanes until the next New Moon. This entire period is called a lunation, and is based on the synodic period. Lunations have been counted since the first one on 16th January 1923. As the phase changes, the time for moon rise and moon set increases on average by about forty-eight minutes each day. However, this value may vary considerably. This is because as the lunar motion increases both in the north-south and east-west directions. For astronomers and astrophotographers, observing away from the city lights, the time of moon rise or set is critical. The Moon’s sheer brightness can cause the loss of all the fainter stars and other interesting celestial and deep-sky objects visible in telescopes. Most have to organise their scheduled observations well ahead to maximise their observing time. Normally during Full Moon few observations are undertaken, and to many astronomers our Moon is viewed as a total nuisance. Each time the moon rises and moon sets can often be found in the local newspaper or yearly ephemeris. The lunar face always roughly points in the same direction towards the Earth as the Moon has the same axial rotation as its period of 29½ days. It keeps this same face as it has been ‘locked-in’ the same direction by the strong tidal interactions between the Earth and the Moon. Other planetary moons are also known to show this effect, which astronomers call captured or synchronous rotation.

Yet this is not strictly true because the observed lunar surface can show 59% of the surface observers on the Earth. This is due to the fact that the lunar orbit is slightly elliptical in nature and also because it is tilted by 515 to the ecliptic. Observers on the Earth sees the Moon appear to 'nod' over the lunar month and can be seen in perspective depending where the Moon lies in its orbit. The term used to describe this effect is called libration. The elliptical orbit of the Moon causes East-West Libration while the tilt of the ecliptic causes the North-South Libration.

The true motion of the lunar orbit varies slightly through its orbit due to the fluctuations in the Earth’s gravitational field. Observations of stars that are sometimes obscured by the Moon, can accurately measure the exact lunar position. Such events are called lunar occultations. As stellar positions are known quite accurately then the position of the Moon can be determined accurately. All the variations that are seen are presently not fully understood, but details about the Earth gravitational field continue to improvement eclipse predictions of both Sun and the Moon. Much of this has been achieved made from many lunar occultations of the years.

The Earth's Tides

One of the most obvious influences of the Moon is the changing tides changing twice a day between high and low tides. The average difference between successive high tides is about 12 hours 24 minutes and 31 seconds. At Sydney's Fort Denison the maximum variation is about two metres, though it may vary from place to place depending on the topography of the ocean's surface. Other influences are caused by tidal flows or storm surges, earthquakes or by nearby cyclones and tornadoes. Countries like the Netherlands and Venice, which are low lying areas, these effects can be very serious as they may cause catastrophic floods.

Astronomically, the cause of the tides is the combination of the gravitational influence of both the Sun and the Moon on the Earth, with the Moon contributing about 75% of the energy, the Sun 25% Over the Earth, the water distribution is 'lop-sided dumbbell' shaped as it is dragged along behind the moon by the Earth's rotation. All tides vary quite differently from place to place depending on the depth of the sea. At certain times of the year usually in December and January, the combination the gravitational pull of the Sun and the Moon can produce king tides. This is caused by the Earth's close perihelion passage from the Sun, often combined with the Moon being closest to Earth.

For the Moon the gravitation influence pulls the water towards it. The axial rotation of the Earth moves through this tide twice each day. The water does not directly align with the Moon but lags behind by almost two or three hours. Hence, Full-Moon at midnight will produce a high tide around 2am to 3am in the morning. Another influence of the Moon is the so-called solid tides, that physically move the Earth surface up and down. The influence is much smaller than the water tides moving about 5 cm twice daily. The greatest influence on Earth is actually in Western Australia.

Also the alignment of the Sun and the Moon also contributes to the levels of low and high tides. When the Sun and the Moon are aligned, near Full Moon and New Moon, the spring tides are produced, when the tides appear slightly higher than normal. When the Moon is at First or Third Quarter, produce the neap tides, and are lower than the spring tides.

Tides have many different variations in different parts of the world. The prediction is based on measurement of tidal effects at particular locations. However, some of the effects can be predictable which has lead to an understanding of the shapes and volumes of the seas.

All tides add frictional forces to both the Earth and the Moon. These forces are complicated, but it is known to slow down the Earth's rotation and speeding up the Moon’s orbital velocity. About four billion years ago, the Moon was closer to the Earth, with the Earth’s rotation being estimated to be about twenty-two hours long. Presently the Moon continues to moving away by about four centimetres per annum from the Earth's surface. Eventually, eons in the future, the Earth will rotate with the same orbital period as the Moon, whose period will be around sixty-two days in length.

The Lunar Surface

Telescopes pointed towards the Moon finds it very different place than Earth. Galileo was first to observe the moon telescopically and was simply astonished by its complexity. The surface is littered with a random distribution of thousands upon thousands of craters. Although the moon to the naked-eye has the appearance of a selenium-grey coloured surface, the moon was once thought to be something similar to that of the Earth. Early astronomers had imagined that these were several 'seas' and these were termed the Latin name singularly mare and as the genitive, maria. Continued usage of is certainly misconceived, as all mare contains no water, but the name has stayed. Our moon is an airless world that changes in temperature well above the boiling and freezing point of water - depending if it is shadow or direct sunlight. The creation of the maria are thought to be ancient lava flows that occurred on the lunar surface three or four billion years ago. The face-parts in the 'Man in the Moon' are examples of the maria.

Telescopes pointed towards the Moon finds it very different place than Earth. Galileo was first to observe the moon telescopically and was simply astonished by its complexity. The surface is littered with a random distribution of thousands upon thousands of craters. Although the moon to the naked-eye has the appearance of a selenium-grey coloured surface, the moon was once thought to be something similar to that of the Earth. Early astronomers had imagined that these were several 'seas' and these were termed the Latin name singularly mare and as the genitive, maria. Continued usage of is certainly misconceived, as all mare contains no water, but the name has stayed. Our moon is an airless world that changes in temperature well above the boiling and freezing point of water - depending if it is shadow or direct sunlight. The creation of the maria are thought to be ancient lava flows that occurred on the lunar surface three or four billion years ago. The face-parts in the 'Man in the Moon' are examples of the maria.

There are many other types of terrain seen on the moon, and to avoid confusion with the lunar (and planetary) features, the neutral language of Latin always used. Most of the these terms, however, are not highly descriptive nor accurate, ie. lakes, marshes and oceans when the Moon has no water. Common examples used on lunar maps include;

Naming of Lunar Craters

irst introduced by Giovanni Riccoioli in 1651, the largest craters have been named after famous astronomers or other people of historical note, as Names like Tycho, Kepler, Hipparchus, Messier, Copernicus etc. Most of the brighter craters have been named after astronomers or legends of the ancient and renaissance periods. The others are named after people in science, the arts and even politicians. Over the years, especially since the exploration of the Moon in the space age, the number of known craters has increased. Often surrounding the larger ones, they are labelled by capital Roman letter. ie. Copernicus A, Kepler D, etc. Names have also been recognised for the invisible portion that cannot be seen from the Earth. Naming lunar features is controlled exclusively by the IAU - International Astronomical Union.

Names and places on the moon can be purchased as lunar maps. Some books also have maps of the Moon within them. All maps are reversed, so identification of the features is similar to the telescopic view. (See The Lunar Map.)

Formation of Craters

Craters range from 350 kilometres down to several centimetres. In general, the craters have high walls sometimes having a dimple in the centre or are littered with debris. Most craters were formed by meteorite impacts and usually are clean and smooth inside the crater with tiny dimples in their centre. Each has been produced by meteorites hitting the surface, liquefying the rock in slurries then setting into the familiar crater shape. These central dimples in craters are indisputably indications of this type of impact. The surface in this collision melts literally setting in the familiar shapes we see today. Most of the lunar craters are very old and formed perhaps between three and four-and-a-half billion years ago, and it is likely groups of craters was formed in the several impact eras during the formation of the Solar System. As the Moon has little atmosphere to speak of, and therefore lunar features cannot suffer the effects of water or wind, enduring for periods exceeding many times that of the age of the Solar System. For this reason the moon is really a snapshot of its own history.

Other craters have been likely made by volcanism, where the internal portions of the moon pushed up the surface into cones. As the Moon's interior cooled, underlying rocks contracted, leaving an exposed dome with space underneath it. A moon-quake, or perhaps meteorite impacts, hits the dome, shattering the dome's shell, so material crashes down in the remaining void, producing the rough, rocky interiors of the craters that are commonplace on the Moon.

Another important feature is that the newer meteor impact craters have radiating white streaks or rays extending across the lunar surface from the crater. This is ejected material and appears from big craters like Tycho and Copernicus. Most of these features are estimated to age between one and three-and-a-half billion years. Features such as rugged mountains, some of these towers over six kilometres high. These can be seen in mountain ranges. Mountain heights can be measured by measuring their shadows on the lunar surface. Other such as rilles can also be seen that look as if water has move through the surface cutting deep channels.

The lunar surface is also covered by layers of micrometeorites and dust to the depth of between two and eight metres, other places maybe twenty metres thick on the Moon. This layer is called the regolith, whose composition is mixture of glass fragments from the impacts and some from the fragmentation of the underlying lunar rock. The footprints, for example, left on the Moon by the Apollo astronauts will not last for all eternity as material from the continued bombardment will erase it - taking at least five hundred million years!

Temperature may reach up to 105^OC at midday, while it can plunge below -160^OC at night. As there is no lunar atmosphere an astronaut’s spacesuit has to have good air conditioner, being both heater and refrigeration unit. Simply walking behind any rock’s shadow causes the temperature to instantly drop by about 265^OC !

The Origin of the Moon

The true origin of the Moon still puzzles planetary astronomers. The Apollo astronauts who landed on the lunar surface, main goal was to solve this particular problem, which was expected to be achieved by taking surface samples of rock and dust. Prior to any of the landings two prominent theories existed. Some believed that the Moon was an independent body, captured in the distant past by the Earth’s stronger gravity. This is called the capture theory. Others thought that the Moon was actual formed from the Earth, either by hit Earth with a large Mars-like object or from the early Earth’s high rotation, with the Moon somehow separating from the Earth’s mass. This latter theory presumes Earth fracturing into to two distinct parts, the second or smaller portions then became the Moon.

To prove if these three theories were correct the lunar rocks could tell. If the rocks were dissimilar in composition, then the first conclusion would likely to be correct. If similar in makeup, then the Moon would prove to be the same as the Earth. The only direct visual evidence of the presumed Earth-Moon connection was the Pacific ocean basin. This ocean covers about half the Earth’s surface and it was assumed that the Moon could have been carved out of this huge expanse.

All the rock samples provided by the twelve Apollo astronauts and three Soviet Luna explorers, has proved to be about the same age as the Earth. The major difference in composition of variations was from lunar meteorite impacts. This seems to have happened in the first billion years of the Solar System. These observed differences have still kept this debate opened.

Observations of the Moon

Observations of the Moon are best made when the Moon is high in altitude when the conditions of seeing and transparency are generally better. Sometimes this cannot be avoided, especially two to four days before and after new moon. The best time to observe the moon is usually around first and last quarter.

At the time of new moon the earthshine can be seen on the lunar face of the darkened moon is illuminated by the light from the Earth’s surface. From the moon, the observed Earth phase is always opposite, so at this time, the near ‘Full Earth𔃅' would be incredibly bright -14 or -15 magnitude! When at low altitudes to the horizon also discloses a yellow colour, simply due to light refracting through more of the Earth’s atmosphere. Observation during full moon becomes limited, as sunlight falls from lunar zenith and makes no shadows and the lunar disk appear almost featureless. Perhaps of interesting at this time are ray features strewn from the brighter craters like Tycho and Copernicus. Sometimes green filter will improve contrast of these features. Daylight observations of the Moon may be achieved using a polarising filter. This partly increases the contrast of the lunar features, but still leaves many craters washed-out by the blue sky’s brightness.

Last Update : 29th April 2006

Southern Astronomical Delights © (2005)

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