Astronomy - The Moon

The Moon

By Margaret Grove

The moon is the earths only natural satellite and is the second brightest object in the sky after the sun. It is also the only object other than earth to have been stepped on by human beings.

We can clearly see, particularly with binoculars and telescopes, many dark patches on the moon’s surface. Ancient astronomers thought these were filled with water and so they are called ‘mare’ Latin for sea. We now know that they are solidified pools of lava.

The moon derives its name from the Romans, Luna and the Greek, Selene and its rhythm has been part of time keeping for thousands of years. Its synchronous rotation means that the far side of the moon was relatively unknown until a probe photographed it in 1959.

Manned Apollo orbits in the late 60s and 70s increased our knowledge revealing a densely crated surface with more highlands and fewer of the dark ‘seas’ than the near side. This indicates a thicker crust where lava could not so easily rise to the surface. 

There is no atmosphere and no oceans but ice left by crashed comets has been found. Other parts of the moon’s surface are very mountainous and their peaks are nearly as high as Everest. The moon is also dotted with craters made by meteorites crashing into it around 4 billion years ago.

It is generally accepted now by scientists that the moon is now some 4.5 billion years old. The earth was still in its molten state when it was hit by possibly an asteroid and it was gravity that drew debris together to form the moon.

This debris must have been at least 14,000 miles away from earth, any closer and it would have crashed back to earth and the moon would not exist. The earth was spinning much faster then so making days and nights much shorter.



It took time for the earth and moon to cool down and become the perfect companions they are today.

The moon orbits the earth on an elliptical path which means there are times when the moon is closer to the earth, Perigee, and rotates faster and times when it is further away, Apogee where it rotates slower. The difference between these two points is about the width of 4 earths.

The earth and the moon have their own centre of gravity but there is a common centre of gravity called the Barycentre. This point is nearer to the circumference of the earth than the centre itself and moves according to the movement of both earth and moon.

Relative to the earth the moon makes one rotation around its axis every 29 and half days on average. This is the same time it takes for the moon to complete one revolution around the earth. This is no coincidence. In the past when the moon was nearer to the earth it rotated faster but one of the effects of the earth’s gravity over millions of years has been to slow down the rotation until the moon has become completely synchronised with earth.

The 29-and-a-half-day orbit is called Lunation or Lunar month. This is the exact time that is recorded between one new moon and the next. The precise measurement is 29 days 12 hours 44 mins. and 2.8 secs. This measurement is an average and not a constant and reflects monthly variations that occur over a long period of time.

Phases of the Moon.

The moon changes its appearance on a very regular monthly pattern. These phases happen because the moon orbits the earth once every 27.3 days. When the moon passes between the sun and the earth we see very little of the sunlight that is reflected from it giving us the new moon. Fourteen days later the moon is on the far side of the earth from the sun and we see it completely illuminated giving us the full moon.

A gradual increase in the visual is called waxing first appearing as a waxing crescent, next the first quarter then to a waxing gibbous increasing to the full moon. The visual decreases to a waning gibbous, the last quarter and waning crescent where it disappears to become the new moon which we cannot see.

The first and last quarter moons mark the halfway point between the full and new moon. It is interesting to note that there is slightly more light reflected from the waning crescent moon than the waxing crescent moon but between the full moon and the quarter phases the waxing period is brighter than the waning period.

The full moon is the brightest as the sunlight strikes the full face of the moon and is reflected back to earth. Another factor has been found increasing the intensity of the moons reflected light while examining moon dust brought back from the Apollo expeditions. It has revealed the role of tiny particles that cling to the surface of Luna sand to amplify rays of light, a condition that has been described as ‘coherent backscattering’.

Under certain conditions such as a full moon the reflection intensifies producing more visible light than during other phases.

The moon rises and sets at different times every day because the civil calendar is based on the solar time table and not the lunar. On average, the moon rise and the moon set are about one hour later each succeeding day, but the time changes considerably from one location to another. Both longitude and latitude have an effect on this change. More northern latitudes being affected the most.

The lunar phase cycle of 29.5 days is longer than the orbital period of the moon, this is because during the moons orbit around the earth it is also moving around the sun and we have to wait a little longer to see the same phase in the sky.

Size of the Moon

Any change in size of the moon is an illusion and only changes slightly when the moon is nearer to the earth on its elliptical path. When the moon is nearer to the horizon it appears to be larger than when it is above [Zenith]. The eye is tricked into measuring the moon against the nearby objects e.g. Buildings, trees and hills etc. This gives the impression of an increased size of the moon.

The Tides

The moon is our nearest celestial neighbour an exerts a constant influence through gravitational attraction. This gravitational attraction is one ten millionth of the gravitational force of the earth. Lunar gravity does not work alone in influencing our tides. They are also influenced by the centrifugal force of the earths spin and gravitational attraction of the sun.

The suns gravitational pull is much greater than that of the moon but because the sun is much further away from earth the moons gravitational pull remains supreme which in turn gives the moon a much greater influence on our tides.

The gravitational force of the moon is tugging upwards on the water while the gravitational force of the earth, which is far stronger, is pulling down at the same time. Instead, the water rises with the tides because of the net balance of the forces i.e. The earth pulling in and the moon pulling out, which averages more in favour of the moon. It does not happen in a perpendicular direction, however, but shows up where the external influence has a greater effect, from side to side.

Spring Tides

During new and full moons, the gravitational force of the sun is in line combining to produce the highest tides.

Neap Tides

During the quarter moons the gravitational forces of the sun and the moon are at right angles partially offsetting each other to produce the lowest tides.

Lunar Eclipse

There are three types of lunar eclipse, a total, a penumbral and a partial eclipse.

The shadow cast during an eclipse has two components, a darker central area [umbra] and a lighter area [the penumbra].


Total Lunar Eclipse

The moon passes completely through the main shadow of the earth. The dark shadow cast by the earth does not completely obscure the moon but changes its colour to a deep copper tone. This colour is created by the filtering effect of the earth’s atmosphere which removes all but red wave lengths of red sunlight.

The colour of the shadow can vary due to factors like the atmosphere, weather conditions and volcanic dust. This spectacle can last for around three hours because the moon and earth are moving slowly in relation to one another and the shadow cast by the earth is so large.

Solar activity can also have an effect on a total eclipse particularly the activity of sunspots and the relative distance between the moon and the sun. The 11-year cycle of solar activity is also known to affect the brightness of an eclipse with the moon appearing dimmer when solar activity is low.

Penumbral Eclipse

This is a partial eclipse that can last around 1 hour when the moon only passes through the secondary shadow of the earth. During the penumbral eclipse, the moons light is dimmed but does not go completely dark as the shadow is not deep enough to block out all the suns light.

Partial Eclipse

The moon enters the secondary shadow then passes through part of the umbra or main shadow. This partial eclipse does not produce the reddish colour of a total eclipse as the secondary shadow is not deep enough to highlight the reflected light from the earth.

Solar Eclipse

When the moon is directly between the earth and the sun it blocks out the sun’s rays.

If the moon is in perigee then we see the wonderful spectacle of a total solar eclipse. If the moon however is at its farthest point, Apogee, away from earth then we have an annular eclipse which shows a ring of sunlight around the edge. If the moon does not take the direct approach across the sun then a dark blob will appear on the edge of the sun giving us a partial eclipse.

Interesting Points.

1] A solar eclipse only occurs 2 weeks after or 2 weeks before a total lunar eclipse.

2] Full moons are the only time lunar eclipses occur.

3] Lunar eclipses can last for a maximum of 3 hours 40 mins.

4] New moons the only time solar eclipses occur.

5] A total solar eclipse can last a maximum 7 mins. and 40 secs. timed from the equator. An annular eclipse can last for 12 mins and 24 secs at most.

A solar eclipse happens at least twice a year but never more than five.

6] A lunar eclipse can never happen more than three times a year.

7] A lunar eclipse is visible over an entire hemisphere where as a solar eclipse is visible in a narrow path that is a maximum of 167miles wide.

8] At any specific geographical location on the globe a total solar eclipse can occur only once every 360 years on average.

9] A solar and Luna eclipse go together in pairs. A solar eclipse is always followed or preceded by a Luna eclipse within an interval of 14 days.

10] The characteristics of one eclipse is repeated every 18 years, one day and eight hours with some minor variations. This long-term cycle is called the Saros cycle.


Rare Moon Effects




The light from the sun reflects off the earth’s surface. When this reflected light from the earth produces visible light on the moon it is referred to as earthshine. When the waxing crescent moon is only a few days old light reflected back from earth can illuminate the full surface of the moon.

Optical Effects

The moon can produce some optical effects when combined with the right atmospheric conditions. Just as light from the sun is refracted through water droplets so the light from the moon can produce the same effect but the colours are less intense. This is a moonbow or Luna rainbow. When moisture is high in the atmosphere ice crystals are formed. If the moon is in the right position it can form a halo or ring around itself.



Moon Dogs or Mock Moons.

These are also produced by the interaction of moonlight and moisture in the atmosphere. With the right combination of humidity and angle, the observer may see bright circular spots on the halo itself caused by the refraction of moonlight through hexagonal shaped crystals. The name for this is parselene.

Rare Moons

Blue moons.

 A Blue moon is the second of two moons that occur in the same month. It can never happen in February. A blue moon occurs approximately 7 times every 19 years and will next appear January 31st and March 31st 2018 then October 31st 2020.





Interesting Points

1] Happens once every 2.7 years

2] 7 Times every 19 yrs.

3] Once every 33 months.

4] Average of 37 every century.

5] About the rarest of all blue moons is the year with two.

6] Once every 100 yrs. a full moon will fall on a leap year, Feb 29th. Last time this happened was 1972 and will occur next in 2048, followed by 2132, 2216 and 2376.

A leap year with no full moon in Feb last occurred in 1608 and the next will be in 2572.

7] The term Blue moon was created by astrologists and not astronomers.

Black Moon

Just as rare as the Blue moon is the Black moon. This is the absence of a new moon in the month. This will next appear in February 2018, 2037, 2067 and 2094

The term was once again created by astrologists and not astronomers.


Snow moon

February full moon is traditionally called the Snow moon.

Harvest Moon

The full moon that is nearest to the autumn equinox

Super moon

This apparent increase in size is due to the moon being in perigee.

Moon Calendars

There are too many to identify here as they differ all over the world. There are names for every month of the year and some depending on religion.

Lagrange Points.

The gravitation of two orbiting bodies produce a unique condition. As in the case of the earth orbiting the sun, 5 specific points in the orbital patterns have the effect of cancelling the gravitational and centrifugal pull on the two bodies. These points are called the Lagrange points after the discovery in 1772 by a French mathematician of the same name. Lagrange points are important spots as they are able to support space stations, spacecraft or permanent colonies in stable orbits without the need for constant refuelling.

The Lagrange points in the earth/moon system are also affected by additional forces from the sun. In order to be unaffected objects would have to be placed in an elliptical orbit.

Unpiloted Moon Exploration

Luna 1 USSR. Landed on Luna surface.1959

Ranger 7 USA. First pre-impact photo 1964

Ranger 8 USA. Transmitted photos. 1965.

Luna 9 USSR. Landed. Transmitted photos. 1966

Surveyor 1 USA. Landed First coloured photos. 1966

Luna 13 USSR. Landed first soil sample 1966.

Explorer 35 USA. Orbited. Magnetic fields. 1966

Surveyor 5 and 7 USA. Landed. Soil tests and photos. 1967.


Apollo Programme.

Started in 1968 USA with Apollo 7. Making orbital tests around the earth.

Apollo 8 1968. First flight to orbit the moon.

Apollo 9 1969. Orbital tests around the earth. First flight of complete Apollo spacecraft.

Apollo 10 1969. Orbital tests and partial descent.


Apollo 12 1969. Landing and surface exploration.

Apollo 13 1970. Flyby. Mission aborted in third day.

Apollo 14 1971. Landing and surface exploration.

Apollo 15 1971. First use of Luna rover. First continuous cover TV programme broadcast of moon walk and extensive scientific study of Luna surface.

Apollo 16 1972. Landing and surface exploration.

Apollo 17 1972. Landing. First geological study of Luna surface.

Moon Rocks

Scientists have discovered many interesting features about the composition and origin of the moon from these rocks, most of them formed by cooling lava. Some rocks are similar to Basalt which is found on earth. Samples were collected in low areas that are observed as Maria from the earth. Rocks from the higher regions of the moon are Gabbro and Norite, similar to rocks of the same names on earth. Although some rocks have some characteristics similar to earth rocks they are recognisably different as they were found not to contain any water.The presence of water has a noticeable effect on minerals in the rocks.

Moon rocks also exhibit crystals of metallic iron because of the lack of free oxygen. Material on the surface of the moon is referred to as regolith or Luna soil but has no organic content. Luna soil forms a layer from 3 to 60 feet deep on the surface. It was created over billions of years by the continuous bombardment of meteorites. Larger meteorites caused visible craters that can be seen from earth. Smaller, virtually invisible craters are formed by particles of cosmic dust. Moon rocks are still being investigated as we speak.

Over the last few years water has been found on the moon which is frozen and trapped in the craters at the Luna poles. Water that has come from the crashing of comets into the moon over time.



Now there is a renewed interest in the moon as scientist are wanting to rethink the possibilities of a return with the idea of building a permanent station using the moons resources. Water that could be turned in to rocket fuel for the purpose of refuelling space craft.

Rockets could be sent deeper into space from the moon thus saving on the enormous amount of fuel [two and half thousand tonnes] it takes to escape the earth’s gravity.

Another idea is to place solar panels on the moon to harness the energy from the sun. The sunlight on the moon is predictable. The energy would be converted into electricity and transmitted down to earth. The amount would satisfy global demand. The cost would be no more than the big oil companies spend over 2 yrs. in the production of oil and gas from the ground. Around £2 billion.

Do We Need the Moon?

The moon is moving away from earth at a rate of 3.78 cm per year. This has been repeated from birth and will continue for the next 4 to 5 billion years.

At the time of its birth both the earth and moon were in a molten state. The moon would have been much closer to earth making it appear very large and would be spinning faster giving us a five-hour day. Even when it had moved away sufficiently and had cooled down enough for the oceans to form, the tides were pulled much higher covering most of the low-lying land. It was a very different place it is today. The pull of the moon was slowing the spin of the earth and continues to do that to this day using the friction between the ocean bulge and the ocean floor.

We only know the 24-hour day due to the length of time we have evolved on earth. All this is due to our moon.

How Do We Know?

Apollo 15 astronauts placed a retroreflector unit packed with small mirrors on the moon.

In New Mexico, a large telescope at Apache point has been firing a series [millions] of laser beams at the units. The response is, one or two photons have been reflected back to earth the distance of which can be measured accurately. This has been a continuous process for the past 40 years and their findings confirm the figure of 3.78 cm is correct.

If it Continues to Move Away, Does it Matter?

As it continues to recede it will look smaller and in turn we will lose one of the world’s greatest spectacles that of a total solar eclipse. The earths spin is slowing down and the days will get longer. When the moon has receded by just 10%, another 24,000 miles, we cannot expect the sun to rise for about 20 hours making the days and nights much longer.

Eventually things will get worse and the stability of our earth will be affected.

The 23 de tilt of our earth give us our seasons. Our animals and plant life rely on them. With further drifting of the moon away from us the angle of the tilt will change over time and cause the earth to wobble. Our earth may also become very wet and if the earth tipped over on to its side for 3 months a year the poles would be under unrelenting sunshine so melting the ice caps. Sea levels would rise 60 meters. Every coastal city in the world would be gone and inland areas that survived would be transformed.

We would have a very dark and cold freezing winter. Over the summer the sun is high in the sky and temperatures soar. Then the pattern is repeated. We as humans could probably survive but other life forms would probably not. Animals could not evolve fast enough to cope with the extremes.

It will take about a billion years before the earth will tip over so we are not in any immediate danger!!

We are indeed enjoying the most stable time of life as we know it, and this knowledge should make us appreciate how fragile the balance is between the Earth and her moon and the Sun.