Astronomy - A Beginner's Guide to the Moon
Credit to Nasa
Unless otherwise stated, photos on this page are credited to Nasa.
My thanks to Nasa - without these images, these webpages would not have been possible
N.B: Please note, all my articles are best read on desktops and laptops
This is the second of four pages which look at the night sky, and what can be seen there by a budding astronomer with just the naked eye or a simple pair of binoculars. This page is the only one of the four devoted to just one object, as we focus on the most conspicuous of all night sights, the Moon.
It is not the most remarkable of all the objects you can see in the night sky. It is not the largest, or the hottest, or the oldest or furthest away.
To many it is just a barren lump of rock. But there is no object in the sky which comes close to matching it in the detail which can be revealed to us simply by the holding of a pair of binoculars to our eyes. Indeed there is perhaps almost as much that can be seen on the Moon with such basic equipment, as in the whole of the rest of the night sky put together. And that fact alone makes the Moon a really excellent subject with which to begin a study of astronomy.
On this page I will locate and describe just a few of the most identifiable features on the surface, and depict these on maps of the Moon.
These Four Pages
The four pages in this series are as follows:
- A Beginner's Guide to the Night Sky - the identification of the types of objects you can see in the sky at night.
- A Beginner's Guide to the Moon - what are the surface features on the Moon which we can see from Earth?
- A Beginner's Guide to the Stars - stars are not all the same. Some are like our Sun, but others that we can see with the naked eye have a true vastness or a brightness which is almost unimaginable.
- A Beginner's Guide to Naked Eye and Binocular Objects in the Night Sky - other outstanding heavenly bodies to look out for in the sky at night.
Contents of Page Two
- Binoculars for Viewing the Moon
- When Looking at the Moon - Use Your Imagination!
- First Impressions - The Face of the Full Moon
- The Moon's Libration (Video)
- Eclipses of the Moon
- The Phases of the Moon
- Viewing the Moon's Features - The Terminator
- The Surface Features of the Moon
- The Most Prominant Maria - Lava Plains on the Moon
- Significant and Easy to Find Craters on the Moon
- Mountains on the Moon
- Man on the Moon
- The Lunar Cycle - The Phases and What to Look For (Video)
- The Earth-Moon Relationship
Binoculars for Viewing the Moon
Some detail on the Moon is of course visible to the naked eye. But hopefully you won't be satisfied with just staring at the Moon for a few minutes with the naked eye; you'll want to see more. A telescope is great for showing fine detail and can reveal mountains and valleys on the surface, but telescopes are expensive and time-consuming to set up, they may give an inverted, upside-down image, and unless your equipment is very sophisticated with motorised tracking, the comparatively rapid rate of movement of the Moon across the sky means that the magnification can become a hindrance - no sooner do you locate the Moon, than it will start to disappear out of the scope's view.
Initially, the best equipment for a beginner is a pair of binoculars - it's easy to find and focus on the Moon, and easy to move your gaze from one feature to the next. For the Moon, as opposed to some of the other heavenly bodies, it's undoubtedly advantageous to use the most powerful instrument you can, perhaps 12x60 or even 20x80. The first figure here gives the magnification, and the second gives the lens aperture which increases the image brightness. The big drawback of such powerful binoculars, is the difficulty of keeping the lenses steady, because the slightest hand shake will exaggerate the movement and cause the Moon's features to dance around the field of view. You must have a steady image, so make sure you can rest your elbows on something comfortable but stable, or - best of all - attach the binoculars to a tripod.
The Moon is the only world beyond our own in the universe which we can study in detail with our naked eyes or with a pair of binoculars. So let’s do it!
When Looking at the Moon - Use Your Imagination!
As well as binoculars, a map of the surface features is essential (the annotated images on this page will do as a starter, although rather more comprehensive maps could be downloaded from the Internet or purchased in educational bookstores). However I would suggest the single most important action you should take before ever you even look up at the Moon, is to put your imagination into gear.
The Moon is just too familiar. Every night one can look up at the night sky and there it is - a great round orb, or a crescent of light, suspended in the darkness. Rather like looking out of the window at the back garden, or the tree at the bottom of the garden, or maybe the house across the street from the garden, the Moon is always there, just one of many familiar objects each of us sees every day or night. Familiarity breeds contempt.
So before you look at the Moon again, just appreciate what it is you are looking at, and just what an extraordinary thing it is to see. Looking at the Earth around you, you may be able to see the horizon a few miles distant (depending on your height and the height of the surface features), or if you climb to the top of a hill, you may be able to view all around the landscape extending for many tens of miles. From a high flying airliner you can see the Earth's horizon a couple of hundred miles distant.
But all of this is as nothing compared to what you are seeing when you look at the Moon. When you look at the Moon, you are looking at something 240,000 miles (about 380,000 kilometres) distant, and you are looking at something more than 2,000 miles (3500 kms) from one side to another. You are looking at great mountain ranges and deep trenches, vast craters and plains. And you can see them in their entirety - not on TV, but in real life. You are looking at something which is quite unlike anything that can be seen here on Earth.
The Moon's Libration
As the Moon orbits the Earth, several factors including speed of revolution and tilt, lead to a slight oscillation or rolling motion - like a wobbling ball. This is called libration, and its rolling effect can be seen in the time lapse video of the full Moon above. One of the effects of this is that the full face of the Moon is not always exactly the same - areas on each of the extreme edges periodically become visible, and then disappear from sight - in effect, we can at various different times see a total of slightly more than 59% of the Moon's surface area.
View of the Moon from the Southern Hemisphere
First Impressions - the Face of the Full Moon
In this section we consider the broad picture of what the Moon looks like, and where it is, and we will concentrate on the Full Moon when the Moon presents a full face to us. One thing that very quickly becomes apparent, is that it is always the same side of the Moon we see. The so-called 'dark side' remains forever hidden to us here on Earth. This is because the Moon rotates on its axis in 29.5 days - exactly the same time it takes for the Moon to complete one revolution of the Earth (this is not really coincidental, but rather results from a gravitational linking of the two motions).
Look at our Moon with the naked eye and you see a patchwork of light and dark areas and a few distinct craters. But look at the Moon through a decent pair of binoculars and the number of craters are multiplied a hundred fold, and many are ringed with ridges and rays of ejecta material. Some of the more prominent surface features will be identified and described later.
By convention we label directions on the Moon in the northern hemisphere as we do on Earth. Thus the left edge is regarded as the West, and the right side is the East, with North and South respectively at the top and bottom.
Viewing in the northern hemisphere, you will be able to find the Moon in the sky towards the south (exact elevation depends on the time of the year, and will be at its highest in the winter). Each night the Moon will rise in the east and seem to move during the course of the night to set in the west - a left to right motion in the sky.
Viewing From South of the Equator
This page is really geared towards observation of the Moon in the northern hemisphere. If you live in the southern hemisphere, you could still use this page, but bear in mind the Moon will be 'upside down' with the crater Tycho at the top. Directions on the Moon are the opposite to those I've described for the Northern Hemisphere. Thus, the Western edge is now on the right side, and the South pole of the Moon is at the top. What's more, the Moon will be positioned towards the north on Earth, and although it will still rise in the east and set in the west, in the southern hemisphere, this will be a right to left movement in the sky.
Eclipses of the Moon
Briefly I will mention eclipses of the Moon. Not to be confused with the far more dramatic eclipses of the Sun, when the Moon is between the Earth and the Sun, eclipses of the Moon occur when the Earth moves directly between the Moon and the Sun. One might expect a situation like this to occur every month, as the Moon orbits the Earth, but in fact the Moon orbits in a slightly different plane to the Earth, and is rarely directly in the shadow of the Earth. Usually it is slightly above or below the shadow of the Earth. Nevertheless, lunar eclipses do occur quite regularly, and if one is forecast, then it is well worth seeing. Gradually the shadow of the Earth takes 'bites' out of the Moon's surface (always a full Moon of course) as seen in the multiple image here. If the eclipse is total, then the Moon may well remain visible as a result of faint sunlight refracted through the Earth's atmosphere. But just as sunlight passing through the Earth's atmosphere at dawn or dusk can make the sky look reddish, so the sunlight now hitting the Moon through our atmosphere can give the Moon an orangey-reddish appearance as shown above.
Phases of the Moon - the Sun Is Shining from the Right Hand Side
The 'smile-like' Crescent Moon near the Equator
The Phases of the Moon
We all know that the Moon goes through a cycle of phases from NEW to FULL and back again to NEW. This cycle takes approximately 29.5 days, and the complete cycle can be divided into four segments or quarters.
1) New Moon - The Moon is dark because it lies between us and the Sun. In the daytime the Moon will be close to the Sun in the sky, and the side facing us will not be receiving the light of the Sun. In the nighttime it will be on the other side of the Earth to us.
2) Waxing - Over a period of just over 14 days, the Moon 'waxes' to full. During this period gradually more and more of the side of the Moon facing us becomes illuminated by sunlight as it moves around our planet. At first we see a thin crescent. (In the northern hemisphere this will be to the right, in the southern hemisphere it will be to the left - in both hemispheres this is considered to be the Eastern edge). Gradually this expands, and when more than half the Moon is in sunlight we term it a WAXING GIBBOUS Moon.
3) Full Moon - Halfway through the lunar cycle, the Moon has orbited half of the way around the Earth. Therefore at nighttime we are between the Moon and the Sun, and the whole side of the Moon facing the Earth is lit by the Sun.
4) Waning - The waxing phase of the Moon is now reversed as the Moon completes its journey round the Earth. Gradually, the side facing us moves into shadow from GIBBOUS WANING, to thin crescent. (The waning crescent will be on the left side of the Moon in the northern hemisphere, and it will be on the right side of the Moon in the southern hemisphere - in both cases this is considered to be the Western edge).
The phases of the Moon do not alter with longitude - they will look identical in New York, Madrid and Beijing. And the timing of the phases of the Moon also does not alter with latitude - when New York experiences a New Moon, so will Lima in Peru. But what does alter with latitude is orientation of the phases. We have already described under 'waxing' and ' waning' above, how the crescent Moon will be reversed in orientation from the northern to the southern hemispheres. And if you live half way between the northern and southern latitudes - ie: near the equator - your view of the Moon will effectively be turned on its side. In the case of the crescent Moon, the crescent will arch upwards - our nearest neighbour in space will resemble a smile!
Viewing the Moon's Features - the Terminator
On the crescent Moon , or indeed at any stage other than New Moon or Full Moon, there is clearly a dividing line between the part we can see, because it is lit by sunlight, and the part which is in darkness. This dividing line is known as the 'Terminator', because it is the terminal edge of visibility (nothing to do with Arnold Schwarzenegger robots from the future). Because the terminator is on the edge of sunlit and shaded regions on the Moon, it represents 'dawn' or 'dusk' on the surface, and it follows that on the terminator, the Sun will be very low in the Moon's sky, at which point it will cast long shadows. The value of this from our point of view is that shadows emphasise changes in relief on a surface, and therefore the terminator is the best part of the Moon to look at to see craters, mountain ranges and the like to best effect. For this reason most astronomers who look at the Moon will choose each night to study the regions of the Moon in the vicinity of the terminator. For a good illustration of this, look at the crescent Moon at the very top of this page - you will see the craters on the curved terminator on the left of the picture are much more distinct than the craters on the right side where the Sun is much higher in the Moon's sky. For an even clearer illustration of the terminator's effect, view the video 'The Lunar Cycle - The Phases and What to Look for', later on this page.
The Surface Features of the Moon
The most obvious characteristic of the Moon's surface, very clearly visible even to the naked eye, is that the Moon is made up of light and dark areas, pock-marked to varying extents with meteor impact craters.
Highlands - The light areas, which make up the majority of the Moon's surface, are called 'terrae' or the 'Highlands' because for the most part this is much higher ground than the dark areas. They also comprise the most ancient surface of the Moon about 4 billion years old. These are very rugged and heavily cratered lands, because they date from the earliest days of the solar system when meteor impacts were much more commonplace than they are today.
Maria - The dark areas are called 'maria' or 'seas', because in past times it was hypothesised that they may represent genuine seas and oceans on the Moon. Now of course it is known that the Moon is essentially a dry world on the surface. So what are the maria? They are comparatively low lying basins originally created by huge meteor impacts and subsequently filled between 4 and 3 billion years ago by massive flows of basalt lava, at a time when the Moon was geologically active. Basalt is very dark in colour, and that is why the maria lava flows are dark grey. Because the maria are slightly younger than the highlands, and these lava flows covered any craters which existed at the time, craters in the maria are fewer in number and less ancient than some of those in the highlands. (On this page I will use the English translations of the Latinised names of the maria, because they are easier to remember, but to be honest, most astronomers use the Latin names, so it would be as well to learn these too.)
Craters and Ejecta Rays - The Moon features hundreds of thousands of meteor impact craters on its surface, most of which are very ancient. They exist today because the Moon has been geologically pretty much dead for more than a billion years, and with no erosion from rivers or wind or ice, there has been almost nothing to degrade the craters. (The Earth has been hit at least as many times, but weathering, earthquakes, soil deposition etc, quickly obliterates craters on Earth).
Some of the Moon's craters exhibit a feature which is readily visible in binoculars, and these are the lines which can be seen radiating out from their rims. They are caused by material ejected from the surface when the meteor hits, and in the case of a large crater, may extend for hundreds of kilometres. One famous crater in the south of the Moon - Tycho - has such prominent rays that they are easily visible to the naked eye.
Mountains of the Moon - Guide books to the Moon will often list features such as mountain ranges and valleys. Perhaps my eyesight is not what it should be, but in all honesty, without a telescope, you may find it rather difficult to see many of these. However, a few are quite prominent, and the most attractive mountain ranges are described elsewhere on this page. (Mountain ranges - not to be confused with the more generalised 'Highlands' which are described above - are believed to have been created by pressure waves and debris raised up by the massive meteor impacts which formed the maria basins - therefore, they tend to occur on the perimeters of the 'Seas').
There now follows a series of maps and videos of the most prominent sights to see.
The 'Seas' or Maria (the Dark Areas) of the Moon
The Most Prominent Maria - Lava Plains on the Moon
Ocean of Storms (Oceanus Procellarum) - This very vast plain is fittingly the only dark area on the Moon described as an 'ocean' rather than a 'sea'. Covering most of the western edge of the Moon, the Ocean of Storms covers an area of about 2 million square kilometres (750 thousand square miles). 'Vast' when talking about the Moon is relative as a description, because the Moon is much smaller than the Earth. The entire surface of the Moon is only a little bit bigger than Africa, and the Ocean of Storms is actually smaller than the Mediterranean Sea. Unlike most of the maria, the Ocean of Storms does not conform to an ancient impact crater basin, but dates back to a huge lava flow nearly 4 billion years ago.
Sea of Clouds (Mare Nubium) - This is a southern plain, immediately above the most conspicuous rayed crater Tycho, and merging into the Ocean of Storms.
Sea of Crisis (Mare Crisium) - This is the most distinctive and attractive of all the dark plains on the Moon detached as it is on the extreme eastern rim of the Moon. The Sea of Crisis is about the size of Uruguay, approximately 550 kilometres (340 miles) in diameter and surrounded by high mountains.
Sea of Fertility (Mare Fecunditatis) - The Sea of Fertility is the most southerly of three similar maria extending down the eastern side of the Moon. This one is about 840 kilometres (520 miles) in diameter.
Sea of Moisture (Mare Humorum) - A distinctive small mare in the south west about 390 kilometres (240 miles) across (similar in size to the State of Ohio).
Sea of Nectar (Mare Nectaris) - This is a relatively small mare close to the Sea of Fertility and the Sea of Tranquility. It is about the size of Iceland.
Sea of Serenity (Mare Serenitatis) - A large 'Sea' on the north eastern aspect of the Moon, approximately 670 kilometres (420 miles) in diameter - similar in size to the nation of Germany. The Sea of Serenity is the most northern of the great eastern maria.The last of the Apollo Moon landings occurred here.
Sea of Showers (Mare Imbrium) - In the north west of the Moon is this large circular plain, roughly 1250 kilometres (750 miles) in diameter. The Sea of Showers is surrounded by mountainous ridges, some of which are visible in binoculars.
Sea of Tranquility (Mare Tranquilitatis) - The most famous named feature of all on our Moon, and the only feature which many people will know. And for one simple reason - the Sea of Tranquility was the great dark plain where Neil Armstrong and Buzz Aldrin first set foot in 1969 (the precise location in the south west of the plain is indicated in the third of these annotated maps). The Sea of Tranquility is in the middle of the three large plains on the eastern side of the Moon.
The Most Prominent Craters to Be Seen on the Moon
Significant and Easy to Find Craters on the Moon
Archimedes - On the eastern edge of the Sea of Showers, Archimedes is about 82 kilometres (50 miles) in diameter.
Aristarchus - A quick glance at the annotated photo of the Moon above will show that the crater Aristarchus has the distinction of being the most brilliantly illuminated (most reflective) spot on the entire surface. It is only 40 kilometres (25 miles) in diameter. (Bear in mind, all craters on the Moon visible in binoculars are much much larger than the famous Meteor Crater in Arizona which is little more than one kilometre in diameter).
Aristoteles - An 87 kilometre (54 mile) diameter crater in the northern region on the Sea of Cold. Just south of Aristoteles is another prominent but slightly smaller crater called Eudoxus (not labelled on the image above, but clearly visible).
Clavius - One of the largest and oldest craters on the Moon, Clavius is a 4 billion year old 225 kilometre (140 mile) walled plain in the extreme south of the Moon. The famous crater Tycho is directly north of it.
Copernicus - Copernicus is probably the most attractive crater on the Moon when viewed close to the terminator, with the prominent rim illuminated against the shadow filled floor of the crater. Copernicus is about 100 kilometres (60 miles) in diameter, and the site of an extensive system of rays.
Grimaldi - On the extreme western edge of the Moon is a large crater which makes a great contrast with the exceptionally bright Aristarchus a little further north. Grimaldi is one of the darkest craters on the Moon, and very easy to spot when the Moon is full.
Kepler - This bright crater, like its near neighbour Copernicus, has a system of rays.
Langrenus - One of the first prominent of craters to become visible on the waxing crescent Moon, Langrenus is about 130 kilometres (80 miles) in diameter.
Longomontanus - This 145 kilometre (90 mile) crater is easily located by its proximity to the famous crater Tycho.
Manilius and Menelaus - This is a nice pair of fairly bright small craters in the east. Manilius, in the Sea of Vapours, is 39 kilometres (24 miles) in diameter. Menelaus is slightly further east, and slightly smaller at 27 kilometres (16 miles).
Plato - One of the most distinctive and identifiable of the craters on the Moon because of its location in the extreme north of the Moon, and because it is a particularly dark crater, about 100 kilometres (60 miles) in diameter.
Plinius and Proclus - These are two craters on the perimeter of the Sea of Tranquility which are not particularly large, but both are easy to find, by virtue of their position. Plinius, a 43 kilometre (27 mile) crater, is sandwiched between the two great Seas of Tranquility and Serenity. Proclus is even smaller at 28 kilometres (17 miles) and lies between the Sea of Tranquility and the Sea of Crisis.
Tycho - In the heart of the southern region of the Moon is one feature which is one of the most conspicuous on the face of the full Moon. The crater Tycho has very dramatic rays emanating from the crater for distances of up to 1500 kilometres (900 miles). Unlike most features which are best seen at or near the terminator, the rays are most visible when the Moon is full. At other times Tycho, which is actually only 85 kilometres (53 miles) in diameter, is less distinctive. Why does Tycho have such prominent rays? Because it is one of the most recent impact craters. A mere 108 million years ago a meteor crashed into this part of the Moon - insufficient time on the relatively inactive surface for the rays to have been been degraded by weathering, or by further impacts.
Mountain Ranges on the Moon
- Apenines and Caucasus Mountains - The Apennine mountain range is perhaps the most distinctive range on the surface of the Moon. It can be seen quite clearly in the photos on this page as a pale, narrow streak between the Sea of Showers, and the Sea of Vapours. The mountains extends for about 600 kilometres (370 miles) and some of the peaks rise as high as 4600 metres (15,000 feet), including Mons Huygens - one of the highest mountains on the Moon. It is believed the Apennines may have formed when land was thrust upwards in the massive meteor impact which later formed the basin of the Sea of Showers. The Caucasus Mountains are a continuation of the Apennines to the north east, where it forms the border of the Sea of Serenity.
- Sinus Iridium and the Jura Mountains - Sinus Iridium or the 'Bay of Rainbows' appears as a bulge on the north western side of the Sea of Showers. It represents the remains of a huge 260 kilometre (160 mile) diameter crater, half obliterated by the even bigger impact which later created the sea of Showers. - that's why Sinus Iridium is a clearly semicircular structure today. All around the edge of the crater is a mountain range generated by the impact. these are the Jura Mountains, and this ring of mountains on the Moon is one of the most visually attractive in binoculars.
Mountains on the Moon --- and Man on the Moon
Man on the Moon
Finally, I would like to mention the sites of the six Apollo Moon landings. Although of course you cannot see anything of the landings with a pair of binoculars (or even a telescope), it may still be of interest to be able to look up at the sky at night and see exactly where people have walked on this alien body, 380,000 kilometres (240,000 miles) distant. The sites are marked in orange on the map above.
- 11 - Apollo 11 - Sea of Tranquility (Mare Tranquillitatis) 20th July 1969. Neil Armstrong and Edwin 'Buzz' Aldwin, with Michael Collins in the Orbiter. It was on this precise spot that mankind first walked on another world, when Neil Armstrong climbed down the steps of the lander on 21st July. As such, I suspect that this spot on the Moon will in future millennia - even more so than today - develop an almost sacred reverence for human beings. No matter where we might one day go, this will become perhaps the most famous spot on any heavenly body.
- 12 - Apollo 12 - Ocean of Storms (Oceanus Procellarum) 19th November 1969. Charles 'Pete' Conrad and Alan Bean. Just a few short months later we were back, this time in the western hemisphere. Conrad and Bean spent more than 7 hours collecting samples at distances of hundreds of metres.
- 14 - Apollo 14 - Fra Mauro 5th February 1971. Alan Shepard and Edgar Mitchell. After the ill-fated Apollo 13 mission, Apollo 14 became the 3rd Moon landing near a small crater. This was the mission where Alan Shepard famously hit two golf balls on the Moon.
- 15 - Apollo 15 - Sea of Showers (Mare Imbrium) 30th July 1971. David Scott and James Irwin. For the first time on this mission, a lunar rover vehicle was used to traverse several kilometres of terrain, in a scenically and geologically interesting area at the foothills of the Apennine Mountains.
- 16 - Apollo 16 - Descartes Highlands 21st April 1972. John Young and Charles Duke Jr. Apollo 16 landed in the highlands near a crater called Dolland. Again, a lunar rover was deployed, and three Moon walks were made.
- 17 - Apollo 17 - Taurus Mountains 11th December 1972. Eugene Cernan and Harrison Schmitt. This final mission landed in a mountainous region on the south eastern rim of the Sea of Serenity. And when they blasted off from the surface on 14th December, the Apollo programme of Moon landings came to an end.
One day we will return.
The Lunar Cycle - the Phases and What to Look For
About This Video
This excellent video (uploaded by aewstudios) shows the complete lunar month from New Moon waxing to the Full Moon and then waning back to New Moon, condensed into just 103 seconds. I will use the video to illustrate the different phases and to highlight how the scenery of the Moon changes with the timeline shown in the video.
How to use the video and text:
1) Where specific times are indicated, it may be a good idea to pause the video exactly at this time to read the notes, in which a few of the prominent features are recorded.
2) Where 5 or 10 second passages of time are indicated, read the notes and then play and replay the video to visualise the changes in the Moon's features :
- 20 SECS : After darkness, sunlight begins to illuminate the thin crescent Moon
- 25 SECS : This is the 'waxing crescent'. The Sea of Crisis is the most prominent feature on the terminator above centre, and the mountainous range which marks the left edge of the 'Sea' is sunlit
- 25-35 SECS : See how the craters in the southern hemisphere show clearly as each appears in turn on the terminator
- 35-40 SECS : Not so distinct, but over these 5 seconds, look at the region in the north between the Sea of Showers and the Sea of Serenity. A thin pale line runs NE to SW. This is the Apennine Mountain Range
- 40 SECS : The 'waxing gibbous' phase. Most prominent near the terminator is the crater Copernicus in which you can see both light and shadow as the oblique rays of the Sun casts only a crescent of light on the crater bed. On the right side of the crater, the bed is in the shadow of the crater rim. Also, at this time, near the north pole of the Moon is the dark crater Plato
- 40-45 SECS: Notice how Copernicus becomes less conspicuous as it moves away from the terminator and the crater bed moves into full sunlight. Notice also how the bright rays of Tycho in the south become prominent during this phase. And on the extreme western edge of the Sinus Iridium you can now see the light line that is the Jura Mountains
- 50 SECS : 'Full Moon'. Compare the very dark crater Grimaldi, which has now appeared on the extreme left, with the small but very bright crater Aristarchus in the '10 o'clock' position. See how prominent Tycho's ray system is now, but also note how many other craters have lost their prominence when they are exposed to the full glare of sunlight
- 55 SECS : As the Moon begins to wane, two craters become very distinctive on the terminator. In particular, the more northerly of these, Langrenus, shows clear shadows on the crater floor, cast by the crater rim
- 1 MIN- 1.05 MIN : The 'waxing gibbous' phase shows better than any other how craters become more and more prominent as the terminator approaches. Look particularly at the southern hemisphere to see this
- 1.10 MIN : As the Moon enters the 'waning crescent' phase, at this precise moment in the cycle, the entire rim of the crater Copernicus is bathed in sunlight, while the crater floor is in shadow
- 1.25 MIN : The side of the Moon facing us is once more in darkness. The Sun is now illuminating the far side of the Moon
The Earth - Moon Relationship
This page is really all about looking at the Moon and identifying features. But it undoubtedly helps to appreciate these features if there is just a little knowledge about the history behind them, and the significance of the Moon to us today. So what follows are a brief few paragraphs about this.
Today it is generally believed that The Moon was actually created as the result of a stupendous collision between a large astronomical planetoid called Theia, and our own planet Earth about 4.5 billion years ago, soon after the Earth's creation. The Earth was almost destroyed in the collision, and a significant amount of its matter was ejected in a massive explosion into outer space. This debris slowly coalesced under the influence of gravity to form a solid ball of rock - our Moon. The Moon therefore is just a little younger than the Earth.
In the early days of the Moon, there was a huge meteoritic bombardment, and most of the craters on the Moon date back to this period, roughly 4 billion years ago. Soon after this, impacts reduced in frequency, but volcanicity led to major outflows of lava into the low lying basins created by the biggest meteor strikes. Thus were formed the maria or 'Seas'. For the past 1 billion years or so, the Moon has been pretty much geologically and atmospherically inactive, so nothing gets quickly eroded away by weather, rearranged by Moon quakes, or covered up by lava. For this reason, almost of the rock surface we can see is much older than that on the Earth, and in the Highlands especially, most of the rocks and the craters date back several billion years.
There is one last aspect of our Moon which is worth briefly mentioning. When you look at the Moon, do not just think of it as a big lump of rock; it's a bit more important than that. The Moon's gravitational pull creates our tides, and tidal regions on Earth are considered by some to have been of major importance in enabling life to emerge from the oceans on to the land. The Moon's gravity also stabilises the Earth's tilt. Without this stabilising influence, our seasons here on Earth would fluctuate hugely. The course of evolution would therefore have been very different. In fact, without that dead globe up in the night sky, the planet we live on would certainly be very different, and we humans might not even exist.
The Moon is a great starting point for an interest in astronomy to develop. To be able to look up at the sky and see a whole other world suspended in space is a reason enough to be intrigued, but to be able to identify great geological features on the surface, and to learn what those features are, makes it truly fascinating.
Next time you have a clear sky and the Moon is visible, take a look at it with a pair of binoculars and just see what you can see.
(And if the Moon isn't visible, well then take a look at some of the other sights covered in my other pages in this series.)
Questions & Answers
© 2012 Greensleeves Hubs