Is this the first picture of the Earth and Moon together? Well one which is sharp and you can see detail on both surfaces. It was taken by the American Lunar Orbiter 1, the first of the five very successful moon missions in 1966. The spacecraft was launched on August 10th and this was shot from a height of 1198 km above the Moon at -14.68° lat 104.34° long. Near the western edge of Mare Orientale.
The frame number is 1102 h2 and was shot on the 65 orbit on 24th August at 9h 10m 17s GMT. This is the centre shot of a three shot series of high resolution pictures.
They can be seen at:
This year, 2009, believe it or not, is the fortieth anniversary year of the first Apollo Moon landings. It was on July 19th 1969, that the Apollo 11 Lunar Module Eagle landed on the Mare Tranquillitatis with Neil Armstrong and Edwin “Buzz” Aldrin on board. While up in lunar orbit, Michael Collins looked after the Command Module Columbia. I can’t believe it was so long ago. Staying up all night to watch the BBC program with James Burke and Patrick Moore in the studio, all watching the feed from Mission Control and trying to clarify the shorthand talk of the astronauts and controllers at Apollo Control, Houston. All the excitement and anticipation of watching history being made. The excitement of watching it on our B&W TV, the CAPCOM voice saying “Houston. You’re go for landing. Over.” The strange worrying messages “Roger. 1202 alarm.” “Sixty seconds.” Then “Thirty seconds.” Then “Eagle looking great. You’re go.” From Eagle, Armstrong saying “Tranquillity Base here. The Eagle has landed.” CAPCOM replied “Roger. Tranquillity. We copy you on the ground. You’ve got a bunch of guys about to turn blue. We’re breathing again.” I think the whole world started to breath again! It was only much later we all discovered that there was only about 10 seconds of fuel left in the tank when they switched off the landing engine, phew! Neil had had to fly the lander over a small crater with lots of boulders onto a smoother area, so had used nearly all the fuel.
There was a buzz in the studio when they decided to do the moon walk early and not have the scheduled rest from the landing. The first B&W TV from the Moon! I don’t remember ever reading a SF book were the first steps were televised to the waiting world. The picture was up-side-down for a few seconds then someone corrected it and the world watched the first steps on the Moon. So Neil and Buzz got to go down in history as the first humans to walk on another world. The footprints they left on the Mare Tranquillitatis will last for millions of years (unless the tourists of the future don’t trample all over them!) They were followed on November 19th by the Apollo 12 mission to the Oceanus Procellarum with Charles “Pete” Conrad and Alan Bean. In all 12 men walked on the moon, but 24 have been out there, 3 of them twice, Lovell, Young and Cernan.
It was President John F Kennedy’s wish back in 1961 May 12th, at his 47 minute State of the Union address when he said: “First, I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth. No single space project in this period will be more impressive to mankind or more important for the long-range exploration of space. And none will be so difficult or expensive to accomplish” It was a bold claim, none of the thousands of engineers and space buffs at the time had any idea how to do it. All they had done up to then was to send small satellites into close earth orbit. The Soviet Union had put Yuri Gagarin into orbit on April 12th 1961, his 108 minute flight had shocked America and they were desperate to match the Russians with even larger rockets.
At the time of Kennedy’s speech only Alan Shepard had made a 15 minute up and down flight in a Mercury-Redstone rocket on May 5th. So getting men to the Moon seemed a long way off. It was not until the next year, 1962 Feb 20th, after John Glenn made 3 orbits of the earth did the Americans feel that at last they were on the way. The Russians had already spent a day in space in 1961 and in August 1962 launched two Vostok’s, 3 and 4 within a day to have two Cosmonauts in orbit at the same time. The Space Race was on.
But on December 19th 1972 the dream stopped. The Apollo 17 Command Module Challenger splashed down in the sea after the longest time spent on the lunar surface by Gene Cernan and Harrison “Jack” Schmitt. Their visit to Taurus-Littrow was our final moon visit until we can get NASA to deliver the next folk to the lunar surface. Of cause out of the woodwork will come the conspiratory “experts” who claimed it was all a fix, a film set in the desert at Area 51. All the “moon rocks” are flakes, all the photographs “false” and all the people on earth were fooled by the Americans. The astronauts are all liars and only pretended to go to the moon, brain-washed?
I have no doubt it could be flaked now. But this was in the 1960’s and what was one of the top rated TV shows at the time with state of the art effects? Thunderbirds. Complete with strings attached. OK, so maybe the NASA organisation had a bigger budget than Pinewood, but it was still years before Photoshop was invented (in 1987), to do photographic retouching and computers at the time had a microscope amount of memory. Remember the Sinclair Spectrum with 16k memory? It came out in 1982!
How would it have been possible to flake the missions with such crude technology? And where did the Saturn 5’s go after launch? Two million folk were around the cape to watch Apollo 11 lift off in July 1969. That would be pretty hard to flake, and the film and video on the moon was impossible at the time to flake, the technology didn’t exist.
It would have been cheaper to go to the moon than try to flake it, and so they did!
By Mike Frost
I am a member of the Solar Eclipse Mailing List (yahoo groups SEML), a group of people worldwide with a passion for all things syzygycal*. As you can imagine, there is plenty of discussion on the best place to observe upcoming eclipses, and the best equipment to record them with. There are plenty of threads on historical eclipses - for example, on New York’s last eclipse, in 1925, when Consolidated Edison, the local power company, stationed an engineer at every intersection on Broadway so that the path of totality could be tied down to a single block. And then sometimes there is whimsy.
Did any eclipse chasers, I wanted to know, manage to get a ticket to see the 2003 Rugby World Cup Final, and then board a flight to see the Antarctic eclipse the very next day? And were they English or Australian? The group’s moderator, Michael Gill, managed to come up with an Englishman who did just that, adding the memorable comment “That’s my idea of a great week-end!”
One very interesting theme came up a few weeks ago - how long until the very last total eclipse? To save unnecessary worry, I’ll start by pointing out that it won’t happen any time soon. Every year or so, the Moon will continue to pass in front of the Sun, and on many of these occasions it will cover the Sun completely, giving rise to that most beautiful and spectacular of phenomena, a total solar eclipse. On other occasions, the Moon’s apparent size in the sky will be smaller than the Sun’s, and so the best that be observed from the Earth’s surface is an annular eclipse, where a thin (but blindingly bright) ring of light surrounds the Sun.
Why are some eclipses total and others annular? The answer is because neither the Moon’s orbit around the Earth, nor the Earth’s around the Sun, is perfectly circular. Both orbits are instead elliptical, like squashed circles. The Earth’s orbit around the Sun is close to circular, nonetheless the Earth is furthest from the Sun in early January, and so the Sun is slightly smaller in apparent size in the sky (this is not why January is colder than July, of course, as any Australian will tell you. The seasons are due to Earth’s axial tilt and so for the southern hemisphere January is mid-summer).
The Moon’s orbit around the Earth is considerably more elliptical. At its furthest from the Earth the moon is 405,696 kilometers away, at its nearest 363,104 kilometers. Any eclipse that takes place when the Moon is close to the Earth will be total, any eclipse at the furthest point in the orbit will be annular. At present total and annular eclipses occur in roughly equal numbers.
So why am I worrying about the possibility of a last ever eclipse? Because the Moon is receding, slowly, away from the Earth. And the reason for this recession? It is caused by the friction of the tides. As is well known, the Moon raises tides in the Earth’s oceans (and for that matter, in the atmosphere and in the Earth’s crust). As the tides are pulled across the Earth’s surface, they slow down the Earth’s rotation rate - very slowly, to be sure, but measurably so.
The length of the day is gradually increasing - it does so at irregular rate, with frequent glitches due to earthquakes, averaging at 170 milliseconds per century. Every few years an extra second is added to the day to keep the day from drifting slowly round the clock. Computer manufacturers don’t like these leap seconds, and there have been moves to detach the day from its astronomical roots, potentially leading to midnight GMT drifting into the middle of the day in England.
So the Earth is slowing down. This means a loss of angular momentum, which is a conserved quantity. The Moon is causing the Earth’s slowdown, and so it is forced to conserve angular momentum by moving gradually away from the Earth.
How fast is the Moon receding? At present, the rate of recession is 38 millimeters per year. You might think this would be impossible to measure, but it has been. Three of the Apollo Moon missions, 11, 12 and 15, left behind reflectors designed to send back laser ranging beams back to Earth. (See below) The Russians also sent a reflector to the Moon on their unmanned Lunokhod rover. The time taken to send a laser beam to the Moon and back to its source can be measured with extreme precision. The calculation of a 38 mm/year recession rate is complicated - don’t forget that both Earth and Moon are rotating, so that deducing how far the Earth’s centre is receding from the Moon’s centre, from measurements between two moving points on the bodies’ surfaces, is not straightforward. Nonetheless, it can be done.
What is even more astonishing was that the 38mm / year recession of the Moon was deduced, with some accuracy (40 mm / yr), nearly 50 years before reflectors were placed on the Moon to measure the recession directly. How could this happen? The Moon’s recession was posited by John Knight Fotheringham in 1920, from an analysis of historical eclipses of the Sun. The slowing down of the Earth seems minuscule, but its cumulative effect over centuries is surprisingly large. The tracks of total eclipses centuries ago shift hundreds of miles to the west or east. The analysis of the historical accounts of eclipses can tie down the location of eclipse tracks, and so deduce how much the Earth’s day has changed and how much the Moon has receded.
If the Earth’s orbit around the Sun doesn’t change, the last eclipse will occur when the Moon’s minimum distance from the Earth widens to 379,590 kilometers. At this point, even when the Earth is at its furthest from the Sun, the Moon will only just cover the Sun. As the Moon approaches the critical distance from Earth, the duration of total eclipses will become steadily shorter, and the proportion of total to annular will become smaller and smaller. The very last total eclipse will probably be for only a fraction of a second.
So when will the last eclipse be? At 38mm / year recession, the Moon’s minimum distance from Earth will reach 379,950 kilometers in 434 million years. However, this calculation is simplistic, for a number of reasons. First, the 38mm/year rate will slow as the Moon gets further away, the size of tides decrease, and the Earth itself slows down. For this reason, 434 million years is a conservative estimate. Jean Meeus, in his book “More Astronomical Morsels”, calculates 1210 million years until the last total eclipse. There are other random effects on the rate of recession. The Earth’s axial tilt affects the rate of recession, the frictional effects change as the continents drift around the globe. Finally, the Earth’s orbit around the Sun changes over the aeons - the ellipticity changes under the gravitational pulls of the other planets, and the Sun’s evolution as a star may alter the Earth’s average distance.
The upshot of all these effects is that we can only guess when the last total eclipse will be - some time in the next billion years or so, but not any time soon.
There is another aspect to the receding Moon, and another reason why I wrote this article. What happens if you calculate the Moon’s distance from Earth back into the distant past? Not just for centuries, or millennia, but for billions of years, right back to the Moon’s formation.
The latest theories suggest that the Moon was formed 30-50 million years after the solar system. Computer simulations suggest that a body the size of Mars smashed into the Earth, nearly shattering the planet, and throwing up a huge amount of debris which eventually coalesced into the Moon. The time for this event was 4.572 billion years ago, as estimated from the decay rates of isotopes in lunar rocks.
How far from the Earth was the Moon when it formed? It is 390,000 km away now, and receding at 38mm per year. Let’s play the tape backwards. 38 millimetres per year is 38 metres per millennium, or 38 kilometres per million years, or 38 thousand kilometers per billion years. In four and a half billion years, the Moon has receded by 171,000 km, under half its current distance.
As you’ll have guessed, however, the calculations are a good deal more complicated than this simplistic analysis. The Moon’s recession rate was faster in its early years, when it was a lot closer to the Earth, and the Earth was spinning a lot faster. All the other complications - axial tilts, varying tidal friction, and so on - still have to be factored in. So we can’t say exactly what the Moon’s recession rate was during the last four billion years; except to note that a recession of only 171,000 km in 4.5 billion years is certain to be a conservative estimate.
But what seems to be an impenetrable calculation is viewed by many astronomers as an opportunity to test models of the solar system’s evolution since its formation. If we accept the independent evidence that the Moon was formed 4.5 billion years ago, we have a very powerful check on our models for the receding Moon. It’s likely that the Moon formed initially quite close to the Earth, and so has receded by, not 171,000 km, but by close to 395,000 kilometers. Any theory that predicts a recession of more than this distance is inconsistent with the 4.5 billion year old age of the Moon.
It turns out that it is quite difficult to produce a model with “only” 395,000 km of recession in 4.5 billion years. Most models of recession, played backwards, put the Moon in near Earth orbit rather earlier than 4.5 billion years ago - or, to be more accurate, 4.5 billion years is at the top end of the possible range of dates.
What does this mean? Well, it’s great constraint on the possible theories. It may be that circumstances have conspired in a particular way - for example, models in which continental drift puts the continents at the poles more often than at the equator fare better than models where the continents stay at the equator (it causes less friction). Likewise, particular behavior for the Earth’s axial tilt will slow the Moon’s recession and so put back its formation date. Most scientists, however, would be unhappy with a model which had to rely on specific, ad hoc, assumptions such as these.
Another possibility, of course, is that the timing for the Moon’s formation is at fault. If the Moon formed, say, three and a half billion years ago, it would be easier to explain where it was today, with the current estimates of recession. However, the estimates of the Moon’s age from isotope decay rule out formation this early, allowing a margin of error of only about 10 million years.
So do our theories of the formation of the solar system need revisiting? My understanding is that the situation is not regarded as critical - the misfit between formation dates estimated from the receding Moon and from other sources is uncomfortable rather than catastrophic, and perhaps further refinements of the models will resolve the tension. The situation is not dissimilar to the situation in the late 1990s, when the age of the universe, as estimated from galactic red shifts, was difficult to reconcile with the age of the oldest stars.
Some models had the oldest stars older than the estimated age of the universe - not a tenable state of affairs! Fortunately, the error bars in the estimates were just big enough to allow both models to co-exist, if somewhat uneasily. In the succeeding decade, further observations have narrowed the error bars down, and the universe is now thought to be 13.7 +/- 0.2 billion years old, comfortably older than the oldest observed stars.
Something like this will probably occur with the receding Moon. We’ll refine the estimates of the age of the Moon from the Moon rocks; we’ll understand better how the Moon’s speed of recession changed over the aeons, and hopefully a coherent picture will emerge, where all independent estimates reinforce each other. And in the meanwhile, the inconsistencies give clues as to which theory might need refining. This is how science works.
Unbelievably though, these inconsistencies have been seized on by our friends the Young Earth Creationists (YECs) - people who believe the universe is only a few thousand years old. The receding Moon was quoted to me by a YEC acquaintance as scientific evidence for the Earth, Moon, solar system and observable universe only being 6000 years old.
Let’s just think about that for a moment. In 6000 years the Moon has receded from the Earth by a grand total of two hundred and twenty eight meters - that’s the length of two and a half football pitches. The receding Moon offers no evidence whatever that the Moon is only 6000 years old.
This doesn’t seem to stop the YEC’s, whose tactic is to pick at the seams. They never offer direct evidence for a young Earth - probably because there isn’t any - but prefer, in the manner of conspiracy theorists, to look for inconsistencies in the established account. A discrepancy in two estimates for the age of the Moon; between say, four, and four and a half billion years old, is taken to discredit both estimates, and therefore by some sleight of hand becomes evidence for the Moon being spectacularly younger - an age not even remotely suggested by either theory. This is NOT how science works.
Does the receding Moon offer challenges to our theories on the age of the solar system? Yes of course it does. It cannot but do otherwise. By its nature it has to place a tight upper limit on the age of the Moon.
Does the receding Moon prove that the Earth, the Moon and the Universe are 6000 years old?
No it doesn’t!
* syzygycal / syzygy: n (sizziji) The dictionary defination says arrangement of three celestial bodies in a straight line, as of the Earth, Sun and Moon at full moon or during an eclipse — -gial adj.
There is an excellent account of the receding Moon and creationist attempts to discredit it at www.talkorigins.org/faqs/moonrec.html This includes a link to a rather unconvincing creationist counter-argument.
Measuring the Moon's Distance
Apollo Laser Ranging Experiments Yield Results
(from LPI Bulletin, No. 72, August, 1994)
Scientists who analyze data from the Lunar Laser Ranging Experiment have reported some watershed results from these long-term experiments, begun 25 years ago when the Apollo 11 astronauts deployed a reflector array in the Sea of Tranquillity. "Using the Lunar Laser Ranging Experiment, we have been able to improve, by orders of magnitude, measurements of the Moon's rotation," said Jet Propulsion Laboratory team investigator Dr. Jean Dickey. "We also have strong evidence that the Moon has a liquid core, and laser ranging has allowed us to determine with great accuracy the rate at which the Moon is gradually receding from the Earth."
The first laser ranging retroreflector was positioned on the Moon in 1969 by the Apollo 11 astronauts. By beaming laser pulses at the reflector from Earth, scientists have been able to determine the round-trip travel time that gives the distance between the two bodies at any time to an accuracy of about 3 centimeters. The laser reflector consists of 100 fused silica half-cubes, called corner cubes, mounted in a 46-centimeter square aluminum panel. Each corner cube is 3.8 centimeters in diameter. Corner cubes reflect a beam of light directly back toward the point of origin.
"Lunar ranging involves sending a laser beam through an optical telescope," Dickey said. "The beam enters the telescope where the eye piece would be, and the transmitted beam is expanded to become the diameter of the main mirror, then bounced off the surface toward the reflector on the Moon."
The reflectors are too small to be seen from Earth, so even when the beam is precisely aligned in the telescope, actually hitting a lunar retroreflector array is technically challenging. At the Moon's surface the beam is roughly four miles wide. Scientists liken the task of aiming the beam to using a rifle to hit a moving dime two miles away.
Once the laser beam hits a reflector, scientists at the ranging observatories use extremely sensitive filtering and amplification equipment to detect the return signal, which is far too weak to be seen with the human eye. Even under good atmospheric viewing conditions, only one photon is received every few seconds.
From the ranging experiments, scientists know that the average distance between the centers of the Earth and the Moon is 385,000 kilometers with an accuracy of better than one part in 10 billion. Laser ranging has also made possible a wealth of new information about the dynamics and structure of the Moon. Among many new observations, scientists now believe that the Moon may harbor a liquid core. The theory has been proposed from data on the Moon's rate of rotation and very slight bobbing motions caused by gravitational forces from the Sun and Earth.
Ranging has also determined that the length of an Earth day has distinct small-scale variations of about one thousandth of a second over the course of a year, caused by the atmosphere, tides, and Earth's core. In addition, precise positions of the laser ranging observatories on Earth are slowly drifting as the crustal plates on Earth drift. The observatory on Maui is seen to be drifting away from the observatory in Texas.
Data also indicate that ocean tides on Earth have a direct influence on the Moon's orbit. Measurements show that the Moon is receding from Earth at a rate of about 3.8 centimeters per year. Ranging has also improved historic knowledge of the Moon's orbit, enough to permit accurate analyses of solar eclipses as far back as 1400 BC. Continued improvements in range determinations and the need for monitoring the details of the Earth's rotation will keep the lunar reflector experiments in service for years to come.
The laser ranging retroreflector was positioned on the Moon in 1969 by the Apollo 11 astronauts.
The Earth and Moon System
By Ivor Clarke
Ever since I was at school, many years ago, I‘ve been fascinated by our Moon. I first started to get interested in space, like most of my generation, by reading Dan Dare’s adventures in the Eagle comic back in 1951. Later I read astronomy books from the local library and then I got hold of a book, The Conquest of Space by Willy Ley with paintings by Chesley Bonestell. This was published in 1952 and it was one of the first serious attempts to depict a flight to the Moon with photographic like realist artwork by Bonestell who had worked for RKO in Hollywood, painting backgrounds for films, so was used to working out the correct sizes of distant objects. He had worked on the 1950 film Destination Moon. He had many of his paintings of space themes in the American Life magazine. All this had inspired me to take a life long interest in our satellite. And when the NASA Apollo moon program got going I was sure I would be able to go there even if it meant a hefty fee in the future. The more I learn about our satellite the more it seems there is to learn. I’m still interested in the Moon but the likelihood of going has slipped a bit. By the way, the anagram of Astronomer is, when you rearrange the letters, Moon Starer.
The Earth / Moon system is unique in our solar system (or so it was thought for many years), it was the only one of its kind. It resembles a two planet binary system, with the Moon unusually large and massive as it is a quarter the size of the Earth. The Earth and Moon average distance is 384,392 km, this is the mean of the elliptical orbit, closest, perigee is 356,410 km and furthest, apogee is 406,679 km. As both bodies are large and fairly near, they exert a huge force on each other. In the case of the Moon this shows as only one side faces Earth with the other turned away because of the gravitational lock the Earth has forced on the Moon, so that it revolves in the same time as it takes to complete an orbit.
Spring and neap tides
The Sun as well as the Moon influences the Earth’s tides, although to a lesser extent. As a result the height of the tides varies with the lunar cycle: at new or full Moon (A) the effect of the Sun reinforces that of the Moon, producing strong “spring” tides; at the first and third quarters (B) the Sun’s attraction partially cancels that of the Moon giving weak “neap” tides. The height of the water level at a given location fluctuates as shown (C) during half a lunation.
So we live on one of a large double planet system with both bodies revolving around a common centre of gravity as they orbit the Sun. Because the Earth is 81 times the mass of our Moon, the common centre of the mass lies not in space, but, lies within the Earth’s radius, 4,800 km from the centre of Earth, or a 1,000 miles below the Earth’s surface in old money. This point is called the barycenter, the centre of gravity of the system.
The size of the Moon to the primary, Earth, is the largest in the solar system, all the other satellites larger then our Moon: Ganymede, Callisto and Io around Jupiter, Titan around Saturn and Triton around Neptune are small in comparison with the large gas giants they revolve around. The mass of these moons will hardly be noticable to such large massive gas giants.
Now we know of one other two body system comparable with ours. In 1978, James Christy discovered Charon, the satellite of Pluto. But both of them are many times smaller and much less massive than our Moon, indeed both of them weigh in at only around one-fifth the mass of our Moon. Pluto is only about 2,400 km in diameter while the Moon is 3,476 km and Charon is probably only 800 km and orbits around 17,000 km away from Pluto in a period of 6.3 days. So both are very much smaller than our Earth / Moon pair. Because of the nearness of the Pluto / Charon pair to each other, it is most likely that they are gravitationaly locked to each other with one face on each forever turned inwards towards each other. We will find out the correct sizes and dimensions when the New Horizons spacecraft gets there in July 2015.
We all know how the tides are rased by a combination of the gravitational pulls of the Sun and Moon, see diagram above. In MIRA numbers 40 and 45, Mike Frost has written about the astronomy of tides and tidal bores and their causes. It is not only the water in the oceans that is pulled by the Moon. The ground to is moved upwards by the Moons pull by a few inches each day as the Earth revolves.
One of the most amazing things about our twin planet system is that we see eclipses. This is because of a fluke of nature. The Sun is 400 times the diameter of the Moon, but it is 400 times further away. Result: both look from the Earth’s surface as the same size! Well nearly. The Moon does not revolve around us in a perfectly round orbit, but in an ellipse, as we have seen. This gives us the different types of eclipse, either total or annular solar eclipses or lunar ones.
One problem with diagrams such as the one below, copied from a standard astronomical work, is that the scale of the diagram is very misleading, If it was drawn to the correct scale to fit into the space above, all you would see would be three dots, two of them very close together.
A good way to see the correct scale of the Earth / Moon system is to make a model. All you need is a golf ball and a 6” grapefruit or 6” ball and place them 15 feet apart. Vola, a model of Earth and Moon to scale. Now you can see why it won’t fit on a page and if you consider that the Moon’s orbit around the Earth is tilted at an angle of just over 5 deg to the ecliptic, this is the plane of the Earth’s orbit around the Sun. You can see why we don’t get a lot more eclipses than we do.
Types of Eclipse
In a lunar eclipse (A), the Moon passes first into the “penumbra” of the Earth’s shadow before reaching the darker “umbra”. An eclipse of the Sun (B) only appears total from the limited region of the Earth’s surface that is covered by the umbral shadow cone: from inside the penumbra the eclipse appears partial and the solar corona is not seen. An annular eclipse (C) occurs when the Moon is near apogee and its shadow cone does not reach the Earth’s surface.
The maximum amount of eclipses in any one year is seven, they can be 5 solar and 2 lunar or 4 solar and 3 lunar. The least number is 2 solar. As the Earth moves round the Sun, every 6 months the Moon’s orbit will cross the plane of the orbit, called the Moon’s nodes. These are the times when an eclipse is possible as the three bodies all line up. At all other times the Moon is either north or south, or above or below if you prefer, the plane of the Earth’s orbit and the shadows from the Moon or the Earth pass harmlessly away from each.
More than 2,000 years ago, it was discovered that the solar eclipses follow a pattern, called the Saros cycle. This is a cycle of 223 lunations, or 18 years, 11 days and 8 hours. If there are 4 leap years in that interval and a day less if there are 5 leap years. A total of 6,585.3 days. This means that the August 1999 solar eclipse was preceded by an eclipse in Norway in July 1945 and one in England in June 1927, all in roughly the same longitude at 54 years and a month difference in time. The other two eclipses in this 54 year period would have been roughly spaced one third of the way around the globe. The next one at our longitude will be September 12th 2053. All these are part of the Saros cycle 145, this started in January 1639 and will finish in April 3009 after 1370 years and 77 solar eclipses, of which 41 will be total. At any one time several Saros cycles will be taking place, all of them start their cycle in the north pole region and work their way down the globe until they pass above the south pole and end.
Sun and the Moon
The pull of the Sun on the orbit of the Moon is responsible for the drift in the Moon’s inclination. The nodes or crossing points go right round the sky in eighteen and a half years. This shift is but one of the many disturbances that the Sun produces in the orbit of the Moon. The Sun’s gravity is trying to pull the plane of the Moon’s orbit down onto the same plane as the Earth’s orbit, but its motion prevents this. What is happening is that the Moon reaches the plane of the ecliptic a little sooner that it would without the Sun’s pull, this results in the node moving backward by 19.5 degrees a year. Each time the Sun passes a node at intervals slightly under 6 months, there is at least one eclipse
Another effect is that the Moon’s perigee advances in the same direction as the Earth’s rotation taking 8.85 years for a complete revolution.
The Sun is not the only body in the solar system to effect the Moon’s orbit, the giant planet Jupiter and to a lesser extent Saturn and the other planets cause perturbations. All of these make defining the Moon’s orbit very difficult. Many millions of man hours of calculation time have been spent over the centuries as mathematicians have tried to calculate the Moon’s position so that longitude could be known to a high degree to prevent ships being lost at sea.
One of the most surprising things is that the Moon is like a planet, which is going round the Sun! If you look at the diagram on below, you will see a scale drawing of the Moon and Earth’s path round the Sun. At no time in the Moon’s orbit is it convex to the Sun. It is only concave, the same as the Earth. So is it orbiting the Earth or the Sun? Well I suppose it’s doing both at once. The Moon does go round our planet in 27.32 days. This is the measure against the fixed stars background. If you measure the time from new moon to new moon it takes 29.5 days, It takes the Moon 2 more days to get back into the new moon position between the Earth and Sun owing to the curvature of the orbit. This is because both are moving round the Sun at 18.5 miles per second or 29.8 km p s, so in 29.5 days we’ve moved 47 million miles, one twelfth part of the orbit. With this scale of drawing, as above, it is easy to see that the Moon and Earth lie very close together in true distance with the Sun being 390 times further away. Only when the true scales of the solar system become apparent do some of the laws of nature make sense. The Moon and Earth are the only pair in our solar system to display this double concave orbit to the Sun for both bodies.
Paths of Earth and Moon Round the Sun
The Sun’s pull on the Moon is more than double the Earth’s pull. Consequently the Moon’s path is always concave to the Sun. It is, however, much more nearly straight at New Moon than at Full; it would be exactly straight at New if the Moon were at two-thirds of its actual distance (see lower diagram). The upper diagram shows the motion from Full to New, that is, for about fifteen days; the interval between the successive positions shown is one and a quarter days. The whole of the Moon’s path may be mapped out by simple repetition of the above diagram (alternately forwards and backwards) about twenty-five times.
The 5 deg angle in the orbit of the Moon to the ecliptic leads to why it is possible to see more of the Moon’s surface than the 50% normally seen. The combined effects of the libration in longitude and latitude and also in a daily or “diurnal” libration mean that around 59% of the Moon’s surface is visible from the Earth over time.
To the early mappers of the Moon it was a problem as many of the limb features would only be visible for a short time each lunation, then it would be cloudy or visible from the other side of the Earth. The shadow of the rising sun on the surface moves on the central regions of the Moon at around 16 km per hour. Compare this with the speed at the equator on Earth’s surface of 1,450 kph. Even though the sun moves slowly over the lunar surface, low relief and small hills and hollows will soon disappear as the sun angle rises. Ask anyone who has had a go at drawing a lunar feature on the terminator, the shadows seem to move quicker than it’s possible to draw them!
The Moon’s libration in longitude (A) is a consequence of the non-uniformity of its speed in its elliptical orbit around the Earth (exaggerated in this diagram). The Moon revolves at a constant rate, so that after one-quarter of the rotational period it has rotated through 90°; at the same time, however, it has moved through 97° of its orbit, so that from Earth the Moon’s central meridian would appear to be displaced by about 7°. The libration in latitude (B) results from the combined effect of the 5° inclination of the Moon’s orbit to the Earth’s equator, and the 1½° tilt between the Moon’s axis and its orbit: as a result the Moon’s axis of rotation is inclined by about 6½° relative to the Earth’s axis so that the north and south polar regions are alternately tilted slightly towards the Earth. Finally, the diurnal libration (C) allows an Earth-based observer to view the Moon from a slightly different angle in the morning as compared with the evening, because his viewpoint changes with the Earth’s daily rotation.
The earliest maps were drawn in the 17th century with the newly invented telescope. Before then it was just what you could see with the Mk1 eyeball. Galileo’s map was not quite the first but it was not very detailed and did not name any areas. The first map to name craters and mare, some of which we still use, was by Giovanni Riccioli in 1651. He named mountain ranges after some of Earth’s ranges and the craters after famous persons, mostly astronomers. Not until 1775 was a fairly accurate map made by Tobias Mayer, but the real “father of selenography” was Johann Schröter between 1778 to 1813 he drew many detailed observations of the surface. The next advance was by Beer and Mädler in 1837, who produced a masterpiece of careful observation in an accurate map which remained the standard work for over 40 years. In the 20th century maps were made by Walter Goodacre in 1930 and by HP Wilkins (with help from Patrick Moore) in 1946. During this time the Moon was largely left to amateurs as it was of no interest to professional astronomers.
The first photographs of the Moon were taken as early as 1840, but the first modern photographic atlas was not produced until around 1900. One was by Pickering in 1904, and showed each part of the Moon in five different lighting conditions. In the early days of photography, it was thought it would take just a few weeks to map the Moon. It took well over a century. The problem was trying to cover all aspects of sunlight on the lunar surface from Earth. This is now not so much of a problem now that the Moon as been photographed in detail from orbit.
The Lunar Orbiter Missions
The first space craft to reach the Moon were Russian, Luna 2 crashed into the surface in September 1959 and Luna 3 in October took a blurred picture of the far side. But in July 1964 the Americans finally got there with Ranger 7 taking over 4,300 pictures as it headed to destruction on the Mare Nubium. It was not until 1967 that the Moon was finally almost fully mapped in detail by the NASA Orbiter’s I to V. The first Orbiter I was launched in August 1966 and Orbiter V a year later in 1967. These 5 space craft worked remarkable well and from their photographs, sites were found to land the Apollo moon LEM’s. The smallest size to be resolved would have been boulders and craters about the size of a small car.
It may be an appropriate moment to look briefly at the Orbiter photographic methods used to obtain these spacecraft images, after all, the Orbiter and Apollo missions were in days of old, before digital watches!
An examination of any Orbiter frame reveals that the whole image is made up of parallel strips across its width, see the very detailed image of the crater Schickard below and the front cover, with one edge having data and framelet information. Each Medium Resolution frame is made up of 27 strips of framelets and each High Resolution frame 86, these being then divided into 3 equal sections. Each framelet has part of the Edge Data Strip encoding which consists of a three digit framelet number, a set of resolving power lines, a 9-level gray scale and a chequered strip of black and white rectangles along the extreme edge. This Data Strip was pre-exposed onto the film before use along with a fine geometric “+” pattern over all of the image area to aid in the detection of processing distortion.
It must be remembered that all of these images were photographed onto a single strip of Kodak high-definition aerial film, 70mm wide and 80m long and developed in lunar orbit by pressing it into contact with a roll Kodak Bimat Transfer Film.
After drying, a high intensity spot of light, 6.5 microns in diameter, was projected through the negative film. This spot scanned a small section just 2.67mm long parallel to the film edge. This was then repeated under the previous scanned line 16,359 times across the 57mm width of the image to make up one framelet strip. The negative was then moved forwards 2.54mm and the scanning process repeated. A read out of the differing light intensity, as the spot moved across the negative, was transmitted back to Earth for recording and reassemble. It is these framelets which make up the strips in the image of each frame. Because only a few frames could be transmitted during each orbit, this operation took about 2 weeks.
The crater Schickard
This view of the crater Schickard is from the Lunar Orbiter IV mission in May 1967. The frame number is IV160h2 and shows how much detail it was possible to get with the Kodak aerial film and in orbit processing. All of these photographs were take years before digital CCD chips were available and remain very useable today. Schickard is not an easy crater to view from Earth as it lies in the far southwest region on the Moon. The crater has been flooded several times with larva on both sides, the darker areas. By doing a crater count the different ages of the flows can be calculated.
At the Lunar Data Processing Laboratory the framelets were reconstructed on to 35mm wide film, this was 7.18 times the spacecraft’s image size and then laid edge to edge to form a complete frame. A high-resolution frame measured 158cm by 40cm before being re-photographed. It is in the accuracy of this framelet assembly that errors seem to have occurred. A quick glance at the chequered edge on most frames will reveal the unevenness of the assembly. This can be seen in the frames as a misalignment of the shape of some small craters. Also as each frame is covered in the regular geometric pattern of tiny crosses, (these look like white dots on the microfilms), any misalignment is apparent.
The next mapping mission to the Moon was the Clementine mission, flown primarily as an engineering test bed for new, lighter, more cost effective image sensor and component technology by the US Ballistic Missile Defense Organization and NASA. It was launched on January 25th 1994. After a eight day check-out period in low Earth orbit, it departed Earth and was injected into a 5 hour, polar lunar orbit on February 19th. By April 24th it had completed two mapping cycles and in May Clementine left lunar orbit soon after orbit 348 to fly by Geographos, a near-Earth asteroid. Unfortunately Clementine suffered an on-board malfunction which spun up the craft to 80 rpm, depleting the fuel supply.
One of the problems with this mission is the huge amount of data sent back. Over two million images! This amount of data can cause amateur overload. It is not possible in an average persons lifetime of spare time to view them all. At a speed of just one picture per minute for 3 hours every evening it would take over 30 years just to look through them all and that’s without a holiday or night off! This is what I mean by overload.
At the moment these two sets of images are the best we have available to use from the internet. Soon other mapping missions are to begin with several other countries sending spacecraft. It is very necessary to restrict the viewing to just a small area or a small set of features when making a selection. Most of the files are JPEG’s and not too large so it is possible to zoom into the Moon on a cloudy night and get to feel like one of the lucky 24 astronaunts who got the best close looks to date.