The Moon, with guest Danie Overbeek

Hi, and welcome to Out of This World, the second program in our monthly series on astronomy.

Slowly orbiting the earth every 29 days, the moon changes from fingernail crescent to full moon. From our earthly vantage point, we have never seen more than just over half of the moon's surface. What do you know about this, our nearest neighbour, this strange and desolate world?

Early humans roamed their territory, foraging and hunting for food. They knew the location of every tree, rock and river for miles around in all directions. Somewhere, way back in history, they looked heavenward and gazed at the moon with a measure of curiosity. It could not have gone unnoticed, for the moon is the second brightest object in the sky next to the sun. Entire nations worshipped it as a deity possessing powers of fertility, and looked to it to make their crops and even their animals grow. Werewolves are supposed to undergo their grotesque transformation under the influence of the full moon and prowl about devouring, infants and corpses.

The English word lunatic is derived from the Latin word for the Moon, Luna. People's insanity was thought to have connections with the phases of the moon. Although the moon has been linked to all sorts of sinister and dark human activities, there are said to be connections too between the phases of the moon and romance.

The Earth's natural satellite certainly continues to have profound physical effects on our planet. Exerting tidal forces strong enough to significantly raise the seas level and to cause measurable tidal displacement of the Earth's solid surface. What effect this body has on people's demeanour and general state of well-being is open to much conjecture and research. Early philosophers pondered the stars, moon, sun and planets. They initially believed that all the objects in the sky were equidistant from the earth and that the earth was the center of the solar system.

From this, they obviously concluded that the stars were fixed objects on a heavenly canvas placed there in certain patterns by the gods themselves. These patterns mapping out characters and events that were taken as portents from the gods. The first study of these objects was performed by the astrologers who usually sat as advisors to the king. The moon, sun, the wandering stars, and the planets had special significance to those people. These objects moved around amongst the fixed stars, and were seen either as gods, or certainly as the tools of the gods.

Our knowledge of astronomy has increased in leaps and bounds. Scientists soon came to the realisation that the earth was not flat, that the planets, the sun and the moon were much closer than the stars, and that there was predictable order in the motion of these bodies, but for a long while, their erroneous belief remained, that the earth was at the center of the entire universe.

Copernicus altered the fundamental foundations of astronomy when he put forward the accurate theory that the sun was at the center of the solar system and that all the other planets orbited around it. The moon came under scrutiny at the time of Galileo when he constructed the first telescope in Padua, Italy, in 1610. He saw that the moon had a scarred and rugged terrain and was not smooth and polished, as some had suggested. Our curiosity remained insatiable. We had to know more about this object that dominated our lives.

In 1835, a journalist Richard Locke wrote a series of articles for a New York newspaper in which he claimed that the moon was inhabited by weird and wonderful creatures that had been observed through William Herschel's powerful telescope in the Cape. The articles were cleverly written and were widely accepted. Of course, today we know better and realise that this was simply a clever hoax.

The moon is the most studied object in the solar system. Earth-based remote sensing equipment has charted and measured it for decades. Last year, eager scientists received more than one and a half million images of the lunar surface from a quarter ton spacecraft, Clementine, making the moon the most precisely mapped object in the solar system. It also holds special significance as the only other place that has been visited by earthlings. In July, 1969, astronauts Armstrong, Alder and Collins were to journey to the moon on board Apollo 11, which placed the first man on the moon. We had finally set foot on another world!

From that first landing, lunar material was brought back to earth for analysis, but this was not the first. Meteorites have been found on earth. Amongst them, some smaller fragments originating from the moon. Our modern calendar was derived from the time the moon took in its passage around the earth.

The best way to observe the moon is with binoculars. With them, you can see the flat, shiny, smooth areas on the moon called mare, which is the Latin word, meaning seas. You can also see high mountains and many enormous craters.endar was derived from the time the moon took in its passage around the earth.

Some of the largest mountains down near the south pole of the moon are comparable in height with Mount Everest. The largest crater, Bailey, is almost four kilometers deep and 295 kilometers wide. Most of the craters you see were formed during the moon's earlier years by impact with other objects. But there is however, evidence of volcanic activity too. The moon has no atmosphere and it has no traces of water.

I met with some keen young astronomers in the grounds of the Old Republic Observatory in Johannesburg.

Now, when we're talking about the solar system, what's important for us to do is to get the scale of dimensions right to appreciate the size of all the objects in relation to each other. So if this beach ball represents the sun, then on the same scale this chickpea would represent the earth and the little mustard seed would represent the moon.

The moon and the earth aren't this far away from the sun. If we had to get the distance scale right, what I'd have to do is ask you to take this beach ball down to that tree over there.

They're well over there. That very tree over there. Off you go.

Is the moon always smaller than the earth?

The the moon is much, much smaller than the earth. And in fact, you'll see once we get the beach ball down to the tree, there we have the sun. And then on the same distance scale and same scale that we have here, we would have the earth and the moon and they would be about that far apart. So, we now have a comprehension of the grand scale of the solar system.

But how long does it take the moon to orbit around Earth?

Now that's something that I'd like to discuss with you, but I think, why don't we go off to the Carlton Center? Go and stand on top of the roof of the Carlton Center. From there, I'll be able to show you both the sun and the moon in the same night sky. How does that sound? Okay, off we go.

Sixteen days earlier, we used the setting sun as our time reference, and noted the full moon's position on the opposite horizon. The moon was rising in the east as the sun set in the west. Clearly, the earth lay between the moon and the sun.

The students and I saw the crescent moon just above the setting sun. The moon had progressed in its orbit from full through new moon to crescent.

Right? What I have here is a model of the solar system. Now from what we know of the solar system, these polystyrene balls certainly are not to scale at all. What we have now, is we have the moon that orbits around the earth, but at the same time, we've got the earth orbiting around the sun. So we, we have a scenario like this where we have the moon going around the earth and at the same time we've got the earth moving around the sun. So we've got this kind of orbital thing happening.

And of course, what we know is going to happen here is that we're going take 365.25 days for the earth to go once around the sun. But it's going to take something like about 27 to 28 days for the moon to go once around the earth. With this model, I'd like to explain how the phases of the moon work. For me to actually demonstrate this and to correlate it to what we saw in the night sky here on top of the Carlton Center.

Tonight, I'd like you just to actually tilt this model on its side, if you don't mind. Let's just get it on its side. Then things look exactly the way we saw it. We saw the sun setting over the African continent, and we can actually get the African continent there for us. And we saw the moon from our vantage point just slightly up in the sky. Now, this helps us to appreciate why we saw just a fingernail of the moon, because we had the sunlight coming across here. It was just setting below the horizon, but the sun's light was also coming up onto the moon and lighting this half of the moon, leaving just this tiny edge that we could see from earth.

What will happen over the next few nights is the moon will start to move little bit by little bit as it orbits around the the earth, and so, in a few nights time, we will have first quarter and we'll see half of the face of the moon lit up by the sun. What happens then is the sun's light is shining on the moon. This half is being lit up, but from the earth we will be able to see half of the face in darkness and half of the face lit up. And where will we see the first quarter directly above us in the sky. Of course what will happen is the moon will continue its journey and as it continues its journey, eventually it'll get like that and we'll see just a little bit of darkness on the moon, and we'll see most of it lit up because the sunlight is shining past us and onto the moon. A little bit later, when the moon has moved around in its journey, we will find the sunlight coming past the earth lighting up the whole face of the moon. For us, that will be full moon.

Now, the interesting thing is that you will never ever get to see the full moon and the sun setting on the same horizon. The full moon will always be rising as the sun sets. And also, what'll happen then, is that the moon will continue its journey around and eventually it'll be down at last quarter. And at last quarter again we will see it, but not at at night, we will get to see it during the day. And finally, the moon comes around to complete a circle, and at that stage, we will be seeing the dark side of the moon, because the sun's light will be coming into and shining on the back of the moon. At that stage, we call that a new moon.

Okay? And the whole process will continue, and 27 to 28 days from tonight, we should see the moon back in exactly the same position as we see it tonight.

Way above the North Pole, we can see how the moon orbits the earth, the sun's rays illuminating the side of the moon closest to it. From our vantage point on earth, we observe the moon's illumination pattern changing over a period of time.

Okay, Thom, every time I look at the moon, I see the same face of the moon. Is it because the moon is standing, is standing still or what?

Oh, that's a very good question. As the moon orbits around the earth, notice what is happening here. We've got one side, you can see the patterns on this side of the moon. And if you were standing on the earth, you'd be able to see those patterns. And as the moon orbits around the earth, notice that it keeps that same pattern facing towards the earth all the way through its orbit. You see that same pattern stays in place. There's one other thing with that as well. Let's pretend that we could go and stand on the sun, and then we could look back at the moon.

When we look back at the moon here, we'd get to get a little bit of a glimpse at this face of the moon. But as it went around its orbit, notice what happens. The moon has turned its back on the sun.

So, from the sun, we are actually looking at the back of the moon, and then it comes around, so we get to see the other side of the moon, and a little bit later from the sun, you get to see the face, and then it goes all the way around. So, in respect to the sun, the moon is in fact orbits once around its own axis in just over 27 days. But with respect to the earth, the moon has always just presented the same face to us. It's what we call a synchronized orbit.

If you measure the elapsed time from new moon to new moon, you will notice that it takes just over 29 days. However, if you measure the time it takes for the moon to travel from a point in the sky back to that point, you'll find that it takes just over 27 days. The two day difference is due to the earth's movement along its orbit.

Hang on, so why don't we get an eclipse every time when the moon is between the sun and the Earth?

So that's a very good question because one would expect that every time the moon was a new moon, in other words we were seeing the dark side of the moon, that we would in fact get a shadow cast by the moon onto the earth, but that's not true. The moon doesn't orbit around the earth on exactly the same plane that it goes around the sun. If it did that every single time we got a new moon, we would have a solar eclipse, and every single time we got the moon on the other side, we'd have a lunar eclipse.

The difference is here that the moon's orbit around the earth is tilted by five degrees. So, that means that at some stage when the moon is down here, which is most of the time, the sun's light is crossing a shadow off of the moon, but the moon's shadow is ending up below or above the earth. It doesn't actually cross the earth. But what's happening is that the earth again rotates around the sun and as it orbits the sun, so its angles change and watch what happens. Now, by the time it's gone on a little in its path, the moon comes around and comes up in its angle and exactly at that place, all three of them line up very, very nicely. Fairly shortly thereafter, what happens, is that we get the same kind of scenario happening where the same happens on the other side. We get the three bodies nicely lined up, and the earth's shadow then falls over the moon and it darkens the face of the moon. You can only ever get a lunar eclipse at full moon because that's when these three bodies are nicely in alignment.

The moon's gravity is a fraction of the earth's, but is enough to significantly raise the seas level, and to a lesser extent, the earth's solid crust, two tidal bulges lie on opposite sides of the earth in alignment with the moon. As the earth rotates on its axis, the tidal bulges sweep across the oceans causing high tide almost every 12 hours.

Amateur astronomers play a significant role in observing and gathering data for the professionals to analyse. I met one such amateur astronomer, Danie Overbeek, at his home where he had prepared a working model to demonstrate to us how he observes the moon.

Danie, you're a well-known variable star observer.

That is right, but I also like to look at occultations of stars by the moon. That's a very interesting pursuit.

Uh, the meaning of the word occultation, what is it?

Occult means dark. And the occultation of an object means that it darkens. When you look at the star that's going to be hidden by the moon, it's a most impressive sight to see it there one moment, and then completely gone a split second later.

Danie, do you need any special kind of equipment like a telescope or anything else?

You don't need a special telescope because these events are rather easy to observe. A small portable telescope like this will be quite sufficient. Unfortunately, you also need to time the exact instant when this occultation occurs, and for that you need a stopwatch which had previously been started against a time signal from the SABC.

So Danie, while you're observing the disappearance and reappearance of the star, you're actually writing down a whole lot of time codes on a piece of paper.

Sometimes, uh, it's necessary to do that in a hurry because there may be a number of stars that are going to be a altered in a fairly short time, and that brings us to the phenomenon of grazing occultations. A grazing occultation is when a star doesn't quite disappear behind the moon or doesn't disappear totally, but simply grazes along the edge of the moon. Now the edge of the moon isn't completely round. It consists of hills and valleys, and it's quite possible to see a star disappearing behind the hill, as it were, on the edge of the moon and reappearing a second or two later only to disappear again, and it may repeat this procedure perhaps even five or six times.

So, Danie, you've done numerous occultation observations. Is this data of any use?

Yes. Our timings are collected and at the end of the year, they're sent to the Japanese National Hydrographic Institute in Tokyo. This institute receives observations from all over the world, and they are then analysed and statistically treated, and are used subsequently to refine our knowledge of the shape of the moon, as well as the position of the moon in its orbit.

Have you ever stood outside on a really dark night away from the city lights and looked at the stars? If so, you were probably amazed to see so many sparkling specks, set against the dark velvet of the sky. At first glance, you may not have found any order in the uncountable objects. Astronomers locate objects using a set of cross-reference coordinates published in star catalogs. We need to follow similar principles if you are to find some of the objects we talk about in this series, and it'll be necessary for us to set a fixed time to synchronise these observations.

What I will do is give you a way of finding the object at that time. The easiest method is for us to use the horizon as a reference. From there, we can describe an object's angular position from it. The only problem would be where along the horizon we should start to resolve this dilemma.

You would need to orientate yourself for this. You'll need to determine the compass points along the horizon, and you can do this by comparing your observation point, say your home, with a map to determine north. Turn so that you now face north. West is on your left, east is on your right, and south will be towards your back. Whenever I tell you where to find objects in the sky, I will give you two coordinates. The first is where to start on the horizon, say north or south. The other would be the object's position in degrees above that point.

Remember that a circle contains 360 degrees, so, from one horizon straight through the very top of the sky and down to the other side would be half a circle, or 180 degrees. From the horizon to the top of the sky, the point we call the zenith, is a quarter of a circle or 90 degrees.

So if I describe an object at a position north, with an elevation of 45 degrees, you would begin to find that object by finding and facing north. From that point on the horizon, follow a line upwards for 45 degrees. Remember that this reference will be valid for a short while only as the Earth's rotation will change the object's position in time. It is valid for a week or so after this program provided you always observe at the same time at night. You may wish to grab a pen and paper as I'll be giving you the coordinates to find Jupiter and the Southern Cross.

An easy way of determining angular distance is to use your hand. Generally, a clenched fist at arm's length will give you 10 degrees from knuckle to knuckle. The width of your finger is about one degree, and the joints on your index finger are two, four, and six degrees respectively. The open hand is about 20 degrees. So, we have 2, 4, 6, 10, and 20 degrees respectively.

As a starting point, I'd like to give you the coordinates for Jupiter and the Southern Cross. Jupiter is 38 degrees above the western horizon at nine tonight just after the show. Two important things to remember. 38 degrees above the western horizon. It is the brightest object in that part of the sky, and you just can't miss it. It lies next to one of the most recognisable constellations, that of Scorpio.

Look for Scorpio just to the left of Jupiter. Its curved tail extends upwards and away from Jupiter itself. The Southern Cross is very beautiful, and is one of the smallest constellations in the sky. It is rich in detail and can be carefully examined with a telescope or even a pair of binoculars. It can be found by facing south, then moving 30 degrees along the horizon in a westerly direction, find it at a low altitude of 12 degrees. The Cross will be lying on its side.

You can confirm that you have found it by looking for the two pointer stars. They stand vertically above the Cross at an altitude of 27 degrees. Most basic astronomy books will contain a map of this and other constellations. Use your constellation map to find some of the most intriguing objects in the Cross itself.

In the news recently, you may have heard of this newly discovered comet that is supposed to be as large as South Africa. Some fear that it may collide with the earth, as did comet Shoemaker-Levy into Jupiter. Let me give you some of the facts as we have them at present: The comet is named after its discoverers, Alan Hale and Thomas Bopp. It is a periodic comet that has been through the inner solar system before. It orbits the sun every 3,000 years or so. It's orbit is very long and stretched out Ellipse, and the comet is part of our solar system, orbiting around the sun.

The comet will reach the closest point to the sun on April the first, 1997. This comet will not hit earth. It'll make its closest approach to us on the 23rd of March, 1997. At that time, it'll be further away from us than the sun is. It'll never cross any point in space that is occupied by the Earth. It just cannot hit the earth. The comet's nucleus is shrouded in gas and dust, and we don't know its actual size. The brightness of a comet is not always directly related to its size.

So, how bright will Hale-Bopp become? We don't know yet.

That's all we have for you tonight, but we'll keep you posted as the comet moves along.

Until next month, wishing you starry nights.