Hot Nectar, Warming Weather and Birds Missing the Spring
In the hot seat this week is Beverley Glover, who will describe how flowers warm their nectar to entice passing pollinators, real life weatherman John Law discusses weather predictions and how to calculate temperature days in advance, and Marcel Visser explains how warming weather and earlier springs spell disaster for migrating birds. Also on the show, we will hear from Katey Walter about a new source of atmospheric methane, and in Kitchen Science, Derek and Dave get their hands wet in the name of discovering how the human body judges temperature.
In this episode
- Rich Pickings For Dinosaur Diggers
Rich Pickings For Dinosaur Diggers
There are still plenty of dinosaurs out there to discover, a new study has concluded...
This good news for any of our listeners who are budding palaeontologists, is according to a new study by Steve Wang, a statistician at Swathmore College, Pennsylvania and Peter Dodson, a palaeontologist at the University of Pennsylvania in the US.
Looking at the number of skeletons found so far from each known dinosaur genus (that's one group bigger than species, so humans are Homo sapiens, which means we belong to the Homo genus and sapiens species) - they plugged those numbers into an established mathematical model which links the fossil sightings to the likely number of unseen dinosaurs.
The analysis predicts that there could have been a total of about 1850 genera of dinosaurs in the world - of which we have so far only found 527 - so there should be plenty more fish - or should that be plenty more plesiosaurs - in the sea.
Sadly, we will never find every last dinosaur, because not all of them will have left fossils behind and the diversity of dinosaurs that we have found is biased by the availability of fossiliferrous rock outcrops. The good news though is that around about 90% of the dinosaurs which are discoverable will probably be found within the next 100 to 140 years.
So, if you're hoping to discover something fiercer than a T-Rex or bigger than a giganotosaurus, then it's definitely still worth getting out there to have a good look!
- Fish Populations Out of Their Depth
Fish Populations Out of Their Depth
Nearly half of the fish that we eat today haven't been caught from seas, rivers or lakes but began life in a farm, just like the beef, pork and chicken that we eat, according to the latest report from the UN Food and Agriculture Organisation (FAO).
There has been a massive boom in the amount of fish that are farmed, a process known as aquaculture: in 2004 we were growing 45 million tonnes of fish compared to 60 million tonnes which were being taken from the wild.
With the number of people in the world climbing ever higher, there will be more and more demand for fish to eat, but the problem is that we are already maxing out the amount of fish we can catch from the sea - even with more fishing boats with bigger and better fishing gear we are still catching about the same about of fish from the seas that we have for the last few decades.
So, maybe the only way we're really going to be able to meet this increasing demand for fish is to grow them ourselves.
This week at the BA Festival of Science in Norwich I went to a session all about "Should we eat fish" - and it's unclear whether this increase in farmed fish is a good or a bad thing for the world.
On the negative side, there worries that farmed fish had spread diseases and parasites to wild fish, and that escapees from fish farms which can genetically contaminate the local wild gene pool. And some farms can generate lots of water pollution from leftover feed and excrement from the fish. And then there's a problem that many of the farmed fish are fed on other fish, which are still being caught from the seas.
But, on the positive side, farmed fish does help to feed people in the developing world and could provide a very important, more secure source of food. And there are some species that are becoming endangered in our seas that maybe we can grow and so leave alone the wild stocks.
And did you know that cod, that favourite ingredient for fish and chips and fish fingers, are now being sustainably grown for the first time in the Shetland Islands? It's early days, but if it takes off, this could help reduce pressure on those wild stocks: maybe one day the Japanese will be farming blue finned tuna for their sushi!
To do this experiment, you will need:
Three bowls or washing up basins large enough to put your hand in
- Warm water (NOTE - be careful not to make the water too hot!)
- Cold water, with a few ice cubes in it
- Medium temperature water
How to do the experiment:
1 - Fill one bowl with warm water, one with iced water and one with medium water.
2 - Put one hand into the warm water and one in the iced water for one minute.
3 - Take your hands out of the water and put them both into the medium water. How does the water feel?
So what's going on?
When you put your hands into the medium temperature water, the hand that's been in the cold water feels warmer while the one that was in the warm water feels cold.
Why is this?
It's because your senses are relative. They don't measure an absolute temperature or an absolute brightness of light; they make their measurements relative to the things around it. In the case of this experiment, the temperature sensors on your hands measure the temperature of the water relative to the temperature of your hand. If the water is warmer than your hand, it feels warm, and if it is colder than your hand, it feels cold.
Measuring temperature relative to a certain object (such as your hand) is fine if your hand stays at a constant temperature - but it doesn't. If you keep your hand in cold water for a long time, the temperature of your hand starts to drop. This means that the reference point to judge temperature by has changed. Water that once felt cold now feels relatively warm compared to your cold hand.
The opposite happens when you transfer your hand from the hot water into the medium water. Your hand is warm and so the medium water feels relatively cold.
The effect is most obvious in this experiment when you transfer a cold and a hot hand into medium temperature water at the same time: one hand is telling you the water is warm while the other is saying that it's cold, even though the actual temperature of the water is the same throughout!
Your hands give two different answers and your senses are confused!
This effect can be seen in other situations too. When people run a bath they generally tend to stir it with one hand. As a result, people should always test the water with the OTHER hand before getting in. This is because the hand that's been stirring will get used to the high temperature and send messages to the brain saying that the water isn't hot. However, as soon as you jump in with cold feet, the water will feel relatively scalding - so be warned!
Another example is when children are told not to wear their coats indoors because they won't feel the benefit. If you get used to being really hot and snuggly inside a coat in the house, as soon as you go outside you feel cold (like the hot hand going into medium water). However, if you keep your coat on the peg while in the house and only put it on outside, the addition of a warm coat to a warm (rather than hot) body means that the shock of braving a cold winter's day is relatively less.
You can even see this effect with your ears! If you've been in a noisy room and then move into another room that's playing a quiet radio, you can hardly hear it at all. However, if you sit there for five minutes you can start to hear the radio loudly, even though it's still at the same volume.
So the moral of the story is: don't believe everything your senses tell you!
- Science Update - Mercury Pollution and Public Transport
Science Update - Mercury Pollution and Public Transport
with Chelsea Wald and Bob Hirshon from AAAS, the science society
Bob - This week the topic is climate change, where all the news seems to be bad. And in fact I'll bring you some bad news later about how climate change may be leading to an influx of mercury in the environment. But first, some good news. We all know that taking public transportation over driving is the right thing to do as far as the environment is concerned. But new research suggests that, in some cases, what's good for the environment may also be good for you.
Chelsea - It's an old debate that's been rehashed countless times: Train commuters claim that their ride is less stressful because they don't deal with traffic; drivers swear they're more relaxed because they're in control. Now new research on New York commuters supports the train riders' side. Environmental psychologist Richard Wener of Polytechnic University in New York and his colleagues asked commuters about their psychological states before and after commutes.
Richard - And what we found was that train commuting was significantly less stressful than car commuting, that it seemed to be largely because it was perceived to be significantly less effortful and also more predictable.
Chelsea - Wener says this predictability is key, because it means people can relax knowing they'll get to work and back home on time. Of course that requires that the commuter trains run more or less on schedule, something that's not true in every city. He adds that the effects of stress are not only psychological but also physical, and the daily commute is a potentially significant but still poorly understood contributor to people's stress levels.
Bob - Thanks, Chelsea. Wildfires raging at unprecedented intensities in the north may be unleashing massive amounts of mercury into the environment. The mercury has accumulated harmlessly over thousands of years in the soils of wetlands, particularly in a type of soil called peat in northern North America. But Michigan State University ecologist Merritt Tueretsky says that in just a few decades, forest fires in these regions have more than doubled in size-a result of global climate change.
Merritt - Forest fires don't just burn forests. And our data show that when peat layers within boreal wetlands burn, it releases very large quantities of mercury into the atmosphere.
Bob - She says the mercury could disperse over long distances and find its way into the food chain, where it could eventually reach animals and people.
Chelsea - Thanks, Bob. That's all for this week. Next week we'll bring you a piano piece composed by Mount Etna. Until then, I'm Chelsea Wald.
Bob - And I'm Bob Hirshon, for AAAS, The Science Society.
- How Flowers Are Turning Up The Heat
How Flowers Are Turning Up The Heat
with Dr Beverley Glover, University of Cambridge
Chris - Here's Beverley Glover from Cambridge University who works on plants and has also found out how they're using jiggery pokery to attract pollinating insects. What's your work about and how is this all achieved?
Beverley - What we're interested in are the adaptations flowers have that make them particularly attractive to pollinators. If you think about a flower, the petals - the bright shiny bit that you like to look at - is really there for only one reason: to attract animals. Different plants have come up with different ways of making those attractive to animals. The one we've been working on most recently is miniature lenses on the petal cells that warm it up a few degrees. We've been doing some work with bumble bees in labs and in flight arenas to test whether they prefer warmer flowers and whether that would be an advantage if the flower could attract more pollinators in the wild.
Chris - And does it work?
Beverley - It seems to work. Having those lens-shaped cells makes the flower warmer and the effect is strongest at dawn and at dusk and we know that bumble bees need extra help at dawn and dusk when it's hard for them to get that big fat body off the ground to fly. So what we've been doing is giving them artificial flowers of different temperatures and seeing whether they prefer the warmer ones and whether they can learn which colours of flowers might be warmer than other colours. It seems as though they can. They can work out that some colours are warmer than others.
Chris - Because the dark colours absorb more energy?
Beverley - Well that would be one way of doing it and it certainly seems that in the wild, from our very early preliminary work, dark coloured flowers are generally warmer than light coloured flowers.
Chris - So why are they all dark then?
Beverley - Because there are all sorts of other ways of doing it and it's not just about warmth. A pollinating animal is interested in being able to spot the flower easily from a distance so it needs visual contrast to the green of the leaves. It also needs to be able to work out where to get into the flower at a short distance, and so there are short distance visual effects. And there's beginning to be some data from work of ours as well about them liking the feel of different flowers, so tactile effects might have a role too. So temperature is just one part of the bag of tricks if you like.
Chris - But why do they like it when it's warmer? Why should that be more attractive?
Beverley - For a bumblebee we think it's about metabolic reward. They need the sugar from the flower to make energy to fly but they, like you on a cold day, might get more energy more quickly from a warm drink than a cold drink. It saves them from using their own energy to warm that nectar up if the flower's already providing it at a warmer temperature.
Helen - And with the bumblebees it's all about being cold blooded. They're not like us mammals; they rely much more on their surroundings to get themselves warm and active for the day's activity.
Beverley - Yes that's right. There's a lot of evidence out there that many insects will back in warm flowers just to get the heat. What we've done that's different is just to show that just warming the flower up by a couple of degrees, just warming the nectar up to give a warmer drink, makes the difference as well. It's not just about sun bathing; it's about getting a warm drink as well.
Chris - How far back in evolutionary terms do you think this goes or is that a very difficult question to answer because there's not a fossil record for plants quite as well as there is for other things?
Beverley - You'd be surprised how good the fossil record for flowers actually is. But it's a difficult question to answer because there's probably a whole range of different ways of warming a flower up and some of them won't fossilise, so for instance tracking the sun. This is not something you'd pick up from a fossil and there are certain plants that do that. The cells we've been looking at, the little lens-shaped ones that warm the flower up, we know that around 80% of flowering plants have those including if not the oldest flowering plant family that's still alive today. Probably the second or third oldest groups have those cells, which suggests that it could have evolved quite early.
Chris - Because insects have been around for quite a few million years. So does that mean that plants have been up to this trick ever since then?
Beverley - It's an interesting question. There's a lot of debate out there as to whether the flowering plants, which are very very rich in species number compared to non-flowering plants, actually underwent that radiation into so many species because the insects were also radiating into so many species at the same time. In the fossil record there are a lot of flowering plants appearing as some of the insect groups expand, so it's possible that that link has been there for quite a long time but it's difficult to prove.
- Climate Change And Migrating Birds
Climate Change And Migrating Birds
with Professor Marcel Visser, Netherlands Institute of Ecology
Chris - Right now we're going to talk to Marcel Visser who's at the Netherlands Institute of Ecology. He's going to talk to us about how climate change is having a fairly major impact on animals that migrate and how they then reproduce. Tell us about your research and how you went about studying this problem.
Marcel - Well we have studied migratory birds in the Netherlands for a long time and one thing we noticed at one point is that the birds are coming in earlier. They were also laying earlier. The pied flycatcher, which comes over from Africa, seemed to be laying its eggs very quickly. In the old days it used to be two to three weeks between arrival and laying, and now it is only a week.
Chris - So birds are leaving Africa, they're leaving according to when they think it's the right time to come back, they're arriving but they're arriving at a time when the seasons have already moved on.
Marcel - Yes, it's very difficult for the birds to predict the conditions when they are all the way over in Africa. The birds use some cue like day length or an internal clock and it used to work very well. But now, by the time they come back, spring has already advanced there quite a bit and it's too late to really take advantage of all the food that's around. There's only a very short period where food is around in the forest, and this is quite important to realise. There's only about two or three weeks where there are a lot of caterpillars around to feed the chicks.
Chris - So how badly are the birds being affected and is this something we can actually do something about?
Marcel - Well in the case of the pied flycatcher we know that the majority of the birds now have their nestlings in the nest too late. We know that by the time they start feeding the nestlings caterpillars, the caterpillars are already on the decline. The other things we have shown is that there are different areas within the Netherlands and some of these areas have a very early food peak, the birds breed early, the caterpillars breed early and everything is early. In other areas, especially where there is poor soil, the peak is a bit later. It is clear that the pied flycatchers in these early areas are really too late, and you can see here that they are declining. The numbers are dropping and in some of these areas the flycatchers have completely disappeared. In the other areas, the poor areas, where food is late, they just arrive on time to raise their offspring and they're just about hanging on. So they're not declining there. It's a very clear effect on population numbers.
Chris - Do you think that this is going to be confined to this species of bird or are other migratory birds other than pied flycatchers likely to be affected?
Marcel - I think it's a very general pattern, especially for the long distance migrants and the ones that come from Africa because they have no idea how things are changing here. The solution, if you ask me what we can do about it, is to reduce the increase in temperatures. That's the only thing we can do.
Chris - It's not really a short term solution though. This is a couple of hundred years of carbon dioxide pollution and if we try and stop it now, it might not be in a do-able amount of time.
Marcel - If we now reduce our CO2 output by 60% and the increase in temperature will still be two degrees in the next 100 years, that is something we can't do anything about. But if we keep going on like we do now, it will be nine degrees, and that's not a trivial difference for the birds. What will happen with these pied flycatchers is that we will see natural selection. The birds that arrive early will get most offspring and these offspring will also arrive earlier as we know that this is heritable. So there is some scope for adaptation in the birds, but the rate of adaptation will be slow. So if we can keep the increase in temperature at a reasonable level, the birds will probably be able to follow that. But if we carry on the way we are going, then the increase in temperature will be so rapid that there is no way that the birds can keep up.
Chris - So are these birds able to adapt to climate change?
Marcel - Well that's actually the thing we're looking at in close detail now. We're looking at the kind of selection there is and the kind of heritability there is. What we can estimate now is that they will be able to evolve in time but it critically depends on how fast the climate is going to change. It's down to the rate of evolution in these birds and the rate that we impose on them.
- A New Source of Atmospheric Methane
A New Source of Atmospheric Methane
with Dr Katey Walter, University of Alaska Fairbanks
Helen - Well let's stick with changing climate and the changing world around us. As the world warms the permafrost is beginning to melt, which is allowing bacteria to change carbon-rich material laid down over 30 000 years ago into the greenhouse gas methane. But how much gas is being produced? Well it's very difficult to quantify because the bubbles come out of thaw lakes, but Katey Walter from the University of Alaska, Fairbanks has used bubble traps to work out how much methane is emerging. It's enough to increase the methane contribution from the northern wetlands by up to 63%.
Katey - This work is all about quantifying a new source of atmospheric methane which was previously not recognised as a large and significant source, and that is bubbling from thaw lakes, lakes where the permafrost is melting and the lakes continue to expand as they melt into that permafrost, that's where they get the name thaw lakes.
Chris - So how have people tried to measure this in the past, or haven't they?
Katey - In the past scientists have measured methane emissions from lakes in two ways, they measure the diffusive emission where methane moves along a concentration gradient, from the sediments into the atmosphere, and they've done that by just measuring the concentration of methane in the surface water of the lakes. Another source of methane from lakes is bubbling, and that's a much more difficult source of methane to quantify because bubbling is very rare both in space and time.
Chris - So what have you done to get these accurate quantifications of them?
Katey - We have the excellent opportunity in Siberia to study bubbling because when the ice forms on the lakes in Autumn it's like putting a piece of Saran Wrap across the surface of the lakes, it traps the bubbles in place as they wobble to the surface and then they freeze into place in the ice. And we can walk across the ice and map out the distribution of point sources and hot spots.
Chris - So you walk out on the ice, you can see where the bubbles are coming up. But then how do you physically work out how much gas is there?
Katey - We've constructed bubble traps out of greenhouse plastic and copper wire and we place those either under the ice or in the summer when there is no ice we just place them floating under the water surface and each trap captures the bubbles that come up continuously. And so we would go out every day and measure the volume of bubbles that had collected.
Chris - So in the grand scheme of things how much methane is this actually contributing to the global environment?
Katey - Well, scaling up, the type of Siberian lake that we were studying, we estimate that methane emissions from these lakes is about 3.8 teragrams per year. Now, these lakes are only a portion of the northern lakes in general so if bubbling is something that happens everywhere then this could be an even much larger phenomenon than just the scope of our Siberia work. And now we see that just adding this small portion of Siberian lakes to the northern wetland emission estimate it increases it by up to 63%, ten to 63%.
Chris - So what are the implications if you add this to the global warming equation, then?
Katey - This is a new positive feedback to global warming. Methane is a very strong greenhouse gas and so as methane is being produced it is trapped in the atmosphere, increasing atmospheric warming which then enhances the thaw and the expansion of these lakes further. So today there are still about 500 gigatons of carbon remaining in this unique type of Siberian permafrost and it's projected that during the next century the majority of that will degrade and that can release tens of thousands of teragrams more carbon into the atmosphere.
Chris - So should this provoke a rethink of what we think is actually likely to happen in terms of global warming in the future, then?
Katey - Well, one component of the general circulation models that is missing is permafrost degradation, and especially with regards to the large pools of carbon that are stored in permafrost. That carbon content is still poorly known let alone these positive feedbacks to climate change that can happen from permafrost degradation. So yes, we do have a lot of rethinking and incorporation of these new sources.
Can you tell me how carbon dating works?
With carbon dating we're talking about different isotopes of carbon. This basically means different forms of the element. Most of the carbon in the world is a form called carbon 12, which means that there are six neutrons and six protons in the nucleus. There is also a heavier but much rarer form of carbon called carbon 14 and that's also slightly radioactive. The fact that it is radioactive means that it will break down over time. When plants photosynthesise, they take on some of this C14 and have an amount of it in their tissues. Once they die, that carbon 14 starts to break down. If you can measure the amount of carbon 14 in that organism, you can tell how long ago it died. It's effective for about 40 000 years.
- I have some frogs in my garden and they come up to me and let me tickle them. Why is this?
I have some frogs in my garden and they come up to me and let me tickle them. Why is this?
I have absolutely no idea! They have very sensitive permeable skins so I wonder why they would want you to touch them. Perhaps they have some parasites they want you to scratch off or something!
- Why are plants green? Why reflect that light rather than use it?
Why are plants green? Why reflect that light rather than use it?
It's an interesting question. Not all plants are green; there are plants in the sea that are brown and red. They photosynthesise in just the same way but they use pigments that trap different wavelengths of light to get that energy. This shows that you can obviously do it with other wavelengths and it's not something specifically about green. It may just be something else about the early green sea plants that were more successful than the early red and brown sea plants that made them more successful and able to get onto land. So although they look the dominant form, the fact that they are green may not make them any better at all; it could be for some other reason.
- Do any animals have chloroplasts and can photosynthesise?
Do any animals have chloroplasts and can photosynthesise?
Not that I know of as their own chloroplasts, but there are more complex multicellular animals out there that pinch the chloroplasts from plants. Quite a few examples are in the cnidarians; that's jellyfish. A little freshwater jellyfish called hydra pinches chloroplasts out of green algae and keeps them in its own gut. It lets them photosynthesise and nicks the sugars that they produce. So there are animals that trick plants out of their chloroplasts.
- Why do some Pinot wines turn out great while others are a disaster?
Why do some Pinot wines turn out great while others are a disaster?
I don't know much about grapes but it's likely to come from the chemistry of the grape, whether that's influenced by the soil or the genetics of the grape. The taste of wine comes from things called secondary metabolites that the plants produce and use to protect themselves against predators and attract animals to eat their fruit. So it's likely that different lines have different chemicals and that gives you different flavours in the wine.
- Why would a windscreen freeze just as the Sun was rising?
Why would a windscreen freeze just as the Sun was rising?
Well I have to say that it's a term I've not actually heard. But the coldest part of the night is just before the sun comes through. We've lost the daytime moisture and there's still a bit of heat in the ground, but it's just as we go through to the very early hours of the morning that we see the coldest temperatures, so about 5 or 6 o'clock. You can also get a lot of condensation and dew at that time of the morning and that's what can cause lots of these problems as well. So it's a combination of the moisture and the cold temperatures that would have caused the windscreen to freeze out.
- If clouds are made of water, why do they look grey and fluffy?
If clouds are made of water, why do they look grey and fluffy?
The main reason is the thickness of the cloud. A nice light fair weather cumulus tends to be quite shallow at times but as it gets bigger and bigger we sense less light coming through the cloud, and that tends to be really dark. So you get these big areas of stratocumulus that cover the sky, not a lot of sunshine gets beneath them so there's lots of shadow beneath the cloud, which makes it appear dark.
- Does rain begin as snow and ice at high altitude in clouds?
Does rain begin as snow and ice at high altitude in clouds?
That's very true in a certain number of cases. Water in the clouds can exist in liquid form even below temperatures of zero. This is in the form of super-cooled droplets of water. But in the very thick clouds where the cloud top temperatures can be around minus 40 degrees Celsius, most of it does start off as ice. As it falls down, the cloud base is still below zero and it falls as snow and hail. But as it falls through and begins to warm up it does melt down and we actually see it on the surface as rain.
- Do fish fart, and if so, where do they come from because you don't see any bubbles?
Do fish fart, and if so, where do they come from because you don't see any bubbles?
Fast repetitive ticks, which is where they got the name from, is fish blowing tiny bubbles from this structure called a cloaca at the rear end. They release these things in pulses at a certain rate and it's believed to be like some form of communication. Communicating by fish farts - fantastic!
- Why do French beans squeak when you eat them but runner beans don't?
Why do French beans squeak when you eat them but runner beans don't?
Not a clue! It'll be to do with what the surface is made of, I guess. I don't know the difference: different wax layers maybe?
- How does the body know that you've donated or lost blood and to make more?
How does the body know that you've donated or lost blood and to make more?
It's all down to the kidney. Your kidneys secrete a hormone called erythropoietin, which stimulates the bone marrow to make new blood cells. What makes the kidney do that? Well the kidney has all these chemical sensors in very tiny blood vessels and it measures how much oxygen is present in the tissue and infers from that how well the red blood cells are working. In other words, how many red blood cells there must be and how well your lungs are working. If your oxygen level drops a little bit, your kidneys assume that you haven't got enough blood and makes this hormone erythropoietin and you make more blood cells. So that means that when you go up a mountain and the oxygen levels drop, your kidney would notice that it's not getting as much oxygen as normal and would therefore boost the amount of erythropoietin. This makes the bone marrow make more red blood cells and solves the problem. That's why you end up with thicker, heavier and denser blood if you spend time at altitude. This is also why athletes like training at altitude because it boosts the amount of oxygen an athlete can carry.
- Are far-away galaxies light years distant still there?
Are far-away galaxies light years distant still there?
Galaxies, of course, are not the same as individual stars. The Milky Way galaxy, for example, contains 200 billion stars and is about 150 000 light years across. This means that a star on the opposite side of the Milky Way from our own solar system has been travelling for 150 000 years before it arrives at the Earth. The next nearest galaxy to our own is the Andromeda galaxy and that's about 3 million light years away, so the light that's coming from there is already 3 million years old. So it's beneficial that some suns are very long lived - suns like our sun live for 10 billion years - so there's more than enough time for light to come from the Andromeda galaxy. But you've asked a good question because space is so vast that inevitably light that's coming to us from stars up there in the sky will actually be signs of a star that's died. One day those stars will wink out because they don't exist any more, but at the moment the light is still coming to us because there's a bit of a delay. So there will inevitably be some stars but I doubt there will be a whole galaxy because they contain billions of stars and they'll all be at different phases of their lifetime.
- Why did the Moon look much larger than normal?
Why did the Moon look much larger than normal?
If the Moon was low in the sky, the reason is due to an optical illusion. When the Moon is high up in the sky you've got nothing in your visual world to compare it with size-wise, so your brain attributes it to being very small. There's nothing in the foreground, you're looking at empty sky and the Moon looks very far away. But when the Moon is near the horizon, then you're seeing the Moon close or in contact with things in the foreground such as buildings, pylons, trees or even people. This fools your brain into thinking that because there's something in the foreground that it does know the size of, it compares it with the Moon and assumes that it must be much larger than it is.