0:00:32 > 0:00:35We've been hearing a great deal about the inner planets,
0:00:35 > 0:00:38but what about those remote members of the Sun's family?
0:00:38 > 0:00:40Uranus, Neptune and Pluto.
0:00:40 > 0:00:44They're not spectacular, though I sometimes feel they're rather neglected,
0:00:44 > 0:00:46because they are fascinating worlds.
0:00:46 > 0:00:48And this is a good time for talking about them,
0:00:48 > 0:00:50because they're all on view now
0:00:50 > 0:00:53and they're all pretty near our position.
0:00:53 > 0:00:56In fact, Pluto came to our position on March 29th.
0:00:56 > 0:00:59And by our position, I mean that Pluto, the Earth and the Sun
0:00:59 > 0:01:01were then in more or less a straight line,
0:01:01 > 0:01:03with the Earth in the middle,
0:01:03 > 0:01:05so that Pluto was opposite to the Sun in the sky
0:01:05 > 0:01:07and well placed for observation.
0:01:07 > 0:01:10Then, Uranus followed to our opposition on April 21st
0:01:10 > 0:01:14and Neptune will do so on June 1st.
0:01:14 > 0:01:16So they're all there for our inspection.
0:01:16 > 0:01:20And, in talking about them, I think will begin with Uranus,
0:01:20 > 0:01:22which is the nearest and the brightest
0:01:22 > 0:01:24and was the first to be discovered.
0:01:24 > 0:01:26So let me show you where to find it.
0:01:26 > 0:01:30We'll begin, as we so often do, with Ursa Major, the Great Bear.
0:01:30 > 0:01:33Well, follow through the line of the bear's tail
0:01:33 > 0:01:36until you come to Arcturus, the brilliant orange red star,
0:01:36 > 0:01:38and then onto Spica, in Virgo,
0:01:38 > 0:01:42and Virgo itself looks rather like a faint and distorted Y.
0:01:42 > 0:01:44And Uranus is here.
0:01:45 > 0:01:47Now, you can see it with the naked eye
0:01:47 > 0:01:49if you know where to look for it.
0:01:49 > 0:01:50I find it a bit difficult.
0:01:50 > 0:01:52People with better eyes can see it quite easily.
0:01:52 > 0:01:55Through binoculars, it looks like a star.
0:01:55 > 0:01:58But through a telescope, it shows a distinct greenish disc.
0:01:58 > 0:02:01And this is how it was first identified.
0:02:01 > 0:02:04It was discovered, way back in 1781,
0:02:04 > 0:02:07by a Hanoverian musician who had come to England
0:02:07 > 0:02:08and taken up astronomy.
0:02:08 > 0:02:11His name was William Herschel and he made telescopes,
0:02:11 > 0:02:13mainly six-inch reflectors.
0:02:13 > 0:02:15And if you want to see a Herschel reflector,
0:02:15 > 0:02:18go and look at this one at the Science Museum, in South Kensington.
0:02:18 > 0:02:22In 1781, Herschel was scanning the sky with one of those telescopes
0:02:22 > 0:02:25when he found an object which quite clearly wasn't a star
0:02:25 > 0:02:28because it did show a disc and it moved.
0:02:28 > 0:02:31And in fact, he thought that it must be a comet
0:02:31 > 0:02:34and he wrote a paper to that effect. But when its path was worked out,
0:02:34 > 0:02:37it was discovered that this was no comet.
0:02:37 > 0:02:41This was a new planet, moving well beyond the path of Saturn,
0:02:41 > 0:02:44which, up to then, had been the outermost known member of the Sun's family.
0:02:44 > 0:02:47And of course, this was a most exacting discovery
0:02:47 > 0:02:49and after some discussion, it was named Uranus.
0:02:49 > 0:02:54Well, I'm afraid you're not going to see very much on Uranus with any telescope.
0:02:54 > 0:02:55I can show you a picture of it.
0:02:55 > 0:02:58And here is one, taken with a major telescope.
0:02:58 > 0:03:00Uranus itself is very overexposed
0:03:00 > 0:03:02and, of course, that ring and those spikes
0:03:02 > 0:03:04are purely photographic effects.
0:03:04 > 0:03:07You can see there some of Uranus's satellites as well.
0:03:07 > 0:03:09But not even a giant telescope
0:03:09 > 0:03:11is going to show you very much on the disc.
0:03:11 > 0:03:15Now, this is not because Uranus is small. It's anything but that.
0:03:15 > 0:03:19It's a giant world nearly 30,000 miles in diameter.
0:03:19 > 0:03:21And as you can see from this picture,
0:03:21 > 0:03:23is a great deal larger than the Earth.
0:03:23 > 0:03:25It's also a very long way away from us.
0:03:25 > 0:03:30Even at the moment, more than 1,600 million miles.
0:03:30 > 0:03:32And so, it's not surprising that Uranus is not bright.
0:03:32 > 0:03:34It is, incidentally, a gas giant
0:03:34 > 0:03:38and rather like a smaller edition of Jupiter or Saturn.
0:03:38 > 0:03:43But one very strange thing is the tilt of Uranus' axis.
0:03:43 > 0:03:45As I think most people know,
0:03:45 > 0:03:49the Earth's axis is tilted to the perpendicular at 23.5 degrees
0:03:49 > 0:03:51and that's why we have our seasons.
0:03:51 > 0:03:54And most of the other planets have tilts of the same nature.
0:03:54 > 0:03:58But Uranus is different, as I can show you from this diagram.
0:03:58 > 0:04:01Here, fist of all, we have the Earth's axis, you can see,
0:04:01 > 0:04:03and the other planets are much the same.
0:04:03 > 0:04:06Mercury is upright, Venus upside down, actually.
0:04:06 > 0:04:08Mars, about the same as the Earth,
0:04:08 > 0:04:10over to the right of your picture now.
0:04:10 > 0:04:12Then Jupiter, pretty well upright.
0:04:12 > 0:04:14Saturn, roughly the same as the Earth.
0:04:14 > 0:04:16And then, we come to Uranus
0:04:16 > 0:04:19where the tilt is more than a right angle.
0:04:19 > 0:04:22And beyond that, Neptune, where again we are back to normal
0:04:22 > 0:04:24and about Pluto we know nothing.
0:04:24 > 0:04:27But this strange axial tilt of Uranus means that, sometimes,
0:04:27 > 0:04:29we see the equator presented to us,
0:04:29 > 0:04:31and sometimes we see the pole.
0:04:31 > 0:04:33And it leads also to a very odd calendar,
0:04:33 > 0:04:37because Uranus takes 84 years to go round the Sun,
0:04:37 > 0:04:40but less than 11 hours to spin on its axis.
0:04:40 > 0:04:44And if you work that out, you can find
0:04:44 > 0:04:49that there are over 65,000 Uranian days in every Uranian year.
0:04:49 > 0:04:53And this alone would make the calendar a bit complicated,
0:04:53 > 0:04:55but the axial tilt makes it worse.
0:04:55 > 0:05:00And each pole has a midnight sun lasting for 21 of our years
0:05:00 > 0:05:03and then, a corresponding period of darkness.
0:05:03 > 0:05:05So the calendar is very odd indeed.
0:05:05 > 0:05:09But I can assure you it doesn't upset the Uranians,
0:05:09 > 0:05:10because there aren't any.
0:05:10 > 0:05:14Uranus is not a world where any Earth-type life can exist
0:05:14 > 0:05:17and because it is made up of gas in its outer layers anyway,
0:05:17 > 0:05:20we simply can't land there and never will be able to do so.
0:05:20 > 0:05:23But I suppose, in the dim and distant future,
0:05:23 > 0:05:28we might be able to land on one of Uranus' five moons.
0:05:28 > 0:05:29All of which, shown in this picture,
0:05:29 > 0:05:32are considerably smaller than our moon,
0:05:32 > 0:05:34but nevertheless, there are solid worlds
0:05:34 > 0:05:37and possibly one day, they may be approached.
0:05:37 > 0:05:39So there is Uranus, as I say,
0:05:39 > 0:05:41quite easily visible with binoculars,
0:05:41 > 0:05:43although the satellites are pretty faint
0:05:43 > 0:05:46and you're not going to see very much on it.
0:05:46 > 0:05:49Now, when Uranus had been found and as I say, this was 1781,
0:05:49 > 0:05:53the Solar System was again assumed to be complete.
0:05:53 > 0:05:56But when a planet is discovered, the mathematicians set to work
0:05:56 > 0:05:59and calculate its orbit or path.
0:05:59 > 0:06:03And before long, it was found that Uranus just was not behaving.
0:06:03 > 0:06:07It was wandering away from the position where it should go.
0:06:07 > 0:06:09So something was very wrong somewhere.
0:06:09 > 0:06:13And the suggestion was made that there might be a more remote planet,
0:06:13 > 0:06:15as yet undiscovered,
0:06:15 > 0:06:17pulling Uranus out of position.
0:06:17 > 0:06:20Now, this suggestion was made in the 1830s
0:06:20 > 0:06:23and a report was actually sent in to Greenwich Observatory.
0:06:23 > 0:06:25And at Greenwich Observatory,
0:06:25 > 0:06:28the Astronomer Royal was a rather formidable gentleman named Airy,
0:06:28 > 0:06:31afterwards Sir George Airy, a great administrator.
0:06:31 > 0:06:35But Airy didn't take the suggestion very seriously and nothing was done.
0:06:35 > 0:06:37Then, in the early 1840s,
0:06:37 > 0:06:41the problem was taken up by a young undergraduate of Cambridge,
0:06:41 > 0:06:43whose name was John Couch Adams.
0:06:43 > 0:06:46Now, Adams had in fact got the key to the whole problem,
0:06:46 > 0:06:49because he knew how Uranus was being tugged
0:06:49 > 0:06:51and he had to find the culprit.
0:06:51 > 0:06:54So he worked out where he thought the new planet must be
0:06:54 > 0:06:56and again, he sent the results in to Greenwich.
0:06:56 > 0:06:58And once again, nothing was done.
0:06:58 > 0:07:00Then, over in France,
0:07:00 > 0:07:03the same problem was attacked quite independently
0:07:03 > 0:07:05by a French mathematician named Le Verrier,
0:07:05 > 0:07:07who was a very brilliant man indeed
0:07:07 > 0:07:10and also supposed, I think, to being one of the very rudest men
0:07:10 > 0:07:11who have ever lived.
0:07:11 > 0:07:13And Le Verrier made the same kind of calculation
0:07:13 > 0:07:15and came to the same result.
0:07:15 > 0:07:18And he sent his calculations in to the Observatory at Berlin,
0:07:18 > 0:07:20who started hunting for the planet.
0:07:20 > 0:07:25Meanwhile, over in England, Airy had at last instigated a search
0:07:25 > 0:07:27and so, the race was on.
0:07:27 > 0:07:28And the continentals won it,
0:07:28 > 0:07:31because, at Berlin, the planet was identified
0:07:31 > 0:07:33on the basis of Le Verrier's calculations.
0:07:33 > 0:07:36So, in fact, Adams finished his work first,
0:07:36 > 0:07:37but it was by Le Verrier's work
0:07:37 > 0:07:40that the new planet, Neptune, was actually discovered.
0:07:40 > 0:07:44And, I may say, this led to quite an international row afterwards
0:07:44 > 0:07:46in which neither of the principles took much part.
0:07:46 > 0:07:51In fact, I'd like to show you too just what the situation was
0:07:51 > 0:07:56and here we've got a diagram of the paths of Uranus and Neptune
0:07:56 > 0:07:58as they were round about that time.
0:07:58 > 0:08:00Uranus the inner, Neptune the outer.
0:08:00 > 0:08:05Before 1822, Neptune was pulling Uranus along,
0:08:05 > 0:08:08and then, after 1822, as now on the diagram,
0:08:08 > 0:08:10Neptune was pulling Uranus back.
0:08:10 > 0:08:13And it was by that kind of calculation
0:08:13 > 0:08:15that Neptune was eventually tracked down.
0:08:16 > 0:08:18Well, in addition to that,
0:08:18 > 0:08:21Neptune has got two satellites and they are both rather interesting.
0:08:21 > 0:08:23They're very different in type.
0:08:23 > 0:08:25The smaller one is called Nereid.
0:08:25 > 0:08:28And that goes round Neptune in a rather peculiar kind of path,
0:08:28 > 0:08:31more like that of a comet than a planet,
0:08:31 > 0:08:32as you can see there.
0:08:32 > 0:08:33It's very strange indeed.
0:08:33 > 0:08:37It's so small you can't see it except with a very powerful telescope.
0:08:37 > 0:08:40Now, the inner satellite is called Triton, very much bigger,
0:08:40 > 0:08:42bigger than our moon, I may say,
0:08:42 > 0:08:45and was discovered not long after Neptune itself.
0:08:45 > 0:08:48And that goes round Neptune in practically a circular orbit,
0:08:48 > 0:08:50but the wrong way.
0:08:50 > 0:08:53And just why that happens is something else we don't know.
0:08:53 > 0:08:56Now, let me show you where to find Neptune.
0:08:56 > 0:08:59We can go back to the same chart we used earlier,
0:08:59 > 0:09:00because at the moment,
0:09:00 > 0:09:02Neptune and Uranus are not all that far apart.
0:09:02 > 0:09:05There again, we have the Great Bear, Virgo and Uranus.
0:09:05 > 0:09:07And here is Neptune,
0:09:07 > 0:09:10on the borders of the Scorpion and the Serpent Bearer,
0:09:10 > 0:09:13and not very far away from the bright red star Antares.
0:09:13 > 0:09:16And then, if you've got the right kind of charts,
0:09:16 > 0:09:18you can identify Neptune with binoculars
0:09:18 > 0:09:21and a fairly powerful telescope will show you that is not a star,
0:09:21 > 0:09:22but as I've said,
0:09:22 > 0:09:25I'm afraid no telescope is going to show you very much on it.
0:09:25 > 0:09:28It's slightly larger than Uranus, slightly denser,
0:09:28 > 0:09:32slightly more massive, and a great deal farther away from the sun
0:09:32 > 0:09:34and so, obviously, it is colder.
0:09:34 > 0:09:37Neptune was tracked down in 1846.
0:09:37 > 0:09:41And once again, the Solar System appeared to be complete.
0:09:41 > 0:09:44But still, there was something just a bit unexplained
0:09:44 > 0:09:47about the movements of Uranus and Neptune itself.
0:09:47 > 0:09:51And this problem was tackled by a very famous American astronomer,
0:09:51 > 0:09:55whose name was Lowell, Percival Lowell, and there he is.
0:09:55 > 0:09:57And he, in fact, is best remembered today
0:09:57 > 0:09:59for his theories about the canals of Mars,
0:09:59 > 0:10:01which turned out to be wrong.
0:10:01 > 0:10:03But he was also a very brilliant mathematician
0:10:03 > 0:10:06and he made the same kind of calculation
0:10:06 > 0:10:08as Adams and Le Verrier had done so long before.
0:10:08 > 0:10:10Only this time, of course, it was more difficult.
0:10:10 > 0:10:14And he worked out a position for a still yet undiscovered planet.
0:10:14 > 0:10:17And since he had a powerful observatory,
0:10:17 > 0:10:19he started looking for it.
0:10:19 > 0:10:21The observatory was at Flagstaff, in Arizona.
0:10:21 > 0:10:22And there's the dome,
0:10:22 > 0:10:25a photograph I took when I was over here some time ago.
0:10:25 > 0:10:28And inside that dome, it's a very powerful reflecting telescope,
0:10:28 > 0:10:32which Lowell used for his observations and which I know well.
0:10:32 > 0:10:34I've used it quite often, so I know how good it is.
0:10:34 > 0:10:38And yet, although Lowell searched energetically,
0:10:38 > 0:10:40the planet just refused to come to light.
0:10:40 > 0:10:43And by the time he died in 1916, still hadn't.
0:10:43 > 0:10:46And for long time after that, nothing more was done.
0:10:46 > 0:10:49But then, in 1930, long after Lowell's death,
0:10:49 > 0:10:53a young astronomer named Clyde Tombaugh had another look photographically
0:10:53 > 0:10:58and he took the photographs on which the new planet, Pluto, was identified.
0:10:58 > 0:11:01And here are the actual discovery photographs.
0:11:01 > 0:11:03Pluto indicated there by the arrows.
0:11:03 > 0:11:05And you can see how much it shifted
0:11:05 > 0:11:09over the period between the times when those photographs were taken.
0:11:09 > 0:11:12Now, when you're talking about a great astronomical discovery,
0:11:12 > 0:11:15it's very nice to be able to speak to the man who actually did it.
0:11:15 > 0:11:19Clyde Tombaugh, whom I know well, was over in England not so long ago
0:11:19 > 0:11:23and while he was here, I asked him, how certain he was about Pluto.
0:11:23 > 0:11:25Well, I was not too familiar with it.
0:11:25 > 0:11:27I was aware of it, of course,
0:11:27 > 0:11:31but in view of the earlier previous searchers,
0:11:31 > 0:11:33I was not sure that it was in that neighbourhood.
0:11:33 > 0:11:36And when I embarked upon it,
0:11:36 > 0:11:39I thought I'd go all the way around the sky
0:11:39 > 0:11:41searching very systematically
0:11:41 > 0:11:44and to see what would come out,
0:11:44 > 0:11:47whether there was a predicted planet or even some others.
0:11:47 > 0:11:50And so, that was the basis for making the search.
0:11:50 > 0:11:52Was it fainter than you expected?
0:11:52 > 0:11:55Yes, it was, as far as Lowell's predictions were concerned.
0:11:55 > 0:11:59He had a mandate of 12 and expected a planet comparable to Neptune.
0:11:59 > 0:12:03Not quite as large, I think he had a mass sine of seven Earth masses.
0:12:03 > 0:12:06And of course, now we know Pluto is much smaller than that.
0:12:06 > 0:12:09Everything seemed fine, but was it?
0:12:09 > 0:12:12Pluto turned out to be very much smaller than expected,
0:12:12 > 0:12:14even smaller than the Earth.
0:12:14 > 0:12:15And a planet of that size
0:12:15 > 0:12:19just could not drag either Uranus or Neptune out of position
0:12:19 > 0:12:20by a measurable amount.
0:12:20 > 0:12:22And yet, it was by those very perturbations
0:12:22 > 0:12:24that Pluto had been tracked down.
0:12:24 > 0:12:27So there's something very strange about the entire thing.
0:12:27 > 0:12:30Either Pluto is larger or more massive than seems likely
0:12:30 > 0:12:32or else the discovery was sheer luck
0:12:32 > 0:12:35or else there's another planet out there somewhere.
0:12:35 > 0:12:36I simply don't know.
0:12:36 > 0:12:38And Pluto has a very strange path.
0:12:38 > 0:12:41It's much more eccentric than those of the other planets.
0:12:41 > 0:12:44It actually crosses Neptune's, as you can see here.
0:12:44 > 0:12:47So it can come closer in than Neptune ever does.
0:12:47 > 0:12:48But because Pluto's orbit is tilted,
0:12:48 > 0:12:51there's no fear of a collision on the line.
0:12:51 > 0:12:53And don't forget too that Pluto takes 248 years
0:12:53 > 0:12:55to go once round the Sun.
0:12:56 > 0:12:58I can show you where Pluto is.
0:12:58 > 0:13:02We can go back to our original diagram showing Uranus and Neptune
0:13:02 > 0:13:05and here is Pluto, again not far from Virgo.
0:13:05 > 0:13:08But you're going to need a powerful telescope to identify it.
0:13:08 > 0:13:13If you would like to see a 1975 chart of its movements, here is one,
0:13:13 > 0:13:15sent in to us by Mr Walter Pennell, of Lincoln.
0:13:15 > 0:13:18And that shows the movements of Pluto during the present year.
0:13:18 > 0:13:21And Mr Pennell also sent us two photographs of Pluto
0:13:21 > 0:13:24taken some time ago and there's the planet on the end of the arrow
0:13:24 > 0:13:27and once again, you can see the motion.
0:13:27 > 0:13:29But as I say, you're going to need a pretty good chart
0:13:29 > 0:13:32and a pretty good telescope to identify Pluto.
0:13:32 > 0:13:34And whether it really is the outermost planet,
0:13:34 > 0:13:36well, that remains to be seen.
0:13:36 > 0:13:39We've got, I think, to wait for the era of space probes.
0:13:39 > 0:13:42But I think you'll agree that unspectacular though they may be,
0:13:42 > 0:13:47these remote members of the Sun's family are certainly not without their interest.
0:13:48 > 0:13:52I must, I think, end this Sky At Night on a personal note.
0:13:52 > 0:13:55We did our first programme in April 1957
0:13:55 > 0:13:58and we showed then the spiked comet, Arend-Roland.
0:13:58 > 0:14:01And I wonder how many people remember that now.
0:14:01 > 0:14:04Since then, we've been on the air once every four weeks
0:14:04 > 0:14:08and we haven't actually missed a month since April 1957.
0:14:08 > 0:14:12And so, this month we come of age, we've been going out for 18 years.
0:14:12 > 0:14:15And I would like to thank very sincerely all those of you
0:14:15 > 0:14:18who have been watching our Sky At Night programmes for this time.
0:14:18 > 0:14:22And I only hope we get the chance to carry on for another 18 years.
0:14:22 > 0:14:24And so, with this anniversary programme,
0:14:24 > 0:14:27I'll say goodbye now and see you next month.
0:14:51 > 0:14:54Subtitles by Red Bee Media Ltd