Monthly Archives: April 2014
Saturn’s opposition in the Balance
Our days lengthen during May until the period of true nighttime darkness dwindles to almost nothing by the month’s end. You might think that astronomers would be tempted to mothball their telescopes, but if they did they would miss the year’s best views of Saturn.
The beautiful ringed planet comes to opposition at a distance of 1,331 million km on the 10th when it lies in Libra, the Balance or Scales, and stands in the south in the middle of the night. The ochre deserts and white north polar cap of Mars are also observable, as are all the other brighter planets at one time or another. There may also be a spectacular meteor shower that has never been seen before.
Look overhead at nightfall to find the Plough and extend a curving line along its handle to reach the star Arcturus shining brightly in Bootes well up in the east-south-east. Continue that line, still bending, into the south-east where Mars is conspicuous and reddish in Virgo, above-right of Virgo’s leading star Spica. By our star map times, the Plough has moved to stand high in the west, Arcturus is high in the south, and Mars is in the south-west.
Following its own opposition on 8 April, Mars is now receding from us, from 96 million to 119 million km during May, and although it halves in brightness from magnitude -1.2 to -0.5 it still outshines Arcturus. Viewed telescopically, its disk shrinks from 15 to 12 arcseconds and only in moments of steady “seeing” can we discern its surface detail. The Red Planet’s slow westerly progress below the famous binary star Porrima halts on the 21st when it reaches a so-called stationary point before tracking eastwards again.
Saturn, creeping westwards in the middle of Libra and bright at magnitude 0.1, stands close to the horizon and beneath Arcturus at nightfall. By our map times, though, it is almost due south at an altitude of nearly 19° as seen from Edinburgh. This is 12° lower than Mars when it transits the meridian, so we see it through more of the Earth’s atmosphere and the seeing is likely to be worse. On the other hand, Saturn’s disk is bigger at 19 arcseconds while its superb ring system spans 42 arcseconds and has its north face tipped 22° towards us. This is a good time to look for the Cassini Division, the 4.800 km gap between the two main rings.
Binoculars show the star Zubenelgenubi, 5° to the west of Saturn, to be an obvious double star, while Zubeneschamali, to Saturn’s north, is held (perhaps mistakenly) by some observers to be one of the few greenish-hued stars in the sky. The Arabic names for these stars mean Southern and Northern Claw respectively and date from an era when they were also associated with the brighter nearby constellation of Scorpius the Scorpion. Use binoculars to scan 11° north of Zubeneschamali for the fuzzy blob of M5, a globular cluster of up to 500,000 stars at a distance of about 25,000 light years. Some observers rate it more highly than the more familiar M13 globular in Hercules and M3 in Canes Venatici, 12° to the north-west of Arcturus.
Jupiter remains prominent, and brighter than any star, in the west at nightfall but is close to setting in the north-west by our map times. At magnitude -2.0, it is tracking eastwards in the middle of Gemini, below Castor and Pollux, and shows a 34 arcseconds disk at midmonth.
Mercury is an evening star as it climbs to stand furthest east of the Sun, 23°, on the 25th. Between the 13th and 29th it stands about 10° high in the west-north-west forty minutes after sunset though it may be hard to spy without binoculars in the slowly-fading twilight. It dims from magnitude -0.6 to 1.0 between these dates. Venus is a brilliant morning star on magnitude -4.1 which rises in the east fifty minutes before the Sun on the 1st and one hour before sunrise on the 31st.
Sunrise/sunset times for Edinburgh change from 05:29/20:52 BST on the 1st to 04:36/21:45 on the 31st. Nautical twilight at dusk and dawn lasts for 105 minutes on the 1st and for all but the middle 24 minutes of the last night of May.
The Moon is at first quarter on the 7th, full on the 14th, at last quarter on the 21st and new on the 28th. The Moon is strongly earthlit when it stands just above Aldebaran in Taurus on the 1st evening. Catch it again below-left of Jupiter on the 4th, near Mars on the nights of the 10th and 11th and Saturn on the 13th and 14th.
The morning of the 24th may see slow meteors streaming away from a radiant point in the dim constellation of Camelopardalis the Giraffe, see north map. The prediction is made by analysts who have back-tracked the motion of a small comet whose official name is Comet 209P/LINEAR. Discovered as recently as 2004, its path carries it between the orbits of the Earth and Jupiter every 5.1 years and it is to pass harmlessly only 8,290,000 km from the Earth on the 29th, the ninth closest approach by a comet on record.
Only a few days earlier, it is thought that the Earth may encounter several streams of particles that were released by the comet between 1803 and 1924. Meteor rates could hit many hundreds per hour, if not storm force, though the peak of activity is predicted between 08:00 and 09:00 BST on the 24th, during daylight for Britain but ideal for observers in N America. Our pre-dawn hours could still be interesting, though.
This is a slightly-revised version of Alan’s article published in The Scotsman on April 29th 2014, with thanks to the newspaper for permission to republish here.
This telescope, hanging on a wall in the Meridian Building at Greenwich, was used to make two of the greatest discoveries of all time in the field of celestial dynamics. But before I introduce these discoveries, I need to create an image of the stage onto which they came.
Measuring the speed of light began with Galileo’s failed attempt of the early 1600’s where he tried to arrive at a result by timing light over about two miles. Moving on, the next person to become involved, in 1676, was Ole Romer, a Danish astronomer who was studying the orbit of Io, the innermost satellite of Jupiter. Romer correctly deduced that difficulties he had in timing this orbit were related to light taking longer to reach earth when Jupiter was further away in its orbit. Working backwards from the observational data he arrived at a speed of light of 200,000 km/ sec. Although this figure is around 33% incorrect – attributed later to the imprecise dimensions of the Solar System known at the time – Romer became the first person in history to measure the speed of light.
Meanwhile, the creation of Greenwich Royal Observatory was underway, and precision measurements of the celestial sphere were being developed. Stellar Parallax was of particular interest as, although Copernicus hypothesised a helio-centric universe in 1543, to date no one had found any proof that this was correct. Existence of Stellar Parallax would confirm this. The effects of Atmospheric Refraction mean that small movements of celestial objects are best measured in the zenith where there is no refraction to interfere with the observational data. As a result, Gamma Draconis, which is in the zenith over London, was a prime candidate for measuring any small movements that a star may make.
It was Robert Hooke in 1669 who invented the Zenith Telescope to study Gamma Draconis in more detail, and he noted that the star “moved by more than 1/100th of a degree” between early July and late October that year. Although compelling evidence for parallax, until a whole year’s data is obtained the question remains open. Hooke went on to design the Monument to the Great Fire in London as a 202ft Zenith Telescope, it being hollow from the basement to a trapdoor at the top that could be opened to reveal the sky. But, even at this time, the monument vibrated under the influence of London traffic and he was unable to carry out any scientific experiments. John Flamsteed in Greenwich built a Zenith Telescope into an existing well in 1679 but the records state he made only one observation.
The problem lay dormant for the better part of 50 years until it was again addressed, this time by two individuals called Samuel Molyneux and James Bradley. Molyneux was a rich “amateur” astronomer, and Bradley was a trained observational astronomer. Molyneux funded a 24ft Zenith Telescope that was attached to a chimney of his mansion at Kew and Bradley ran the experiment.
After one year’s observation Gamma Draconis was shown to indeed move with time – in an almost cyclical manner – but this could not be mathematically related to parallax – something else was happening.
The diagram above shows the movement of Gamma Draconis, in green, in 1726. The red curve is the path that Bradley calculated the star would take if they were observing parallax. Although they seem to superimpose quite well, there is a phase shift in their timing and it is not possible for the green curve to represent stellar parallax.
There is also a vertical green line in the image. To measure the horizontal (Right Ascension) movement of the star a precision Sidereal Clock is required, but Molyneux and Bradley did not have one. So they had to be content with only observing and recording the north/ south movement of the star.
Bradley took stock and realised that the telescope they were using was not suitable to carry the experiment forward. It had so little travel at its lower end that only two stars could be observed. Using a star catalogue Bradley established that if the lower end of the telescope could be moved by 5 degrees in each direction over 70 stars could be observed. He further noted that he would be able to see Capella – one of the brightest stars in the sky – if the movement was over 6¼ degrees.
He therefore had a new, 12½ ft Zenith Telescope made, but he had this installed in the house in which he was living with his aunt, in Wanstead. He wanted to make copious measurements and by having it where he lived he would not have the difficulty of travel to Kew. He began using this telescope in August 1727. He measured over 50 stars and at least 10 on a very regular basis.
Unfortunately, Samuel Molyneux, whilst in attendance at the House of Commons where he was an MP, fell seriously ill in April 1728 and he died a few days later. This is particularly sad as Bradley was on the cusp of making one of the greatest discoveries in the history of celestial dynamics.
Bradley came to a conclusion on his discovery in late 1728 when sailing with a friend on the Thames. By comparing the constant wind on the Thames with the constant light stream from Gamma Draconis, the movement of the yacht with that of the earth in its orbit, and the slight movements of a vane on the yacht mast with a telescope trained on Gamma Draconis (having to be moved as the earth changed direction in its orbit) Bradley came up with a theory now known as the “Aberration of Light”. He correctly deduced that when the earth is moving in a non parallel manner to the light from a star that the telescope would have to be tilted slightly to receive the light from that star.
He calculated that the largest angle that the telescope would have to be tilted by – when the earth was moving perpendicularly to the light from the star – was 20.2 seconds of arc. This value is called the Constant of Aberration and is denoted by the Greek letter Kappa. Its currently accepted value is 20.49552 seconds of arc, less than 1.5% different from Bradley’s initial result. Knowing this angle, Bradley found the speed of light to be 20,210 times the speed of the earth in its orbit. Bradley, however, did not give a value for the speed of light in his 1729 paper. It relies on a knowledge of the radius of the orbit of the earth. Perhaps Bradley was not sufficiently convinced that the value of the radius known at this time was of sufficient accuracy for him to quote a speed of light. But what is evident is that, given an accurate radius, Bradley’s data yields a result within 1.5% of the currently accepted value.
Bradley noted that the movements of Gamma Draconis were more complex than could be explained by the Aberration of Light. Indeed had the star movements been solely due to the aberration of light the above green curve that the star took in 1726 would have been a closed curve. So he kept his experiment going – indeed he ran it until 1747 – over 20 years since he started out with Molyneux in 1725. In 1747 Bradley published a second paper on his experiment as the secondary effects on the movement of the star were found to be related to “Nutation” – a wobbling of the earth’s axis due to the gravitational pull of the moon. One of the cycles of the moon is 18.6 years long – hence the length of time Bradley needed to take observational measurements to see this effect. The behaviour of the star over this period is demonstrated in the diagram below.
These were not his only accomplishments, however. Bradley became Savilian Professor of Astronomy at the University of Oxford in 1721, and the third Astronomer Royal at Greenwich in 1742, both of which he held to his dying day in 1762. In both of these he made further significant astronomical observations and discoveries. His telescope still hangs today on a wall in the Meridian House at Greenwich where it can be seen by the public.
In conclusion, I feel that the current exhibit at Greenwich does not do justice to a telescope that made two of the greatest discoveries of all time in the field of Celestial Dynamics.
This article is an adaptation of Bruce Vickery’s talk of the same name, which was given to the Astronomical Society of Edinburgh on the 7th of March 2014. Bruce is a member of the Society and one of his personal interests is the development of software to demonstrate the dynamic behaviour of the celestial sphere. This software was used to graphically illustrate the principles involved in the discovery process throughout his talk, and examples of this are included in the article above. Our thanks to Bruce for sharing his knowledge of this subject with us in such an interesting and informative manner.
Mars shines brightly at opposition in Virgo
Six years have passed since Mars was as close and bright as it is this month, but two other planets outshine it and a fourth, Saturn, will soon be at its best for the year. There are also two of 2014’s four eclipses but, as with the second pair in October, neither is of much interest for observers in Scotland.
For the moment, our evening sky retains a flavour of stellar feast we enjoyed over the winter. Orion is still on show in the south-west at nightfall below the conspicuous planet Jupiter. Orion’s Belt now lies almost parallel to the horizon, a line along it pointing to the left towards Sirius, our brightest nighttime star, and to the right towards Aldebaran and the Pleiades in Taurus. By our star map times, though, Orion has all but sunk below our western horizon.
Jupiter, however, continues as our brightest evening object bar the Moon. As it slips 3.5° or seven Moon-widths eastwards in the middle of Gemini during April, it fades a little between magnitude -2.2 and -2.0 and its telescopic diameter shrinks from 38 to 35 arcseconds. The earlier in the night that we catch it, the higher it stands and the sharper the view of its cloud-banded disk. By our map times Jupiter is some 30° high in the west and on its way to setting in the north-west four hours later.
The month begins with impressive views of the young earthlit Moon in the west at nightfall. It is only 5% illuminated on the 1st as it stands 14° high forty minutes after sunset. Look for it below the Pleiades on the 2nd, below the Aldebaran-Pleiades line on the 3rd and 6° below Jupiter on the 6th as it nears first quarter.
Mars reaches opposition on the 8th when it lies 93 million km away and shines at magnitude -1.5 so that its orange-red beacon rivals Sirius in brightness if not in colour. By definition, it stands opposite the Sun in the sky so that we find it climbing from the eastern horizon as the evening twilight fades to pass 28° high on Edinburgh’s meridian two hours after our map times. As the arrow on our south map shows, Mars tracks 10° westwards in Virgo during April, from 5° above the magnitude 1.0 Spica today to lie 1.6° below-left of the famous binary star Porrima as the month ends.
Often the day of opposition is when a planet is closest to us but Mars is approaching the Sun in its orbit and is 450,000 km closer to us on the 14th than on the 8th. Through a telescope, its ochre disk is 15 arcseconds wide and shows dusky markings and the dwindling white smudge of its north polar ice cap, tipped about 22° towards us.
The full Moon lies below Mars on the evening of the 14th and is approaching Spica as it sets for Edinburgh at 06:08 BST on the 15th. Only 14 minutes before this, and while it is less than 2° above the west-south-western horizon in the twilight, it begins to enter the outer shadow of the Earth, the penumbra. Sadly, we have no hope of seeing any dimming of the lunar disk before it sets. Observers in the Americas are much better placed to view the resulting total eclipse of the Moon which is total from 08:07 until 09:26 BST (03:07 to 04:25 EDT).
Sunrise/sunset times for Edinburgh change from 06:44/19:51 BST on the 1st to 05:32/20:50 on the 30th while the duration of nautical twilight at dawn and dusk stretches from 84 to 105 minutes. After first quarter on the 7th, the Moon is full during the eclipse on the 15th, at last quarter on the 22nd and new on the 29th when a small area of Antarctica and perhaps a few penguins experience an annular eclipse of the Sun. A partial solar eclipse is visible from Australia and the southern Indian Ocean.
On course to reach opposition in May, Saturn rises at Edinburgh’s east-south-eastern horizon at 23:31 on the 1st and only 36 minutes after sunset by the 30th, climbing to pass 18° high on the meridian four hours after our map times. Improving from magnitude 0.3 to 0.1, it edges westwards in Libra and draws ever closer to the Moon overnight on the 16th-17th when Saturn’s disk is 18 arcseconds wide while its stunning rings span 42 arcseconds.
Mercury is hidden in the dawn twilight until it passes around the Sun’s far side on the 26th. Venus, brilliant as a morning star, rises in the east-south-east seventy minutes before sunrise on the 1st and in the east only 51 minutes before the Sun on the 30th. Dimming from magnitude -4.3 to -4.1, its gibbous disk shrinks from 22 to 17 arcseconds in diameter.
It is less than a month since results from NASA’s WISE spacecraft appeared to rule out any Jupiter or Saturn-sized planet lurking unseen in the outermost solar system. Now we learn that a new dwarf planet, dubbed 2012 VP113, has been found to have an orbit that comes no closer to the Sun than 80 times the Earth’s distance, further than any other known object in the solar system. Thought to be a ball of rock and ice perhaps 450 km wide, it may be six times further away at its farthest, and take perhaps 5,000 years to complete each orbit.
Surprisingly, 2012 VP113’s orbit is similarly orientated to those of some other remote bodies, including the only other comparable object, Sedna. There is speculation that this is because they are influenced by a larger undiscovered world, perhaps a super-Earth, even further out.