Posts Tagged ‘planet’

Morning Conjunction: Venus and Jupiter, August 2014

August 16, 2014 Leave a comment

On Monday morning, 18 August 2014, in the eastern sky before sunrise you’ll see a very close conjunction of the two brightest planets, Venus and Jupiter.

They’ve been shining brightly in the pre-dawn sky for a while now, but as they trace out their separate orbits around the Sun they appear to move relative to one another, Venus the faster of the two. And they’re getting closer every day, until on Monday 18 August they’ll be at their closest, only 12 arcminutes apart, about one third of the diameter of the Moon.

This is closest conjunction in 15 years, and will be a very striking sight in the morning sky, but you’ll need to be up and about early to see it, about an hour before sunrise, around 0450 BST (sunrise is around 0550BST for most of the UK – Orkney gets an earlier sunrise at 0535, while the southwest of England have to wait till around 0605).

If you’ve got a pair of binoculars and a tripod, or even better a telescope, it’s really worth looking at these two planets. Venus is the brighter of the two, shining about twice as brightly as Jupiter through the morning twilight, but if you can magnify them (and you’ll catch them in the same field of view in a pair of binoculars), then Jupiter will be around three times the diameter of Venus (30 arcseconds compared to 10), and you’ll see Jupiter’s four largest moons as tiny points of light near the giant planet.

Don’t worry if you’re clouded out, or if you sleep in, on Monday morning; they’ll be close together in the pre-dawn sky for a few days afterwards too.

Venus-Jupiter conjunction this morning, 0450BST 16 August 2014

Venus-Jupiter conjunction this morning, 0450BST 16 August 2014

Screenshot 2014-08-16 17.42.53

Venus-Jupiter conjunction 0450BST 17 August 2014

Venus-Jupiter conjunction at its closest, 0450BST 18 August 2014

Venus-Jupiter conjunction in close up, 0450BST 18 August 2014


Mercury at maximum elongation, 25 May 2014

May 13, 2014 1 comment

The planet Mercury is the most elusive of all of the naked eye planets. It orbits nearest the Sun, and so always rises just before the Sun or sets just after it, appearing in the glow of twilight. For much of Mercury’s orbit it isn’t visible at all, lying too close to the Sun in the sky.

To see Mercury at its best you have to wait until it’s as far as possible from the Sun in the sky; what astronomers refer to as its maximum elongation. When Mercury is at its maximum eastern elongation it’s visible just before sunrise; when it’s at its maximum western elongation its visible just after sunset.

At the moment Mercury is nearing its maximum western elongation and so makes a perfect evening target.

Mercury in the WNW, at Maximum Elongation, 25 May 2014

Mercury in the WNW, at Maximum Elongation, 25 May 2014, created using Stellarium

Mercury’s range of maximum elongation is between 18° and 28°, and in this particular apparition it’s furthest distance from the Sun is 22.7°. This occurs on 25 May 2014. Between now and the end of May look west just after sunset to try and catch a glimpse of Mercury shining at magnitude +0.4. It’ll be low in the sky, very low, but if you look towards the west, find Jupiter shining brilliantly, and follow a line down to the right at an angle of approx. 45° you should see Mercury a few degrees above the horizon.

If you’re trying to observe it through a telescope then make sure you wait until the Sun has well and truly set below the horizon. Mercury exhibits phases like the Moon and Venus which can be seen through a telescope but shows no other detail through an earth-based scope; on 25 May the disk of Mercury facing the Earth will only be 40% illuminated, making a fat crescent shape. Mercury’s angular size is the smallest of all the planets save distant Uranus and Neptune.

If you’ve ever seen Jupiter or Saturn through a telescope then you’ll know that they look spectacular despite their relatively meagre size. On 25 May, for example, Jupiter will appear to have a diameter of 33 arcseconds (written 33″), Saturn 19″, Venus 15″, Mars 12″, and Mercury a paltry 8″.

And you can actually see all five of these planets on the night of 25 May (or any night between now and the end of May. Mercury is the trickiest to find, but Jupiter will be blazing low in the east, Mars high in the south, Saturn lower in the south-east, and if you’re keen to get up before sunrise you’ll see Venus low in the east. (Uranus and Neptune are dawn objects too at the moment).

Saturn at Opposition 2014

April 28, 2014 Leave a comment

On 10 May 2014 the planet Saturn will be at opposition, making it ideally placed for observation. To be honest, though, Saturn will be a feature of our night sky throughout the spring and summer, only vanishing into the twilight glow of sunset in September. However, at opposition Saturn rises when the sun sets and sets when the sun rises, meaning it’s in the sky all night long.

Saturn, image by Kenneth Crawford and Michael A. Mayda

Saturn as it might look through a large telescope, image by Kenneth Crawford and Michael A. Mayda

Saturn looks like a bright star in the east at sunset, shining at magnitude 0, making it a little fainter than the other bright planets up there at the moment, Jupiter (at around magnitude -1.5) and Mars (at around magnitude -1), but still brighter than most other stars in the night sky, shining about as brightly as the star Arcturus.

Saturn is the furthest planet we can see with the naked eye (unless you head somewhere very dark and strain your eyes to catch a glimpse of Uranus), lying around 9 astronomical units from us (approx. 827 million miles). The reason we can see it shining so brightly is that it’s quite reflective (reflecting 47% of the Sun’s light that shines on it) and VERY big.

The disk of Saturn will appear larger (just) than the disk of Mars when seen through a telescope (18.7 arcseconds for Saturn compared to 15 arcseconds for Mars), but its rings stretch further, subtending 44 arcseconds.

Saturn really is the jewel of the solar system. It’s the planet that most people recognise, and I would bet that it ranks pretty high on most people’s bucket lists of “things to see through a telescope”. If you have a ‘scope, or know someone who does, it’s worth taking a look as Saturn arcs overhead this spring and summer.

You’ll also catch a glimpse, if observing with a small telescope, of Saturn’s largest moon Titan, the second largest moon in the solar system, larger the the planet Mercury. Saturn has 62 major moons, and countless smaller ones (the rings after all are made up of billions of pieces of ice and dust, mini-moons) but only Titan is visible through small scopes. To see the next four brightest (Dione, Enceladus, Tethys and Rhea) you’ll need a decent sized scope, say 8″.

Saturn, Mars, and Arcturus make a prominent triangle in the south at midnight, 10/11 May (created using Stellarium)

Saturn, Mars, and Arcturus make a prominent triangle in the south at midnight, 10/11 May (created using Stellarium)



Kepler-186f – Earth’s Exoplanet Twin

April 18, 2014 1 comment

Astronomers yesterday announced the discovery of the first Earth-sized planet found in the habitable zone of its star. Revelling in the name of Kepler-186f this “twin Earth” was discovered by the Kepler telescope, adding to the 1800 or so exoplanets we’ve already detected.

Artists Impression of Kepler-186f

Artists Impression of Kepler-186f – we’ve no idea what it actually looks like…

The Kepler telescope surveys many stars at one time looking at whether the light we receive from those stars dims temporarily. If it does then that could mean its being blocked out by a planet passing across the face of the star. The dip in star light is tiny, a fraction of one percent of the star’s light, but nonetheless we can get a lot of information about the planet and its orbit from this dimming of its parent star.

By measuring how long the star’s light dims for we can work out how fast the planet is going, and therefore how far from the star it is. By the amount of the star’s light that is blocked out we can tell how big the planet is. In fact we can use mathematical techniques to strip out information from a complicated dimming pattern to work out these factors for a family of planets.

And indeed the parent star in this case, Kepler-186, has five planets going round it, named, from closest to furthest, Kepler-186b, -c, -d, -e, and -f. Only the last of these though is orbiting far enough from the parent star to be in the Goldilocks Zone, the region around a star where it is not too hot, not too cold, but just right for liquid water – a prerequisite for life on Earth at least – to exist. And not only that, but the amount of starlight that Kepler-186f blocks out tells us that it’s very similar in size to the Earth, which means it must be a rocky planet like our own rather than a gas planet, as gas planets are much bigger than the Earth.

Kepler-186 system compared to the inner solar system

Kepler-186 system compared to the inner solar system, showing the HZ in green

The parent star Kepler-186 is much smaller than the Sun; it’s a red dwarf star with a mass of 0.48 M(solar masses), a radius of 0.47 R(solar radii), and a temperature of around 4000°C compared to the Sun’s 6000°C. This means that Kepler-186’s Goldilocks Zone (also known as the habitable zone, or HZ, green above) is much nearer the star than is the case in our solar system. In fact all five of Kepler-186’s planets orbit their star closer than Mercury orbits the Sun, with the most distant Kepler-186f orbiting at a distance of 0.356AU compared to Mercury’s 0.387AU, going round its star every 130 days.

Might there be life?

No one would have believed in the first years of the twenty-first century that this world was being watched keenly and closely by intelligences greater than man’s and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. With infinite complacency men went to and fro over this globe about their little affairs, serene in their assurance of their empire over matter… Yet across the gulf of space… intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.

– an unlikely scenario, borrowed from H.G. Wells’ War of the Worlds

As soon as this planet was discovered (yesterday!) the Search for Extra Terrestrial Intelligence (SETI) trained their Allen Telescope Array on the star in the hope of hearing a message from an intelligent civilisation. So far: nothing. However in order to be detectable to us here on Earth the Keploids would have to be transmitting at 10x the power we do when beaming signals at potential alien civilisations.

Another route to detecting life – any kind of life, not just the intelligent kind – is to use powerful telescopes to study the planet’s atmosphere. If there’s oxygen there then it must be being produced by plant life; if there are industrial pollutants there (like CFCs that don’t occur naturally) then something would have to be making them. However our scopes are not powerful enough to see the atmosphere of Kepler-186f yet, partly because it’s so far away: 490 light years from us.

E.T. Phone Kepler-186f

Even if we did find evidence of intelligent life on this twin Earth, it’s so far away that communicating with it would be terribly slow. Limited as we are in this universe to sending signals at the speed of light, this planet is 490 light years away, and so the conversation would go something like this:

US: “Hello, how are you guys?
[wait 490 years for them to get the signal]
[wait for them to translate the message]
[wait 490 years for their reply to reach us]
THEM: “Fine thanks, how are you?” [980+ years later…]

Visiting Kepler-186f

As you can imagine if it takes light that long to get there, it would take our spaceships even longer. The furthest we’ve ever sent a spacecraft out into space (Voyager 1) is 19 billion km, which sound pretty far, but is only 35 light minutes away. And Voyager 1 has been traveling for 37 years. 37 years for 35 light minutes. That means it would take Voyager 1 around 270 million years to get to Kepler-186f…

Finding Kepler-186 in the sky

Where can you find Kepler-186 in the sky? The short answer is: you can’t. It’s far too distant and faint to be seen with anything other than the most powerful of telescopes, but you can see roughly where it is by looking in the constellation of Cygnus the Swan.

Cygnus rising in the NE at midnight, created using Stellarium

Cygnus rising in the NE at midnight, created using Stellarium

Cygnus is low in the north-east as the sky darkens, rising to high in the east by dawn, and looks like a large cross, with the long leg of the cross representing the swan’s neck, the short leg of the cross being its tail, and the two arms of the cross being its wings. The bright star in the “right wing” (the higher one) is called δ Cygni and Kepler-186 is near this star, towards the tail of the swan.

Other Kepler-186fs

The discovery if this twin Earth is very exciting, but it’s just the very start of our exploration of exoplanets (planets beyond our solar system). The star that Kepler-186f orbits is a red dwarf, a very typical star. approximately 70% of the 300 hundred billion stars in our galaxy are of this type (called M-type). If only one in a thousand of these stars has a planet like Kepler-186f that still leaves 200 million Earth twins in our galaxy, and some of them might be closer to us, making them easier to study, and perhaps to talk with…



Mars at Opposition 2014

This week the red planet Mars reached opposition, making it best placed for observing. Opposition is, as the name suggests, the point where a planet is directly opposite the Sun in our sky.

This means that Mars is up all night long at the moment, rising as the Sun sets and setting as the Sun rises, and so you should be able to spot it whatever time of night you’re out.

Mars reached opposition on 8 April, but on 14 April it will reach its closest approach to Earth, at a mere 57.4 million miles!

On that night – and on nights near that date – the planet Mars will shine very brightly at magnitude -1.5, brighter than anything else in the night sky except the Moon (which is Full on 14 April, and sits near Mars) and Jupiter.

Mars in the sky at midnight on 9 April

Mars in the sky at midnight on 9 April

Mars also looms larger than normal when seen through a telescope, at a whopping 15″ (15 arcseconds = 0.25 arcminutes = 1/240 of a degree!). Stargazers with a decent sized telescope, good observing skills, and good observing conditions should be able to make out the north polar cap of Mars which is tilted towards us at the moment.

Through a small scope you might catch it looking like this:

Mars as seen through a small telescope

Mars as seen through a small telescope

Uranus at Opposition 2013

October 3, 2013 Leave a comment

The gas giant planet Uranus, the seventh planet in our solar system, reaches opposition today at 1558 BST (1458 UT), meaning that this is the best time of the year to find this elusive planet.

Opposition is the name astronomers give to the point at which a planet is directly opposite the Sun in the sky. This means that the planet rises as the sun sets, gets to its highest in the sky at midnight, and sets again when the sun rises, meaning that it’s in the sky all night long.

The exact instant of Uranus’ opposition this year occurs on 3 October at 1458 UT, but Uranus moves so slowly against the background stars that there will be ideal observing conditions all month long.

Uranus was the first planet to be discovered after the invention of the telescope. It was first seen in 1781 by Sir William Herschel. The reason it hadn’t been seen before was that it is not a naked eye planet… But actually it is. At opposition tonight Uranus shines at magnitude 5.8, which is right at the edge of what’s possible to see with the naked eye. You’ll need really dark skies to see it, and perfect eyesight too, but even if you can spot it without a telescope it will only look like an incredibly dim star. Through a telescope you might make out a tiny blue-green disk, only 10% the diameter of the much nearer and much larger Jupiter.

To find Uranus look for the constellation of Pisces high in the south at midnight UT, or 1am BST. Uranus rise to between 30° and 40° above the horizon, depending on your viewing location in the UK. Here’s a handy finder chart (courtesy of Stellarium):

Screen Shot 2013-10-03 at 13.57.54


Morning Mercury, December 2012

December 2, 2012 1 comment

Over the next few mornings you’ll be able spot the most elusive of the naked-eye planets, Mercury, low in the south-east just before sunrise.

Mercury is hard to find, and most days isn’t visible at all. Since it orbits so close to the Sun, when seen from Earth it never appears very far from the Sun in the sky. You can only catch it for a few days at a time when it’s furthest from the Sun in our sky, at a point called its maximum elongation. And even then it’s not that simple to find, as it will always be quite low on the horizon, hidden amongst twilight.

As Mercury whizzes round the Sun (it takes 88 days to make one complete orbit) sometimes we see it in the morning and sometimes in the evening. The amount of time between one morning appearance and the following evening appearance is around six or seven weeks. However Mercury isn’t very clearly visible at every maximum elongation (in some the Sun is much nearer the horizon so the sky is much brighter, making it harder to find), and even when it is clearly visible you’ll only catch sight of it on the few days before and after the date of maximum elongation.

Mercury’s next maximum elongation in of 4 Dec 2012, when it’s quite far (21°) west of the Sun, and quite bright (magnitude -0.3) making it quite easy to spot over the next few mornings.

How to find Mercury

If you have clear skies, head outside around 0630 and find somewhere with a good clear SE horizon (Mercury rises around 0630 and only gets a few degrees above the horizon by the time the Sun’s light begins to significantly brighten the sky).

Luckily there are two other planets up near Mercury right now, namely Venus and Saturn. Both of these planets are brighter than Mercury and higher in the sky, and together all three form a straight line leading diagonally down to the horizon. Find brilliant Venus, the brightest thing in the sky except for the Sun or the Moon, and then look for Saturn up and to the right, and Mercury in the opposite direction, down and to the left.

This photo, taken by the excellent Paul Sutherland, shows how the three planets lined up this morning (2 Dec) when viewed from the UK.

Mercury, Venus and Saturn in the morning sky. Image credit Paul Sutherland.

Mercury, Venus and Saturn in the morning sky. Image credit Paul Sutherland.

Transit of Venus

May 23, 2012 2 comments

This year, on 5 and 6 June 2012, there is a very rare astronomical occurrence: a transit of Venus across the face of the Sun. There have only been six of these transits ever observed before – in 1639, 1761, 1769, 1874, 1882, and in 2004 – and this year’s transit is the last for 105 years!

So what exactly will you see, if you’re lucky enough to catch this last-chance-to-see event? If you’re able to look at the Sun safely you’ll see a tiny black dot moving slowly across the surface – that dot is the planet Venus! NASA has the exact times of the transit from major cities. Importantly, this transit is best seen from the Pacific. Observers in north and central America will see only the start of the transit before the Sun sets, while those of us in Europe will only catch the end of it if we’re up at sunrise.

UK observers: set your alarms! You’ll see the transit between sunrise and 0536 BST, at which point Venus begins leaving the Sun’s disk, taking about 18 minutes to do so.

Venus is 6000km across – just a little smaller than the Earth – and at transit it will be around 43 million km away, directly between us and the Sun. The Sun is 1.4 million km across and around 150 million km away. This means that, seen from Earth, Venus is only about 58 arcseconds in diameter, while the Sun is 1891 arcseconds across, about 33 times the apparent diameter of Venus. So: Venus small dot; Sun big bright ball.

Also, we know how far from the Sun Venus is (107 million km), and how long it takes to orbit the Sun (225 days), so we can work out how long it should take to pass across the Sun’s disk (around 6.5 hours). However the start and end times for the transit vary depending on where on Earth you’re observing, with observers in eastern Canada seeing Venus start to cross the Sun’s disk a whole thirteen minutes earlier than observers in Australia! This is because Canadians are looking at the transit from a slightly different angle than Australians.

Why transits of Venus are (were) important

If you have observations from two widely spaced points on the Earth’s surface, and if you time the start and end of transit accurately at each, you can work out the solar parallax, that is, the difference in position of the Sun when viewed from two different points on Earth, the two points being one Earth radius apart. (Hold your thumb up, close one eye, and obscure a distant object; now switch eyes, and your thumb appears to move with respect to the distant object. That’s parallax).

From the solar parallax, if you know the Earth’s radius, you can work out the Earth-Sun distance (known as the astronomical unit) using high-school trigonometry. This was important to astronomers in the 18th century, as up until then all we knew were the relative distances between all the planets in our solar system, not the actual distances. Once we had one measurement within the solar system – the astronomical unit, say – we could work out how far away everything else was.

The technique of using transits of Venus to work out the solar parallax was first suggested by Edmund Halley in 1716, after he had observed a much more common (although still only 13 times per century) transit of Mercury from the island of Saint Helena. Halley knew that Venus would give much more accurate measurements than Mercury, since it was closer to the Earth and so the angles would be easier to measure. He also knew that the next transit of Venus would happen in 1761, and urged future astronomers to make observations world-wide and thereby calculate the solar parallax, and from that the astronomical unit.

This was duly done, and a value for the astronomical unit of 153 million km was calculated. Later transits in the 19th century yielded a value of 149.59 million km. The current accepted value, calculated from telemetry from space craft is 149.60 million km, so the transit method worked pretty well.

Daytime Venus

May 16, 2010 2 comments

Now that winter’s dark nights are far behind us, and as astronomers begin to pack their scopes away until the autumn, it’s worthwhile knowing a few daytime observing targets, and none is more elusive than a daytime sighting of Venus.

That’s right, at midday over the next few days, you can see Venus in the brilliant blue sky, assuming you have the patience (and no clouds).

Venus is the only thing other than the Moon to be visible to the naked eye against the blue daytime sky, and it really does feel quite bizarre to be looking at a bright “star” in the daytime sky.

Venus is just to the left of the Sun, and when the Sun is at it’s highest point due south it wil be around 30° to the left, at the same height above the horizon as the Sun. The crescent Moon will be further to the left. If you can see the Moon, then Venus is about 1/3 of the distance from it to the Sun. You will probably have to stand and scan the sky for some time before you see it – it certainly won’t be very obvious, but it is there.

Venus in the daytime sky - screen capture from Starwalk for the iPhone

Venus in the daytime sky - screen capture from Starwalk for the iPhone

This picture shows what you can expect to see, but please note that the stars won’t be visible – the Starwalk app for the iPhone shows what stars are up even in the daytime, when they’re not visible.

Please also note that the Sun is in Taurus, so if you were born today that would be your starsign. But of course astrology is a load of bull.

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