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Partial Lunar Eclipse Tonight, 25 April
There’s a partial lunar eclipse tonight, visible from the UK, as well as from the rest of Europe, Africa, Asia and Australia.
It won’t be hugely dramatic, as it’s only a partial eclipse of the Moon, not a total one. Even total lunar eclipses are far less grand than total solar eclipses, unfolding over several hours rather than minutes, and turning the Moon a deep red rather than making it vanish altogether.
And for partial lunar eclipses, like tonight’s, all we’ll see is a slight darkening of the edge of the Moon, what we call the “limb”.
Nevertheless it’s worth watching out for if you have clear skies. And the best thing of all is that light pollution isn’t really an issue; you’ll see it just fine from a city.
Here are the timings:
Penumbral Eclipse Begins: 18:03:38 UT
Partial Eclipse Begins: 19:54:08 UT
Greatest Eclipse: 20:07:30 UT
Partial Eclipse Ends: 20:21:02 UT
Penumbral Eclipse Ends: 22:11:26 UT
Remember that these times are in universal time (UT) which is the same as GMT, so add one hour on for BST.
Best time to look is between 9pm and 9:20pm BST.
Image from NASA’s eclipse site.
Constellation of the Month: Leo the Lion
Head outside during April just as the sky gets properly dark and sitting high in the south is the constellation of Leo the Lion.
Leo is well-known as it’s one of the signs of the zodiac, and therefore one of the constellations through which the planets, Sun and Moon pass over the course of the year.
Leo is also well-known due to its most prominent feature, a pattern of stars within the constellation (called an asterism) known as The Sickle, which looks like a backwards question mark, with the bright star Regulus as the dot.
Regulus is known as the king star, and is one of the brightest stars in the sky, shining blue-white in late winter and spring.
Within the constellation of Leo are two groups of galaxies, marked as 1 and 2 on the chart above.
1. The Leo Triplet: M65, M66, and NGC3628
2. The M96 Group: Including M95 & M96
Any of these galaxies can be seen with even a small telescope, but their detailed structure can only really be seen in larger scopes.
Enjoy the spring skies, and happy galaxy hunting!
Maps and descriptions like this one for each of the 88 constellations can be found in my new book, Stargazing for Dummies. Click on the image on the right for more info.
Spring Equilux 2013
Today, Sunday 17 March 2013, it is the Spring Equilux throughout the UK (and possibly elsewhere too*) meaning that there are almost exactly 12 hours between sunrise and sunset.
Sunrise
This date differs from the Spring, or Vernal, Equinox (1102 GMT on Wednesday 20 March 2013) for a variety of reasons, which I explain in a previous post but here is a list of sunrise / sunset times for a variety of towns and cities throughout the UK:
| Town / City | Sunrise | Sunset |
| Aberdeen | 0617 | 1817 |
| Glasgow | 0627 | 1825 |
| Belfast | 0633 | 1831 |
| Newcastle | 0615 | 1815 |
| Manchester | 0618 | 1817 |
| Birmingham | 0617 | 1816 |
| Cardiff | 0622 | 1821 |
| London | 0610 | 1809 |
As you can see the time between sunrise and sunset is not exactly 12 hours everywhere but this is the day of the year when that is closest to being true everywhere*. Yesterday the sun rose a couple of minutes later and set a couple of minutes earlier, and tomorrow the sun will rise a couple of minutes earlier and set a couple of minutes later, as the days lengthen.
Also, the reason that sunrise and sunset do not occur at the same time everywhere* is due mainly to the longitude of the town; the further east a town is the earlier it sees the sun in the morning, and the earlier it loses it again at night.
So happy Equilux everyone*!
* interestingly, the equilux does not occur on the same same day for everyone, it depends on your latitude. The closer you are to the equator the earlier the date of your equilux. For example the equilux in most US cities occurred yesterday, 16 March, and in cities near the equator there is never a day with exactly twelve hours between sunrise and sunset! Take Quito, the capital city of Ecuador (latitude 0 degrees 14 minutes south) for instance. The length of day there only ever varies between 12 hours and 6 minutes long and 12 hours and 8 minutes long!
Russian Meteor 15/02/13
News reports have recently come in of a huge meteor exploding in the air over the Russian cities of Yekatarinburg and Chelyabinsk (about 200km apart), injuring hundreds of people. It’s worth clarifying some of the facts in this matter:
The object that exploded was a meteor, a lump of space rock passing through the Earth’s atmosphere. In this particular case the meteor appears to have exploded around 10km above the ground, over the city of Chelyabinsk.
The shockwave from the explosion damaged some buildings, shattered windows, and set off car alarms. It appears that most of the injuries came from the broken glass, not from the meteor itself hitting anyone.
Showers of fragments from the meteor have been reported too, falling after the explosion over a large area of Russia.
The meteor poses no risk to us any more; it’s all burned up, and it was a one-off random event. Such things are not that rare, happening once every few years, but this one just happened to fall over a populated area.
This meteor was unrelated to asteroid 2012 DA14 that is due to pass by the Earth later today.
UPDATE: A 6m diameter crater has been found in the ice & snow of Lake Chebarkul where the meteorite is thought to have landed:
Perihelion 2013
At around 0500 GMT on 2 January 2012 the Earth was at perihelion, its closest approach to the Sun this year.
If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.
The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).
The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.
This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.
The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (2 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.
Tau Ceti, a nearby Sun-like star, MAY have an Earth-like planet
One of the closest Sun-like stars to us, Tau Ceti, in the constellation of Cetus the Sea Monster, MAY have a family of five Earth-like planets orbiting it, one of which MIGHT be in the star’s circumstellar habitable zone (CHZ), otherwise known as the “goldilocks zone” where it’s not too hot, not too cold, but just the right temperature for liquid water to exist.
Tau Ceti lies only 12 light years away from our solar system, which in astronomy terms is just next door. There are only 19 stars closer to the Sun, and only one of these is a Sun-like star, Alpha Centauri, which lies only 4.4 light years away.
Tau Ceti is a bit smaller than the Sun (0.8 times the Sun’s radius), is cooler (5350K compared to the Sun’s 5780K) and less luminous (0.5 times the Sun’s brightness), and so the CHZ in which the Earth-like planet MIGHT orbit is much closer to the star than the Sun’s CHZ, around half the Earth-Sun distance, approximately 75 million kilometres.
The five planets that MIGHT have been discovered are labelled Tau Ceti b, c, d, e, and f, and the Earth-like planet is the fourth from the star, e.
Why all the MAYBEs and MIGHTs? Well, that comes down to the method by which the planets were detected. They were discovered by observing the star Tau Ceti, and watching for wobbles caused by the gravitational pull of the orbiting planets. Now all five potential planets are similar in size to the Earth, between two and six times the mass of the Earth, but still are tiny compared to the star, and so the wobbles they cause the star to make are very small, almost indistinguishable from noise in the data. Further studies of the star’s wobble might show that some, none, or all of these potential planets might just be artifacts in the data.
How to find Tau Ceti in the sky
Tau Ceti is visible in the sky this month, lying low in the south around 8pm. To find it you have to star hop from the distinct constellations of Orion and Taurus to the much less obvious Cetus.
Find Orion, with the three stars of Orion’s belt pointing up and to the left to the bright star Aldebaran in Taurus. Aldebaran lies in a V-shape collection of stars called the Hyades making up Taurus’ head. This V-shape arrow points down and to the right to a bright-ish star called Menkar in Cetus, lying low in the south. Lower and to the right is the brightest star in Cetus, Diphda, and the fainter star to the left of this is Tau Ceti. Phew!
The Fireball of 21 September 2012
Did you see it? Plenty of UK-based stargazers did; a huge, bright, long-lasting fireball streaked through the sky last night, 21 September, at 2255BST (2155UT).
The fireball was caught on meteorlog’s meteor cam
I saw it as I was driving home from a stargazing trip to Loch Tay in the Highlands of Scotland. The first glimpse I caught was behind some trees, and I clearly saw the very bright (estimated magnitude -5), yellow fragments of a space rock that was disintegrating as it burned up in our atmosphere. It was traveling westwards from the SW, and look to be about 40° above the horizon. I lost sight of it as I drove, but once I’d turned a bend in the road, a full ten seconds later, I caught the last few fragments burning it.
Straight away I pulled over and tweeted to see if anyone else had seen in, and very soon a flood of observations came in. The team at the Kielder Observatory actually had a group of people up observing, who all saw it.
Huge fire ball from east at 9.55 UTC heading west mag -6 to -7
—
Kielder Observatory (@kielder_obs) September 21, 2012Mike Alexander, who runs the Galloway Astronomy Centre, had a group of guests out stargazing then too, and they also saw it, but even better they heard it!
@darkskyman @virtualastro Still buzzing from fireball. Can't get over hearing sonic boom minutes later - beauty of a silent rural location
—
GallowayAstro (@Gallowayastro) September 22, 2012By the time I got home, around 0030BST this morning, only an hour and a half after it happened, there were reports of it on BBC Radio 4, with people all over the northern UK and Ireland reporting the same thing, a disintegrating fireball burning through the night sky for around 20 or 30 seconds.
By the time I woke up this morning it had made it onto BBC News.
So what was it?
A few people suggested that it might have been man-made space debris, an old satellite burning up as it de-orbits, but this isn’t the case, for a couple of reasons. First it was traveling east to west, and satellites don’t orbit in that direction. Secondly, Mike (above) reported hearing a sonic boom approx 150 seconds after it faded. Both of these observations point to the fact that it was a large chunk of space rock, a meteor. When meteors are as bright as this one we call them fireballs.
The Return of Darkness
With summer coming to an end in the British Isles we start the return to the dark skies of autumn and winter. Depending on where you are in the country you will have been without truly dark skies for many weeks, maybe even months, as summer evening twilight lasts throughout the night during the summer.
This all-night-long twilight is almost gone throughout the UK, indeed anywhere on the mainland UK can see astronomically dark skies around 1am at the moment. Only the furthest north outpost of the British Isles still doesn’t have that opportunity.
On the island of Unst, the furthest north of the Shetland islands, lies the UK’s furthest-north town, Skaw, at 60°49′N and 00°47′W. This tiny village will see astronomical darkness return at 0043 on 24 August, lasting only 46 minutes until at 0129 the sun’s light begins to creep into the sky again.
Midsummer sunset (at 2241) from Unst. Image Credit Brydon Thomason, Shetland Nature Ltd.
The last time that astronomical darkness was seen at Skaw was on 18 April, over four months ago! Indeed this settlement is so far north that between around 13 and 29 June each year they never get out of civil twilight, meaning that the sky’s bright all night long!
Compare this with the furthest south town in the British Isles, Saint Clement in Jersey, in the Channel Islands. Astronomical darkness returned to Saint Clement on 4 July this year, having been absent since 8 June; only four weeks without true darkness!
Such is the effect of differences in latitude that these two settlements, separated by 1299 km, have such hugely different seasonal swings between summer and winter.
Curiosity
In the early morning hours (UK time) of Monday 6 August, NASA’s latest Mars rover, the Mars Science Laboratory, or Curiosity to its friends, will land on the red planet after an eight month journey from Earth.
The Mars Science Laboratory, Curiosity
Curiosity is the largest rover ever sent to Mars – it’s about the size of a Mini – and has a huge array if scientific instruments, which will enable it to complete its science missions: to determine if Mars could ever have supported life; to study Mars’ geology; to study Mars’ climate; to plan for a human mission to Mars.
Curiosity will touch down on Mars after a not-entirely-risk-free landing procedure, which uses a heat shield, parachute, engine, and sky crane, a system by which the lander separates from the sky crane, attached by a tether. The sky crane will use its engines to slow it down to almost a dead-stop, and lower the rover gently onto the surface of Mars.
If you want to watch the landing live, NASA and others are streaming it live. Landing is scheduled for 0631 BST, so you’ll have to tune in a bit before that to watch the whole process. You can also follow Curiosity on Twitter.
And if you want to see the red planet yourself, it’s visible low in the west just after sunset, forming a beautiful triangle with Saturn and Spica, the brightest star in the constellation of Virgo. Mars is the right-hand most of the three bright points of light. You’ll only just catch a glimpse of Mars after the sky darkens enough for it to appear, and before it sets around 2245 BST.
Transit of Venus
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.
