Uranus at Opposition 2013

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):

 

Autumn Equinox 2013

Today, 22 September 2012, marks the moment of the Autumn Equinox. At 2044 UT (2144 BST) the Sun will cross from the northern hemisphere sky to the southern, and we’ll begin the slow approach to the Winter Solstice on 21 December.

The equinoxes (one in spring and one in autumn) are the two instances every year when the Sun makes that crossing from north to south and vice versa, and they’re commonly thought to be the days when day and night are equal length, but they’re really not, for reasons I’ve outline before:

1. astronomers measure the timings of equinoxes, sunrises and sunsets based on the middle point of the Sun’s disk in the sky, so when you read a sunrise time it means the time that the centre of the Sun’s disk rises above the horizon. For a few minutes before that time the top of the Sun’s disk will already have risen, giving “daylight”.
2. Even before this happens the sky is lit up by the Sun below the horizon, and we experience twilight. Most people would think that the sky is bright enough to call it “daytime” long before the Sun pops above the horizon, during the phase of civil twilight.

So today, even though day and night are said to be equal on the equinox, the “daytime” (i.e the start of civil twilight) started about 0630BST in Glasgow (where I am) and will end this evening around 2000BST, giving me 13.5 hours of “daylight”. (Londoners will have from about  0615 until 1930BST, or approx. 13.25 hours of “daylight”).

The day this year where I have exactly 12 hours of “daylight” (i.e. between the morning start and the evening end of civil twilight) is 11 October and this day is called the equilux. (In London the equilux falls on 12 October).

A Bright Nova Appears in Delphinus

Last night a bright nova was discovered in the constellation of Delphinus. It’s bright by nova standards: you normally need telescopes to see novae but this can can be seen with the naked eye – just! – and is easily spottable through binoculars. At the time of writing it has been observed at magnitude 6.3 by Koichi Itagaki, of Yamagata, Japan, and at magnitude 6.0 by Patrick Schmeer, of Bischmisheim, Germany. This means that under dark skies, free from light pollution, with good seeing conditions and good eyesight, it’s within the limit of human eyesight. If you’re in a city though, or if your eyesight isn’t perfect, you’ll need binoculars.

UPDATE 16/08/13 at 1525UT
The British Astronomical Association e-bulletin 00757 is reporting that observations have been submitted to the AAVSO database suggesting the Nova Delphini 2013 has brightened to magnitude +4.5, making it an easy naked-eye object from rural and many suburban sites.

UPDATE 17/08/13 at 0630UT
The AAVSO light curves suggest that Nova Delphini 2013 is dimming, and is currently at magnitude +4.9. This is still naked eye under dark skies and an easy binocular object from cities but get outside as soon as it’s dark and clear; it’s going to keep dimming and soon won’t be naked eye.

UPDATE 18/08/13 at 1430 UT
Although it has dimmed slightly from its maximum brightness of +4.4 magnitudes, it has stayed at +4.9 magnitudes for almost two days now, meaning it’s still naked eye.

UPDATE 22/08/13 at 1330 UT
Nova Delphinus 2013 has dropped below +5.5 magnitude, and will probably drop below human eye detectability in a few days time (it’s already a non-naked-eye object except in very dark sky sites).

Here are some finder charts for the nova, produced using the excellent (free!) Stellarium package.

Nova Delphini 2013 marked with a +

Your first task will be to locate the small constellation of Delphinus. Luckily, that’s really easy at the moment. It’s high in the south around midnight (SE in the evening), and right next to the prominent stars of the Summer Triangle. The brightest stars in Delphinus make up a tiny diamond shape in the sky. Got it? OK, here’s where it gets a little trickier.

Three steps to Nova Delphini 2013 marked with a +

Step 1: Find the diamond shape of Delphinus, shown in the lower left portion of this star chart, with the bright stars of the diamond α, β, γ, and δ labelled (along with ζ nearby).

Step 2: Draw a line from the lower left star of the diamond, δ, past the upper right star, α, but missing it slightly to the “left” of α. Continue for approx. five times the α-δ distance. Here you’ll find another four stars in a diamond of almost exactly the same shape and orientation as (albeit slightly smaller than) the bright diamond of Delphinus. These stars are all really faint. Their magnitudes are marked on the chart above, and they’re all at the very limit of naked eye visibility. Use binoculars if you can’t see them directly.

Step 3: Continue your line onwards, through the lower left star of this fainter diamond to the upper right star, and now take an approximately 45° turn to the “right”, past a very faint star (magnitude 7.85) to the new nova!

(The bright star in the top left of this star chart is 29 Vul, magnitude 4.8)

Nova means “new”, a term coined in 1572 by astronomer Tycho Brahe after he discovered a “new star” in the constellation of Cassiopeia. But these stars aren’t new at all. In fact their brightness is a result of a giant explosion on the surface of a dead white dwarf star.

White dwarf stars form when small stars die and collapse down into a much smaller volume. If there’s another star nearby then the gravity of the white dwarf star can draw some hydrogen gas from the surface of its neighbour onto its own surface. This gas builds up until there is a sufficient quantity of it that it undergoes runaway nuclear fusion, igniting, flaring off, and temporarily brightening the otherwise very faint white dwarf.

No one’s quite sure how this new nova will develop. It might brighten further, or it might begin to dim over the course of days or weeks. All the more reason to get out an find it as soon as you have clear skies. Happy nova hunting!

My Perseids Meteorwatch

On Monday night one of the year’s most spectacular meteor showers is set to peak.

You’ll get a great view of it if you’re somewhere dark with clear skies. But those of us stuck in the city can see plenty too.

I’ll be heading to the grounds of Glasgow Science Centre to view it, and you’re welcome to join me. A few things first:

If it’s cloudy or raining DON’T COME! I won’t be there.

I’ll be there from around 10pm, probably till around midnight.

If you want a great view then head somewhere dark, but if you’re in Glasgow and don’t want to head out of the city this is a decent site.

This ISN’T a formal event; I’ll be hanging out there and you’re welcome to come under your own steam. You’ll be responsible for your own safety and comfort; bring extra clothing, a torch, and a deck chair if you have one.

You can park in Glasgow Science Centre car park for £3. Public transport at that time of night is pretty sparse.

Glasgow Science Centre will NOT be open, so there’s no access to refreshments or toilets etc.

Thanks to Glasgow Science Centre for letting us use their outside space.

August Asteroids: 3 Juno and 7 Iris at Opposition

This month two of the brightest asteroids, 3 Juno and 7 Iris, will be at opposition in our skies, giving a great opportunity for asteroid hunters to track down these lumps of space rock.

Bear in mind though that you (almost certainly) won’t be able to see them with the naked eye, and that you’ll need binoculars on a tripod or a telescope to find them properly. And even then they’ll just look like very faint stars. But they’re not stars; they’re asteroids, lumps of rock in our solar system orbiting the Sun between Mars and Jupiter.

How big and bright are they?

3 Juno and 7 Iris are amongst the largest of the asteroids, a few hundred kilometres along any one axis. This might seem pretty big but they’re tiny compared to the planets, and so don’t reflect nearly as much light back to us, and are therefore much fainter.

Their magnitudes vary depending on how far away they are from us. They both vary between around seventh and eleventh mag; at their brightest 3 Juno is magnitude 7.4 and 7 Iris is magnitude 6.7. This only occurs under perfect conditions, and this year’s oppositions for both asteroids won’t have them presenting their very brightest aspect. The generally-accepted view is that the human eye can only see down to magnitude 6, but in exceptional circumstances – very dark skies free from light pollution, and very good atmospheric conditions – and with exceptional eyesight, you might just be able to see 7 Iris when it’s closest to us, and at its brightest.

When can I see them?

They’re visible all month but the best time to look at them is when they’re at opposition. That means they’re directly opposite the Sun in the sky, and therefore rise as the Sun sets and set as the Sun rises, getting to their highest due south around midnight.

3 Juno reaches opposition on Sunday 4 August 2013 and it’ll brighten up to magnitude 9. You’ll need a scope, a good star map, and patience to track it down.

7 Iris reaches opposition on Friday 16 August 2013, and it’ll be brighter than 3 Juno, but still not near its best, gaining magnitude 8 during this year’s opposition. Again, a good star chart and telescope is needed.

Where can I see them?

They are both visible in the lower part of the southern sky in the constellation of Aquarius, but you’ll need very detailed star maps to help you find them. 7 Iris is only a degree or so away from the brightest star in Aquarius, β Aquarii, on the night of opposition, making it a bit easier to find. The British Astronomical Association computing section has downloadable star-charts to help you find these asteroids, and others.

How will I know that I’m looking at an asteroid?

The short answer is: you won’t, at least not at first. Asteroids, even the brighter ones like 3 Juno and 7 Iris, will only ever appear as tiny specks of light when seen through a telescope, just like the millions of other tiny specks of light, the stars. However if you observe them over the course of a number of nights around opposition, and mark their position on a star map, then you’ll notice that their position changes relative to the “fixed” stars, as they circle the Sun and move through space.

Don’t be put off if you don’t manage to find them. While you’re out hunting for them don’t forget you can check out lots of other amazing sights through your telescope. Why not have a go at finding the Ring Nebula in Lyra, high overhead this month.

Good luck, and happy asteroid-hunting!

Perseids 2013: The What, How, Where, When, and Why

Here’s a simple guide for observing the Perseids 2013 meteor shower this year, covering five basic questions:
What is the Perseids meteor shower?

The Perseids meteor shower is the most reliable of the active regular meteor showers that happen throughout the year. A meteor shower is a display of meteors (or shooting stars) where you see lots of them in the space of just a few hours. The Perseids occurs around the same time each year, in mid-August, and during the peak of the shower meteor rates increase from just a few an hour (the background rate that you’ll see on any clear, dark night) up to maybe 100 or 200 meteors every hour for observers in the perfect location. Meteorwatchers in the UK will probably see dozens per hour from dark sites, dropping to a few an hour (still worth watching for) in towns and cities.

How can I observe the meteor shower?

You don’t need any special equipment to observe a meteor shower; just your eyes. Try and get as far from city lights as possible (out into the countryside if you can, or into a local park if not), and get comfortable. You might want to bring a reclining deck chair with you, as that makes meteorwatching much more civilised! Just lie back and take in as much of the sky as possible. If you’re lucky enough to see a good display of meteors, you might see as many as one a minute, maybe more!

Where should I look?

Meteors streak across the whole sky, so you don’t need to look in any specific direction, but of course if you’ve got a tall building or tree that’s blocking the view, or a streetlight nearby that’s a bit glare-y, then put these to your back. The Perseids meteors all appear to streak from a point in the sky (called the radiant) in the constellation of Perseus (hence the name) which rises in the east about 10pm local time, climbing to its highest in the sky towards dawn.

When is it happening?

The peak of the meteor shower will probably happen some time around 1815 and 2045 UT (1915 and 2145 BST) on Monday 12 August 2013, although there are uncertainties here. The peak could happen any time between 1415 BST 12 Aug and 0215BST 13 Aug. This means that observers in the UK might catch the peak of the shower, if it happens after the sky darkens on 12 August. Even on the nights on either side we’ll still see plenty. In fact the peak of the Perseids is several days wide, so you can start meteorwatching early, and carry on well after 12 August, so that even if this weekend is cloudy you’ll almost certainly have a chance to see some Perseids. Whatever night you’re out you’ll see more the later you’re up. Starting after dusk, the meteor rate will increase each night as Perseus climbs higher in the sky towards dawn.

Why do meteor showers happen?

Meteors are tiny bits of space dust streaking through our atmosphere. These motes of dust float about in space and as the Earth orbits the Sun it hoovers them up. Sometimes the Earth passes through a particularly dense clump of dust, and we get lots of meteors, in a meteor shower. These clumps of dust are left behind by comets as the orbit the Sun, their streaking tails leaving behind a trail of tiny rock particles. The comet that left behind the space-rocks that we’ll see in the Perseids meteor shower is called Swift-Tuttle, after the two astronomers that discovered it in 1862.

Summer Solstice 2013

The northern hemisphere summer solstice occurs today, 21 June 2013 at 0504 UT (which is actually 0604 BST in the UK).

But surely the summer solstice is just the longest day. How can it “occur” at a specific instant?

That’s because we astronomers define the summer solstice as the instant when the Sun gets to its furthest north above the celestial equator. Or to put it another way, the instant when the north pole of the Earth is tilted towards the Sun as far as it can.

And this happens at exactly 0504 UT on 21 June 2013.

It’s important to remember though that while we are in the midst of summer, the southern hemisphere are experiencing their winter solstice, and their shortest day.

And how much longer is our “longest day”? In Glasgow, my home town, the Sun will be above the horizon for 17h35m12s today (21 June), a full one second longer than yesterday, and six seconds longer than tomorrow.

Mercury, Venus, Jupiter in the evening sky, May 2013

This evening, and for the next few evenings, just as the sky begins to darken after sunset, you’ve got a chance to see three of the five naked-eye planets side by side.

The two brightest naked eye planets (Venus and Jupiter) are close together, separated by only a few degrees, closing to 1° on 28 May (in what we call a conjunction). This should make them very easy to spot, low in the NW from around 30 minutes after sunset. In fact they’re close enough together that you could fit them both in one binocular field of view.

Mercury, however, might be trickier to spot. As the faintest naked-eye planet it will lurk in the twilight sky unseen for many people, just above the two brighter planets.

Remember, if you’re observing with binoculars or a telescope make sure you wait until the Sun has fully set

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!

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.