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

After the Peak: How Long Does a Meteor Shower Last?

August 13, 2014 Leave a comment

I had a question on Twitter today asking whether it was worth while looking out for any Perseid meteors tonight after the shower peaked yesterday. (Here’s my original post about the Perseids 2014 and why the Moon will interfere this year meaning fewer meteors will be visible).

While individual meteors are short blink-and-you’ll-miss-them events, meteor showers themselves last many days, sometimes weeks. The rate of meteor activity builds up in the days before the peak, and tails off afterwards. The peak itself lasts a few hours, maybe a day or so for some broad-peaked showers.

Take a look at this graph of activity for last year’s Perseids meteor shower, when the Moon didn’t interfere:

Perseids 2013 Rate (from imo.net)

Perseids 2013 Rate (from IMO.net)

The number of meteors per hour under ideal conditions (known as the ZHR, the Zenith Hourly Rate) built up to around 20meteors//hour during the month before the peak. The rate started to increase around 9 Aug, doubling to 40meteors//hour around 11 Aug, before tripling again to 120meteors//hour around 13 Aug at the peak. The drop off was quicker, but even for the two days after the peak the rate was still above 20meteors//hour.

Look at this year’s graph, where fewer meteors have been visible due to light pollution from the Moon:

Perseids 2014 Rate (from IMO.net)

Perseids 2014 Rate (from IMO.net)

The shower peaked this year around 50 meteors/hour, less than half last year’s peak rate. Nonetheless in the four days leading up to the peak the rate has been above 20 meteors/hour. So you can expect to see some Perseids tonight or tomorrow before the rate drops back to nearer background levels.

Indeed the Moon, which is now waning past Full, will interfere less tonight, and even less tomorrow, so if you have clear skies tonight (13 Aug 2014) or tomorrow night it’s well worthwhile getting outside and looking up!

Perseids Meteor Shower 2014

August 10, 2014 Leave a comment

This month sees the most reliable meteor shower of the year; the Perseids. However this year the near-full Moon will be in the sky too, brightening the sky so much that only the brightest Perseid meteors will be visible. It’s still worth looking up over the next week if you have clear skies, in case you might spot a bright Perseid.

You can begin watching for Perseid meteors now, and the shower will last until late-August, but the peak of the shower occurs on Tuesday 12 August 2014, which means that the nights on either side of this will be best for meteorwatching, albeit with interference from the moon.

Perseus at 0200 13 August 2014

The best time of night to watch the meteor shower is from around 2200 onwards on both 11 and 12 August 2014, once the radiant, the point from where the meteors appear to originate, rises above the horizon.

The number of meteors that you will observe every hour depends on a number of factors:

  • the density of the cloud of dust that the Earth is moving through, that is causing the shower in the first place;
  • the height above the horizon of the radiant of the shower, the point from which the meteors appear to radiate;
  • the fraction of your sky that is obscured by cloud;
  • the naked-eye limiting magnitude of the sky, that is a measure of the faintest object you can see.

The Perseid meteor shower has a zenith hourly rate (ZHR) of between 50 and 200. This is the number of meteors that you can expect to see if the radiant is directly overhead (the point in the sky called the zenith), and you are observing under a cloudless sky with no trace of light pollution.

However conditions are rarely that perfect. In the UK, for example, the radiant of the shower will not be at the zenith; it will be around 30° above the horizon at midnight, and 45° above the eastern horizon at 2am.

Assuming a clear night, the other factor is the limiting magnitude of the sky, a measure of the faintest object you can see. Man-made light pollution will be an issue for most people. From suburbia the limiting magnitude of the sky is ~4.5 (around 500 stars visible), so you will only be able to see meteors that are at least this bright; the fainter ones wouldn’t be visible through the orange glow. In a big city centre your limiting magnitude might be ~3 (only around 50 stars visible); in a very dark site like Galloway Forest Dark Sky Park the limiting magnitude is ~6.5 (many thousands of stars visible), limited only by the sensitivity of your eye. So in most cases it’s best to try and get somewhere nice and dark, away from man-made light pollution.

This year though the full Moon is a great leveller, giving everyone a limiting magnitude of around 3.

The calculation that you need to make in order to determine your actual hourly rate is:

Actual Hourly Rate = (ZHR x sin(h))/((1/(1-k)) x 2^(6.5-m)) where

h = the height of the radiant above the horizon

k = fraction of the sky covered in cloud

m = limiting magnitude

Let’s plug the numbers in for the Persieds 2014.

ZHR = 100 at the peak, say.

h = 30° at 0001, 45° at 0200, 60° at 0400

k = 0 (let’s hope!)

m = 3 (pesky moon!)

So your actual hourly rate at 0200 under clear dark skies is

(100 x sin(30))/((1/(1-0) x 2^(6.5-3) = 4.4 meteors per hour at 0001
(100 x sin(45))/((1/(1-0) x 2^(6.5-3) = 6.2 meteors per hour at 0200
(100 x sin(60))/((1/(1-0) x 2^(6.5-6.5) = 7.6 meteors per hour at 0400

Remember though that these numbers might be lower if the ZHR drops off after the peak.

It is of course worthwhile having a look on the days leading up to the peak, when the numbers of meteors will be gradually increasing towards this rate.

*UT = Universal Time = GMT, so for UK times (BST) add one hour to these

British MP Wants Astrology To Play A Part In Health Policy

July 25, 2014 9 comments

<rant>

Conservative MP David Tredinnick has recently spoken about his belief in astrology, and the fact that he thinks it should be integrated into UK health care.

It’s worth saying at the outset that astrology is bullshit.

The MP for Bosworth, Mr Tredinnick, is well known for his whacky beliefs, so much so that he is often referred to as the MP for Holland and Barrett, the high street purveyor of supplements, homeopathic remedies, and flim-flam.

What makes Mr Tredinnick’s comments so worrying is that he sits on the health committee and the science and technology committee in the House of Commons.

What makes this even more worrying is that these posts are elected by other MPs, so this believer in all things pseudo-science was deliberately placed on these committees by his fellow MPs.

It’s hard to imagine anyone less suited to judging the merits of health and science policy than a man who thinks that the stars and planets somehow influence our lives. This is a pre-scientific notion, and has no place in a modern society.

Mr Tredinnick has attempted to head off criticism from the reality-based community by referring to those rationalists and skeptics who disagree with his fanciful notions as “bullies” who had “never studied the subjects”.

On the contrary, most astronomers have studied the universe to a far deeper degree than most astrologers, and have come to the realisation that:

  • there are no mechanisms by which the stars and planets can influence our lives
  • one only needs conjure up a new mystical mechanism to account for astrology if there is significant evidence that it works
  • there is no evidence that astrology works: none

</rant>

Summer Solstice 2014

The northern hemisphere summer solstice occurs today, 21 June 2014 at 1051 UT (which is actually 1151 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 1051 UT on 21 June 2014.

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.

Categories: Time and Date

A New Supernova in M106

A brand new* supernova has flared up in a nearby galaxy, M106, according to astronomers. This supernova is located very near the bright core of the galaxy, as can be seen in the image below, making it a little trickier to see, and also making it harder to get a light curve to tell us more about it.

Potential Supernova PSNJ12185771+4718113 Credit: Gianluca Masi, Francesca Nocentini and Patrick Schmeer

Potential Supernova PSNJ12185771+4718113
Credit: Gianluca Masi, Francesca Nocentini and Patrick Schmeer

We do know that it’s a type II supernova, the kind that happens when an old supergiant star suddenly stops fusing elements in its core and collapses under its immense gravity. This collapse is so rapid that the outer shell of the star rebounds off the core in a huge explosion which rips the star apart, scattering its constituent elements into the cosmos, and temporarily brightening it significantly.

This supernova we discovered in April and recently imaged using a 17″ telescope on 21 May 2014. At the moment its brightness is put at around magnitude +15, which makes it pretty hard to spot with anything other than a very large telescope and very dark skies. Anyone in the UK desperate to see it can book a visit to the Scottish Dark Sky Observatory or the Kielder Observatory; both facilities are well worth the effort anyway, and have scopes more than big enough to see the new supernova.

* The galaxy that this supernova is in is 23.5 million light years away so technically it went supernova 23.5 million years ago, and the light has only just got to us here on Earth. Galaxy M106 is the in the little-known constellation Canes Venatici, which despite its lack of any bright stars is still easy to find lying “beneath” the tail of Ursa Minor, the Great Bear.

The location of M106 in Canes Venatici

The location of M106 in Canes Venatici

 

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).

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