Today, 23 September 2014, marks the moment of the Autumn Equinox. At 0229 UT (0329 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:
- 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”.
- 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 0625BST in Glasgow (where I am) and will end this evening around 1950BST, giving me approx. 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).
Join me this Thursday night / Friday morning (18/19 September 2014) for some Twitter-based stargazing!
Many people will be staying up late or even overnight to watch the results of the Scottish Independence Referendum come in. To while away the wee small hours I thought it’d be fun to do a bit of stargazing too!
If you find yourself staying up late then join in by following me on Twitter @darkskyman, and using the hashtag #indyrefstargazing. I’ll kick things off around 9pm on Thursday as the sky darkens, and throughout the night I’ll be describing what’s up in the sky, and how best to see it, winding up around 6am as the dawn breaks. By then we’ll be eagerly awaiting the result of the referendum, which is estimated to be around 7am.
Join in, it’ll be fun! You can send me questions via Twitter too, or just say hello. You don’t need any equipment to take part: just your eyes and clear skies. If you’ve got binoculars, dig them out, as you’ll be amazed how much more you can see.
*** UPDATE 0630UT 13 September: Overnight, some UK aurora watchers caught sight of some northern lights. Reports from Northumberland, N Norfolk, and those parts of Scotland not shrouded in fog. Now it’s night time in N America, activity rates have dropped off but it’s worth keeping an eye on Spaceweather.com for the Kp index to get back up to storm level (5+), as well as the NOAA Spaceweather Now page for the Bz component to turn S. Both of these have to happen in order for a good aurora display.***
Stargazers in the northern UK should look out for northern lights tonight and tomorrow, Friday 12 and Saturday 13 September 2014.
Two large solar eruptions blasted material off the Sun on 9 and 10 September, and that material has been hurtling through space for the last couple of days. The first batch of it hit our atmosphere in the small hours of this morning, resulting in some moderate northern lights displays seen from North America. The second batch has arrived this afternoon, and could possibly trigger a dramatic display of northern lights overnight and tomorrow night.
If you’ve never seen the northern lights (aurora borealis) before then this is an ideal opportunity to catch them. It’s unlikely that this display will be as good as the once-in-20-year display we saw back in February this year, but you never know. It’s hard to predict these things until they actually happen.
If you want to see the northern lights there are a few things you can do to increase your chances:
1. Find an observing site with a clear northern horizon
2. Get away from light pollution; put towns and cities behind you to the south (i.e. head to the northern edge of your town or city, preferably further)
3. Be patient. Aurorae can be faint and indistinct at first, and you need to let your eyes dark adapt to see them properly
Many thanks to the always-excellent Astronomy Now magazine for this story. Their full article is here, and is well worth a read.
In the early hours of the morning of Wednesday 10 September (at around 0306BST) stargazers in the northern part of the British Isles have the chance to witness a star disappearing, if only for a few seconds.
The star in question – HIP 22792 in the constellation of Taurus – is faint, though, and so you won’t see it with your naked eye. The good news is that you can see it through even a modest pair of binoculars mounted on a tripod, and it’s easily seen through a telescope.
So why is it blinking off and on again? In fact it isn’t, but for those few seconds a much smaller but much closer object, an asteroid called (569) Misa will pass in front of it, perfectly obscuring it for observers in a 90km wide band running from Galway in Ireland all the way up to Peterhead near Aberdeen. Observers within this band will see the occultation, and those near the centre line (including me in Glasgow and stargazers in Sligo, Londonderry, and Dundee) will get the best view, with the occultation lasting longest (3.6 seconds!).
How to find HIP 22792
The faint star in question is located in the constellation of Taurus, and will be around 38° above the eastern horizon at the time of occultation. Luckily there are plenty of bright stars nearby to signpost you there.
Here’s a close up of the area in question:
The stars labelled in red (by me) are the signposts to HIP 22792. Stars a and b (ι Tau and τ Tau) are both naked eye in anything other than bright city light pollution, shining at magnitudes +4.6 and +4.3 respectively. This makes them very easy to spot in binoculars. Draw an imaginary line between a and b and cut it with a perpendicular line moving in the opposite direction from the bright star of Aldebaran, and the next bright-ish star you come to is labelled c, HIP 22743, which shines at +6.5. Continue along this line past a faint star (unlabelled) shining at +7.4, then double that distance again to find the target, HIP 22792, which is the faintest so far, at +7.6. The stars labelled d and e are there for reference, and are at +5.8 and +6.3 respectively.
Practical tips for finding HIP 22792
If you have a telescope with in-built goto and tracking you’re good to go but for the rest of us we need to do a bit of prep.
- At the very least you’ll need to mount your binoculars on a steady tripod, or have your scope aligned, so that you can track the target for several minutes.
- Finding it may take some time so don’t just fall out of bed expecting to locate it easily. Give yourself at least 20 minutes of set-up (or more, if you’re new to this!)
- Make sure you’re observing from a site that has a good eastern view, that isn’t obscured by buildings or trees
- As always, the further you can get from the glare of street lights the better.
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.
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:
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:
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!
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.
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