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Total Lunar Eclipse 15 June 2011

June 13, 2011 40 comments

The first total eclipse of the Moon of 2011 occurs this Wednesday evening, 15 June 2011, and it will be the longest lunar eclipse in over a decade. However the views from the UK (and Europe) will be constrained by the fact that the Moon will be below the horizon for much of the eclipse, and will rise fully eclipsed, or in some cases even coming out of eclipse. It’s still worth having a look though: just try to find somewhere with a very low and clear SE horizon, as this will be the direction in which the Moon will rise, and it will be in eclipse only while it is VERY low (only a few degrees above the horizon).

Lunar Eclipse, December 21, 2010, by Jiyang Chen

A lunar eclipse occurs when the Moon, in its orbit around the Earth, passes into the Earth’s shadow, as cast by the Sun. You might imagine that this would happen once every lunar orbit, or once a month. That it does not is due to the fact that the Moon’s orbit around the Earth is tilted by around 5 degrees compared with the Earth’s orbit around the Sun. So in most orbits the Moon passes above or below the Earth’s shadow.

However, once in a while (there are at least two lunar eclipses each year) the orbital planes will align so that the Moon passes through the Earth’s shadow, sometimes just grazing it, in which case we get a partial lunar eclipse, and at other times passing right through the shadow, when we get a total lunar eclipse.

The Earth’s shadow has two distinct regions, forming two concentric circles [correction, the shadow is two cones, not circles – need to think in 3D! (see comments)]: the inner, darker, part of the shadow is called the umbra, and objects within this part of the shadow receive no direct light from the Sun. The outer, lighter, part of the shadow is called the penumbra, and objects within this part of the shadow can receive direct light from the Sun, but part of the Sun’s disk will be obscured by the Earth, and so less light than normal falls on the object.

The umbra and the penumbra

For the lunar eclipse of 15 June 2011 the Moon will pass very deeply into the darker umbra, making this an especially dark – and long – eclipse.

There are several distinct phases of a lunar eclipse, as the Moon travels through the penumbra and umbra. For this lunar eclipse the total time during which the Moon is at least partially in the Earth’s shadow is 219 minutes, and 100 minutes of this is spent entirely within the umbra, i.e. in total eclipse.

The phases of the 15 June 2011 total lunar eclipse (via NASA's eclipse website)

These phases are given the names: P1, the time when the Moon’s disk enter the penumbra; U1, the time when the Moon’s disk enters the umbra; U2, the time when the entirety of the Moon’s disk is within the umbra; U3, the last time when the entirety of the Moon’s disk is within the umbra; U4, the last time when part of the Moon’s disk is within the umbra; and P4, the last time when part of the Moon’s disk is within the penumbra.

The timings for these instances are well known. In the following table are: the timings for P1, U1, U2, U3, U4, and P4; the time of greatest eclipse (i.e. where the Moon is closest to the centre of the umbra); and the local times of moonrise for a variety of places around the UK

Eclipse Contact Contact Description Time (UT) Time (BST) Moonrise Time (BST) (UK Location)
P1 Penumbral Eclipse Begins 17:24:34 UT 18:24
U1 Partial Eclipse Begins 18:22:56 UT 19:22
U2 Total Eclipse Begins 19:22:30 UT 20:22
Greatest Eclipse 20:12:37 UT 21:12 21:12 (Channel Islands)
21:14 (London)
21:26 (Cardiff)
21:33 (Manchester)
21:56 (Belfast)
21:58 (Glasgow)
U3 Total Eclipse Ends 21:02:42 UT 22:02
22:19 (Orkney)
21:23 (Shetland)
U4 Partial Eclipse Ends 22:02:15 UT  23:02
P4 Penumbral Eclipse Ends 23:00:45 UT  00:00

UT = Universal Time = GMT
BST = British Summer Time = GMT+1

As you can see, the UK is far from ideally placed to view this total lunar eclipse, but the further south and east you are the better your chances of seeing something. The Moon will rise well past U2 across the UK, and everywhere except the Channel Islands and SE England it will rise well past greatest eclipse. Indeed in the north of Scotland the Moon will rise after the total eclipse phase ends (i.e. past U3).

Observers in the Channel Islands and in the SE of England will have around 50 minutes of total eclipse to observe although the Moon will still only be a few degrees above the horizon at U3), and here in Glasgow I’ll have about 4 minutes between moonrise and U3!

While in total eclipse no direct sunlight will fall on the Moon, but we will still be able to see it illuminated a dull red colour. How can this be, if there is no sunlight shining on it to light it up? It is due to the fact that the Sun’s light is refracted, or bent, through the Earth’s atmosphere. The red light from the Sun’s spectrum is refracted the most, and so it is this light that will illuminate the Moon during a lunar eclipse. In effect, the light you will see on the Moon is the combined light from all the sunrises and sunsets on Earth, being focused onto the Moon by the lensing effect of the Earth’s atmosphere.

A detailed information sheet for this eclipse (and others) is available (pdf) on the NASA Eclipse website.

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Twilight vs. Light Pollution

May 4, 2011 4 comments

A few weeks ago I was waiting for it to get dark so I could go out into my garden and use my telescope. I decided to wait until the end of astronomical twilight (when there is no light from the Sun left in the sky) so that the sky was “properly dark”, but of course given that I live in a city (Glasgow) the light pollution from street lights means that it never actually gets “properly dark”.

So I decided to figure out exactly how long I should wait after sunset before going out to observe, or put another way, at what point does light pollution take over from twilight as the dominant source of light in the sky?

Sunset behind Glasgow Science Centre

I carried out this little experiment on 28 April 2011, in my back garden on the Southside of Glasgow, under the following conditions:

Sunset Time: 2050 BST
Civil Twilight Ends (Sun 6 degrees below horizon): 2134 BST
Nautical Twilight Ends (Sun 12 degrees below horizon): 2232 BST
Astronomical Twilight Ends (Sun 18 degrees below horizon): 2358 BST
Longitude: 55.866 N
Latitude: 4.257 W
Sky conditions: 100% clear

[Incidentally, sitting out in my garden for four hours as it darkened was an absolute delight: I saw, as well as the emerging stars, many bats, some ducks, two foxes, and two passes of the International Space Station!]

Using a Sky Quality Meter I took readings of the sky brightness at the zenith every two minutes. The SQM-L makes measurements of the sky in magnitudes per square arcsecond, i.e. brightness per unit area in the sky. As a general rule, in city centres you would expect readings of 16-17, while in dark places you can get readings of 21-22. The higher the number the darker the sky. The darkest reading possible under a starry sky is around 22, as at that point the starlight itself becomes the limiting factor.

From experience I know that in my garden the darkest reading possible is around 18.5, so I decided to continue taking readings until I got fifteen in a row that were above 18.4, i.e. for half an hour the sky had not been significantly darkening. I got my first reading of above 18.4 at 2244 and the sky did not appreciably darken over the next 30 minutes.

At 2244 the sun was 13 degrees 07 minutes (13.117 degrees) below the horizon.

I graphed the results to see how they looked, and placed them alongside the projected results if I were under a dark sky free of light pollution i.e. so that the results could get as low as 22.0 at the end of astronomical twilight at 2358 BST

SQM-L Readings vs Time

Result: After sunset, and throughout civil twilight (Sun between 0 and 6 degrees below the horizon) and nautical twilight (Sun between 6 and 12 degrees) the dominant source of light in the sky is sunlight.

However shortly after the end of nautical twilight light pollution became the dominant source of light in the sky, when the Sun was a little more than 13 degrees below the horizon.

This means that, while observing from my garden in Glasgow, I shouldn’t wait much later than the end of nautical twilight to go out observing, since beyond that point the sky will not significantly darken.

CAVEAT: It should be noted that this information is really only relevant for my specific circumstances, i.e. the light pollution in your sky may be better of worse than mine, and mean that the point at which it begins to dominate twilight is different for you.

Happy Spring Equilux

March 18, 2011 5 comments

Today, Friday 18 March 2011,  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

Sunrise

This date differs from the Spring, or Vernal, Equinox (2321 GMT on Sunday 20 March 2011) 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 0626 1826
Belfast 0632 1833
Newcastle 0615 1815
Manchester 0617 1818
Birmingham 0616 1817
Cardiff 0621 1822
London 0609 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 does 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, 17 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!

The Stars of Spring

March 5, 2011 2 comments

As winter fades and spring arrives the stars in our evening sky change and new stars, not seen over the winter, make a reappearance.

Two of those stars are Arcturus, in the constellation of Bootes, and Spica, in Virgo, both of which will become early evening objects in April (in early March Spica doesn’t rise until 2130, three hours after sunset).

Spica, or the “ear of wheat” is held by Virgo the Virgin, and its presence in the evening sky has long been used by ancient astronomers as an indication that spring is coming.

Stars of Spring, screen capture from Stellarium

Stars of Spring

To find these stars start with the curved handle of the Plough and continue that arc down to Arcturus. Spica is the same distance again beyond Arcturus. You can use the common mnemonic: “Arc to Arcturus and spike down to Spica”.

These stars should be easy to spot as they’re very bright, Arcturus being the fourth brightest star in the night sky at -0.04 magnitude (only Sirius, Canopus and Alpha Centauri are brighter, and only one of these – Sirius – is visible from the UK), and Spica is the 15th brightest at +1.04 magnitude.

Arcturus is so bright for two reasons: (1) is it intrinsically quite bright, being an orange giant star (the Sun would fit inside Arcturus 17000 times), and (2) it is quite close to us, being only 37 light years away.

Spica, by comparison, is intrinsically even brighter (it too is a giant star, but it is a super-hot blue-white giant compared to the rather cooler orange Arcturus), but it is a little smaller (“only” 400 Suns would fill it) and much further away, around 260 light years distant.

Twelve hours of “daylight”

March 1, 2011 4 comments

Today, throughout the UK, civil twilight began almost exactly twelve hours before it will end this evening meaning that we have, for the first time this year, twelve hours of “daylight”. Summer is on its way!

Sunset on Sark, 11 September 2010

Sunset on Sark, 11 September 2010

City Civil Twilight Dawn Civil Twilight Dusk
Glasgow 0632 1828
Manchester 0622 1821
London 0614 1814

Of course some of this “daylight” is what we call twilight, but if you’re outside between these times you will certainly think that the sky is bright, and that the day has begun.

Technically the equinox (“equal night”) doesn’t occur this year until 20 March, with the equilux (“equal light”) occurring a few days before that (it varies around the world but in most of the UK the equilux occurs on 18 March 2011). For a detailed explanation of equinox, equilux and twilight times see my blog post from last March.

Mercury and Venus – See them while you can

April 6, 2010 2 comments

For the last week or so, and for the next few nights, Mercury and Venus are putting on a lovely display low in the western sky just after sunet. The closest two planets to the Sun, Mercury and Venus appear to move very quickly from night to night against the background stars, and so are never in the same place for very long.

Mercury is always difficult to spot as it is often quite faint and is so close to the Sun that it either sets very soon after sunset or rises only just before sunrise. For that reason it’s always good to have a brighter marker to locate Mercury by; for the next few days that marker is Venus.

Last week on April 3rd and 4th, Mercury and Venus were at their closest to one another, seperated by only a few degrees (or a bit more than a thumb’s width held at arm’s length), and Mercury was at its brightest.

Mercury and Venus
Mercury (faint) and Venus low in the west

As the month of April draws on, Mercury will set earlier and earlier, getting fainter and fainter, until by the middle of the month it will be all but lost in the western twilight.

So get out tonight (or the next time it’s clear over the following week) and have a look.

You can find out more about observing Mercury and Venus on the British Astronomical Association’s website

Categories: General Astronomy

Five Useful iPhone Apps for Astronomers

March 26, 2010 1 comment

Here’s a brief overview of the five iPhone apps that I, as an astronomer, find indispensible (in no particular order):

Weather Pro (£2.39)

Let’s face it, no weather forecast is 100% right, but this app gives you more info than most, allowing you to figure out in advance whether there’ll be clear skies.

Magic Hour – formerly VelaClock (£2.39)

So, it’s going to be a clear night. Now you need to know: when is sunset? when does astronomical twilight end? Magic hour is your app for that. Also displays moonrise and moonset times, this is the perfect app for figuring out exactly when your skies will be dark.

Starwalk (£1.79)

The very best of all iPhone astronomy apps, Starwalk has an exquisite interface, and is packed full of features. Great for beginners wanting to find their way around the sky, and for experts who want to dig a little deeper.

Satellite Visibility (£1.79)

Showing iridium flares, ISS & Hubble passes, and many other sats too. Impress your friends by knowing where and when (to the exact second) satellites are due to pass overhead.

Reeder (£1.79)

OK, so not immediately anything to do with astronomy, but with this RSS aggregator and subscriptions to some astronomy blog feeds (Bad Astronomy, Universe Today…), you’ll be able to keep abreast of all breaking astronomy news.

So only two of these are directly related to astronomy, but I’d be lost without any of these, and for a total cost of just over a tenner these apps are great value for money.

Equinox, Equilux, and Twilight Times

March 20, 2010 23 comments

On or around 21 March each year it is the Spring Equinox in the Northern Hemisphere, the day when eggs can be stood on their ends, when the Sun crosses the celestial equator, and when night and day are equal length the world over.

But this last part isn’t entirely true, for a variety of reasons.

Equinox

Equinox is latin for aequus (equal) and nox (night) meaning, roughly, “equal night”. The moment of the equinox is defined as the point at which the centre of the Sun’s disk crosses an imaginary line in the sky called the celestial equator, the projection of the Earth’s equator out into space.

The Sun, courtesy of NASA

The Sun (and the Moon and all the planets) move along a line in the sky called the ecliptic, the projection of the disk of the solar system out into space. These two lines, the equator and the ecliptic, cirlce the sky, and because the Earth’s axis is tilted at 23.5 degrees the angle between the equator and the ecliptic is 23.5 degrees, and the two circles meet at only two points, called equinoctial points.

Over the course of the year the Sun, as seen from Earth, appears to make one complete circuit around the ecliptic, as the Earth in fact orbits the Sun. And so on two days each year the Sun’s path crosses the equator. This means a number of things:

1. that an observer at the equator will see the Sun directly overhead at mid-day on the equinoxes
2. that the Sun will rise due east and set due west on the equinoxes (on all other days the Sun will rise either north or south of east, and set north or south of west)
3. the length of day and night are nearly equal

On this last point, they are not exactly equal, for two reasons:

1. the Sun appears as a disk in the sky with a radius of around 16 arcminutes, and so the top of the Sun appears to rise while the centre of the disk is still below the horizon, and the instant of the equinox is measured with respect to the Sun’s centre, and
2. the Sun’s light is bent, or refracted, in the Earth’s atmosphere, so that rays from the Sun can light you up even before the Sun rises, and keep you lit after it sets, with the degree of refaction being around 34 arcminutes

These two factors combine to mean that the Sun will appear to have “risen” when the centre of the disk is still 50 arcminutes (16 + 34) below the horizon, making the amount of daylight longer than the expected 12 hours. How much longer depends on where on Earth you are, but in the UK the length of the day is approx. 12 hours 10 minutes, rather than exactly 12 hours.

Equilux

Because of this effect, the days on which the length of day and night are exactly equal, called the equilux, occur a few days before the spring equinox and a few days after the autumn equinox. This date will vary depending on where on Earth you are, and indeed equiluxes do not occur at all close to the equator, whereas the equinox is a fixed instant in time.

Twilight

But the story doesn’t end there. Even on the days of equilux, the sky will have been bright for some time before the first rays of the Sun hit you, and will remain bright for some time after the last rays disappear from view. This time of day is called twilight, starting at dawn and ending at dusk.

There are, in fact, three kinds of twilight.

Civil twilight is what most people mean when they talk about twilight. It starts in the morning when the centre of the Sun’s disk is 6 degrees below the horizon, and ends at sunrise. In the evening civil twilight stars at sunset and continues until the centre of the Sun’s disk is 6 degrees below the horizon. During civil twilights the sky is still bright enough that, in general, artificial illumination will not be needed when doing things outside. In reality though, most councils switch lights on a fixed time after sunset (say 30 minutes) and turn them off a fixed time before sunrise, rather than relying on the Sun’s angular distance below the horizon.

Nautical twilight is when the Sun’s disk is between 6 and 12 degrees below the horizon. During nautical twilight it is still possible to distinguish the sky from the distant horizon when at sea, thus allowing sailors to take measurements of bright stars against the horizon. Most of us would consider this “dark”, but it is still technicaly twilight.

Astronomical twilight is when the Sun’s disk is between 12 and 18 degrees below the horizon. During astromical twilight you’ll no longer be able to tell the sky from the distant horizon when at sea, but crucially for astronomers, there is still light in the sky. Not much – indeed most of us would say it’s properly night time at this point – but the faintest objects in the sky, such as nebulae and very dim stars, will only be visible after astronomical twilight ends. Also, if you intend to measure how dark the sky is using a Sky Quality Metre, say, it is important to wait till after astronomical twilight.

So how much daytime does this twilight add? Only during civil twilight can the sky really be considered “light”, and so we can say that the day begins (at the point we would call dawn) at the start of morning civil twilight, and ends, (at the point we would call dusk) at the end of evening civil twilight. This means that on the Spring Equinox, the “day” will be around 13 hours and 15 minutes long in the UK (depending on where you are).

The date at which the “day” (including dawn and dusk) is 12 hours long in the UK (and therefore when we had equal amounts of daytime and nighttime) occurs around 01 or 02 March, over two weeks before the so-called equinox!

To find out your local sunrise / sunset / twilight times visit www.timeanddate.com

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