Winter Solstice 2011

The northern hemisphere winter solstice occurs on 22 December 2011, at 0530 GMT At this point the Earth’s north pole will be tipped away from the Sun. As seen from Earth, the Sun will stop its slow daily decent south in our sky – over the past six months the Sun’s mid-day height above the horizon has been decreasing steadily – and once again turn north, getting higher in the sky at noon each day, until it gets to its highest point in midsummer 2012.

The actual day of the winter solstice – in this case 22 December 2011 – is commonly known as midwinter, the shortest day, and is the day when the Sun spends least time above the horizon. The further north of the equator you are, the more profound the effect. Indeed if you live within the arctic circle the Sun won’t actually rise today.

I’m not that far north, but by most standards I’m pretty far north, in Orkney delivering a midwinter astronomy festival. Orkney sits between 58°41′and 59°24′ North, and on midwinters day the Sun rises around 0905 and sets around 1515, and only spends 6h10m above the horizon. The winter nights in Orkney are long and dark.

But the morning after midwinter, the days will be lengthening. For many cultures then, midwinter symbolised the rebirth of the year, and ancient peoples often built monuments to celebrate the returning of the light.

And people in neolithic Orkney built some of the most incredible midwinter monuments that still exist. I’ll be spending this afternoon inside the 4700 year old chambered cairn at Maeshowe, built so that the passageway – which one has to crawl through to get into the inner chamber – points directly towards sunset on the shortest day.

Maes Howe sections

Given clear skies, the last rays of midwinter sunlight stream into the burial chamber for a few moments before the sun sets.

Maeshowe midwinter sun, Charles Tait

The Orkney poet George Mackay Brown said of midwinter at Maeshowe:

The most exciting thing in Orkney, perhaps in Scotland, is going to happen this afternoon at sunset, in few other places even in Orkney can you see the wide hemisphere of sky in all its plenitude.

The winter sun just hangs over the ridge of the Coolags. Its setting will seal the shortest day of the year, the winter solstice. At this season the sun is a pale wick between two gulfs of darkness. Surely there could be no darker place in the be-wintered world than the interior of Maeshowe.

One of the light rays is caught in this stone web of death. Through the long corridor it has found its way; it splashes the far wall of the chamber. The illumination lasts a few minutes, then is quenched

Winter after winter I never cease to wonder at the way primitive man arranged, in hewn stone, such powerful symbolism.

Geminids Meteor Shower 2011

The best view of the Geminids Meteor Shower this year will be after midnight on the nights of 13/14 and 14/15 December, however the hourly rate of meteors will be drastically reduced due to the presence of a waning gibbous Moon which will be high in the sky at the peak meteor times, and right next to the part of the sky that the Geminid meteors will appear to radiate from (called the radiant).

A Geminid Meteor (image credit Wally Pacholka / AstroPics.com / TWAN)

The light from the Moon will drown out all but the brightest meteors, and so rather than seeing 80+ meteors per hour (under cloudless skies, free of light pollution) that rate may be reduced to only a few an hour. This still represents an increase in the normal background rate of meteors, but is far from the spectacular display usually seen during this reliable performer.

To maximise your chances of seeing meteors you should plan to spend a few hours outside after midnight. A reclining lawn chair is ideal, allowing you to observe in comfort, and wrapping yourself in blankets should stave off much of the midwinter chill.

You should face your lawn chair away from the Moon, so you are not in its direct glare. Although the radiant of the shower will be behind you if you do this, the meteors will streak all over the sky, and so there is no real harm in facing away.

More information about the Geminids meteor shower can be found at the Meteorwatch website.

Leonids Meteor Shower 2011

On the night of 17/18 November 2011 the Leonids meteor shower reaches its peak. This annual performer is associated with Comet Temple-Tuttle, which orbits the Sun once every 33 years leaving a trail of debris as it goes. Once a year the Earth passes through this trail, and we see a meteor shower.

Leonids 2001

This year’s Leonids shower is hampered by the last quarter Moon which sits just to the right of the radiant of the Leonids, in Leo. Despite this there is good reason to observe the shower this year, as the International Meteor Organisation suggest there might be as many as three peaks of activity.

Throughout November the rate of Leonids will increase, with the main peak occurring at 0340 GMT on 18 November, at which time the Zenith Hourly Rate may be 20+. For observers in the UK, observing under cloudless skies, away from light pollution, this translates as an hourly rate of ~14, but the Moon will interfere and reduce this value somewhat. Two other peaks may also occur, at ~2100 on November 17, and at ~2300 on 18 November, with similar rates. This means that both the nights of 17/18 and 18/19 November may offer good opportunites to observe this shower.

The Leonids has the distinction of being the most dramatic meteor shower that I’ve ever seen, as I observed the Leonid meteor storms every year from 1998 to 2002, when we saw hundreds of meteors each night at the peak of the shower. These storm peaks are predictable, and occur every 33 years, associated with the pass of comet Temple Tuttle, as it refreshes the trail of debris that cause the meteors. The next pass of Temple Tuttle is due 2031, so we’ve a long wait for the next storm.

Interestingly, the Leonid storm of 1833 was truly stunning, with rates estimated to be around 100,000 per hour across North America.

To view the Leonids, find a dark spot, away from light pollution, sit on a reclining deck chair facing as large an area of the sky as you can manage, wrap yourself in a blanket, and enjoy the view. For observers in the UK the meteor shower radiant will rise around 2200 GMT on 17 November and will be high in the SE by 0400 on 18 November.

If you want to make observations of the Leonids that might help scientists better understand the shower, you can do so via the Society of Popular Astronomy, or the British Astronomical Association. Lots more info can be found at the Meteorwatch website.

Perseids: Last Chance to See?

Tonight (9/10 August 2011), between Moonset and the start of twilight, might present the best opportunity to view the Perseids meteor shower, at around 0200, although the peak of the shower doesn’t occur until the night of 12/13 August 2011.

The International Meteor Organisation has been recording the increasing ZHR (activity level) of this year’s Perseids meteor shower, and it currently stands at ZHR=20. It is expected to increase over the next four nights to ZHR=50-200 at the peak on 12/13 August.

However, tonight is the last night of the shower where the Moon will be absent from the sky for part of the night, setting before 0200. After the Moon sets there will be a period of time before twilight begins where the sky will be free of any natural light pollution, so assuming you can get away from man-made light pollution you’ll maximise your chances of seeing meteors.

How long you will have under dark skies will depend on where in the UK you are, with observers in the far south of England having until 0330, while those in southern Scotland having until 0230. Observers north of central Scotland will not experience truly dark skies for several days yet. (To find your local Moonset and twilight times visit timeanddate.com)

If you can get somewhere very far from light pollution (like Galloway Forest Dark Sky Park, or Exmoor National Park) then you’ll have the best chance of seeing lots of meteors tonight. Let’s take these locations as demonstrations of how the actual hourly rate will vary between tonight and the peak at the weekend.

Observing Night	Hours of true darkness (Glentrool)	Hours of true darkness (Exmoor)	Limiting Mag (due to Moon)	ZHR (estimate)	Actual Hourly Rate
09/10 August	0148-0225	0207-0322	6.5	25	22
10/11 August	none (due to Moon)	none (due to Moon)	4.5	30	7
11/12 August	none (due to Moon)	none (due to Moon)	4.0	35	5
12/13 August	none (due to Moon)	none (due to Moon)	3.5 (Full Moon)	100 (peak)	11* (peak)

*The peak ZHR has been taken here to be 100, although it may be as much as twice this, meaning that even under a Full Moon you’ll see the same at the peak as you will under dark skies tonight.

So if you can get somewhere truly dark tonight between Moonset and the start of astronomical twilight (and you might get less than an hour of these perfect conditions) then your Actual Hourly Rate tonight may be better than that during the peak, due to the Full Moon then.

For those observing in towns and cities this won’t be such a big issue, as man-made light pollution will impose a limiting mag on the sky tonight, which will reduce tonight’s Actual Hourly Rate rate to ~3, compared to ~11 at the peak.

So if you get the chance head out tonight, and indeed every night this week, to look for Perseids in the wee small hours. This is the most reliable shower of the year, and one that even the Full Moon cannot ruin completely!

Perseids Meteor Shower 2011

This month sees the most reliable meteor shower of the year; the Perseids. You can begin watching for Perseid meteors now, and the shower will last until mid-August, but the peak of the shower occurs in the small hours of Saturday 13 August 2011.

Perseus under dark skies

Perseus under moonlit skies

Unfortunately this year’s shower will be obscured by the full Moon which occurs on the same day, and so it won’t present its usual excellent display.

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. Observing from Glasgow, as I will be, the radiant will be around 60° above the eastern horizon at 2am. (In the far south of the UK it will be a few degrees lower, and in the far north a couple of degrees higher).

Assuming a clear night, the other factor is the limiting magnitude of the sky, a measure of the faintest object you can see. Even if the Moon were not in the sky, man-made light pollution would be an issue for most people. From my garden the limiting magnitude of the sky is ~4.5 (around 500 stars visible), so I 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.

However a full Moon introduces natural light pollution that can be as bad the man-made glare in a city centre, with a limiting magnitude of ~3 (it is hard to estimate what the limiting magnitude will be exactly, but this is a decent estimate).

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 2011.

ZHR = 100, say (might be as low as 50 or as high as 200, so our final answer might be out by a factor of two in either direction)

h = 60°

k = 0 (let’s hope!)

m = 3 (for the full Moon)

So your actual hourly rate under clear skies is

(100 x sin(60))/((1/(1-0) x 2^(6.5-3) = 7.7, or 8 meteors per hour. This might be out by a factor of two, so you might see as few as 4 per hour, or as many as 16 per hour.

If the full Moon wasn’t present we might expect somewhere around 80 meteors per hour.

So, this might be a poor show compared to moonless Perseids displays, but you will still still see plenty of shooting stars if you’re out for a few hours around the peak time (between 2200 UT* on 12 August and 0300 UT* on 13 August 2011).

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.

You can keep track of the increasing ZHR at the International Meteor Organisation website.

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

Scots in Space

I was delighted to hear that two groups from Glasgow were winners in last night’s UK Space Conference‘s Arthur Clarke Awards 2011.
Clyde Space, a “leading supplier of small and micro spacecraft systems”, was given the Arthur Clarke Award 2011 for Achievement in Space Commerce, while the University of Strathclyde’s Advanced Space Concepts Laboratory, which “undertakes frontier research on visionary space systems”, was given the Arthur Clarke Award 2011 for Achievement in Space Research.

Congratulations to both, and it’s exciting to me as a Scot and a resident of Glasgow that these two groups, located within 5 miles of one another, are leading the UK in space research and commerce.

Aphelion 2011

At 1600 BST (1500 GMT) on 4 July 2011 the Earth will be at aphelion, its furthest from the Sun this year.

The Sun

If that sounds confusing to you, and has you wondering why the Earth is at its furthest from the Sun in Summer, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s winter 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 (its closest approach to the Sun, which occurred on 3 January 2011) Earth is 0.983AU from the Sun, while during aphelion 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.

Earth

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 northern hemisphere midsummer (21 Jun) almost coincides with aphelion (4 Jul) is simply that; a coincidence. Given that fact, there is no reason to be surprised that aphelion occurs so close to northern hemisphere midsummer: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.

To take this explanation even further, we can calculate how much variation in incident sunlight there would be in two scenarios: 1. an imaginary scenario where the seasonal variations in temperature are due to the tilt of the Earth’s axis but where the Earth goes round the Sun in a perfectly circular orbit and 2. an imaginary scenario where the Earth’s axis isn’t tilted, but where its orbit is elliptical in the same degree as ours actually is.

1. The Sun appears at its highest point in our sky each day at noon. The highest it ever gets is at noon on midsummer. The lowest noontime altitude occurs at noon on midwinter. In Scotland the Sun is around 55 degrees above the horizon at noon on midsummer, and only 10 degrees above it at noon on midwinter. The amount of energy from the Sun radiant on a fixed area is proportional to the sine of the altitude, so the ratio of the solar energy radiant on a square metre of Scotland between midsummer and midwinter is

sin(55) / sin(10) = 4.72

So here in Scotland we get 372% more energy from the Sun in summer than we do in winter, due to the tilt of the Earth’s axis.

2. If the Earth’s axis was not tilted, then we would only experience temperature differences from the Sun depending on how far or near we are from it. In this case, the amount of energy from the Sun radiant of a fixed area is proportional to the square of the distance from the Sun, so the ratio of the solar energy radiant on a square metre of Scotland between perihelion and aphelion is

(1.017/0.983)^2 = 1.07

So we get 7% more energy from the Sun at perihelion than we do at aphelion, due to the differing distances to the Sun. From this you can see that, while the distance to the Sun has some effect on how much heat we receive, it is a very small effect compared to that produced by our axial tilt.

Summer Solstice 2011

The northern hemisphere summer solstice occurs today, 21 June 2011 at 1816 BST (1716 UT).

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 tipped most directly towards the Sun.

And this happens at exactly 1816 BST on 21 June 2011.

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 17h35m15s today, a full 6 seconds longer than yesterday, and two seconds longer than tomorrow.

The Lowest Full Moon of the Year

Tonight (actually around 0130 tomorrow morning) the Full Moon will reach its highest point due south, just an hour and a half after the eclipse ends. Despite being at its highest in the sky, you’ll still struggle to see it, as it is very low down. In fact the Full Moon nearest the Summer Solstice is the lowest Full Moon of the Year.

Full Moon by Luc Viatour http://www.lucnix.be

First, let’s begin with the definition of “Full Moon”. A Full Moon occurs when the Moon is diametrically opposite the Sun, as seen from the Earth. In this configuration, the entire lit hemisphere of the Moon’s surface is visible from Earth, which is what makes it “Full”. There is an actual instant of the exactly Full Moon, that is the exact instant that the Moon is directly opposite the Sun. Therefore when you see timings listed for the Full Moon they will usually include the exact time (hh:mm) that the Moon is 180° round from the Sun (we call this point opposition).

Here’s a list of the times of all Full Moons between June 2011 and June 2012:

Month	Date of Full Moon	Time of Full Moon (UT)
June 2011	15 June	2014*
July 2011	15 July	0640*
August 2011	13 August	1857*
September 2011	12 September	0927*
October 2011	12 October	0206*
November 2011	10 November	2016
December 2011	10 December	1436
January 2012	09 January	0730
February 2012	07 February	2154
March 2012	08 March	0939
April 2012	06 April	1919*
May 2012	06 May	0335*
June 2012	04 June	1112*

* UK observers should add on one hour for BST

As you can see from this table, the instant of the Full Moon can occur at any time of day, even in the daytime when the Moon is below the horizon. So most often when we see a “Full Moon” in the sky it is not exactly full, it is a little bit less than full, being a few hours ahead or behind the instant of the Full Moon. I’ll refer to this with “” marks, to distinguish this from the instant of the Full Moon (they look virtually identical in the sky).

The Moon rises and sets, like the Sun does, rising towards the east and setting towards the west, reaching its highest point due south around midnight (although not exactly at midnight, just like the Sun does not usually reach its highest point exactly at noon). And like with the Sun the maximum distance above the horizon of the “Full Moon” varies over the year.

The Sun is at its highest due south around noon on the Summer Solstice (20 or 21 June) and at its lowest due south around noon on the Winter Solstice (21 or 22 Dec) (of course the Sun is often lower than this, as it rises and sets, but we’re talking here about the lowest high point at mid-day, i.e. the day of the year in which, when the Sun is at its highest point that day, that height is lowest…)

And because Full Moons occur when the Moon is directly opposite the Sun, you can imagine the Moon and Sun as sitting on either sides of a celestial see-saw: on the day when the Sun is highest in the middle of the day (in Summer), the Moon is at its lowest high point at midnight; and on the day when the Sun is at its lowest high point in the middle of the day (in Winter), the Moon is at its highest high point at midnight.

This means, in practical terms, that Summer “Full Moons” are always very low on the horizon, while Winter “Full Moons” can be very high overhead.

Here’s a table of the altitude of the “Full Moon” when due south. Remember the times in this table don’t match the exact time of the Full Moon, but instead have been chosen as the closest in time to that instant, and so have be labelled “Full Moon” (in quotes).

Month	Date of Full Moon	Time of Full Moon (UT)	Time/Date of “Full Moon” due S	Time from/since instant of Full Moon	Altitude due S (degrees)**
June 2011	15 June	2014*	0127BST 16 June 2011	+4h13m	10° 05′
July 2011	15 July	0640*	0012BST 15 July 2011	-7h28m	10° 24′
August 2011	13 August	1857*	0126BST 14 August 2011	+5h27m	19° 19′
September 2011	12 September	0927*	0049BST 12 September 2011	-9h38m	31° 49′
October 2011	12 October	0206*	0053BST 12 October 2011	-1h13m	44° 16′
November 2011	10 November	2016	0005GMT 11 November 2011	-3h49m	53° 24′
December 2011	10 December	1436	0030GMT 11 December 2011	+9h54m	56° 03′
January 2012	09 January	0730	0006GMT 09 January 2012	-7h24m	53° 36′
February 2012	07 February	2154	0031GMT 08 February 2012	+2h37m	43° 47′
March 2012	08 March	0939	0000GMT 08 March 2012	-9h39m	35° 37′
April 2012	06 April	1919*	0145BST 07 April 2012	+5h26m	21° 45′
May 2012	06 May	0335*	0102BST 06 May 2012	-3h33m	15° 20′
June 2012	04 June	1112*	0047BST 04 June 2012	-11h25m	11° 49′

* UK observers should add on one hour for BST
** The altitude here is based on my observing location in Glasgow, Scotland. You can find out how to work out how high these altitudes are here.

As you can see from this table, the highest “Full Moon” due S this year occurs at 0030 on 11 December 2011, when the Moon will be over 56° above the southern horizon (approximately the height of the midsummer mid-day Sun which culminates at 57°34′).

Compare this to the “Full Moon” this month, just after the eclipse, in the morning of 16 June, when the Moon barely grazes 10° above the horizon, and you can see just how low the midsummer Full Moon can be.

In fact the closeness of summer “Full Moons” to the horizon means that this is an ideal time of year to try and observe the Moon Illusion.

UPDATE: Here’s a very cool speeded up video of the Moon cycling through its phases, as see by the LRO spacecraft:

Happy Spring Equilux

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

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!