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The Moon Illusion

Originally posted on Dark Sky Diary by Sreve Owens @Darkskyman

I’ll make a prediction: on or around 19 March, when the so-called “Supermoon” occurs, at its closest approach to Earth in two decades, people will indeed report that the Moon looks much bigger than normal. But it won’t really be much bigger in the sky at all. It’s all in our heads!


"Wow, the Moon's even bigger than that tree!"

You’ve probably all seen it before, a huge Full Moon sitting on the horizon. Time and again I have had people ask me why the Moon is so much bigger some times than others, and the answer is: it isn’t, really.

The Moon orbits the Earth in an elliptical orbit, meaning that it is not always the same distance from the Earth. The closest the Moon ever gets to Earth (called apogee) is 364,000km, and the furthest is ever gets (perigee) is around 406,000km (these figures vary, and in fact this Full Moon on 19 March 2011 will see a slightly closer approach of 357,000km).

So the percentage difference in distance between the average perigee and the average apogee is ~10%. That is, if the Full Moon occurs at perigee it can be up to 10% closer (and therefore larger) than if it occurred at apogee.

This is quite a significant difference, and so it is worth pointing out that the Moon does appear to be different sizes at different times throughout the year.

But that’s NOT what causes the Moon to look huge on the horizon. Such a measly 10% difference in size cannot account for the fact that people describe the Moon as “huge” when they see it low on the horizon.

What’s really causing the Moon to look huge on such occasions is the circuitry in your brain. It’s an optical illusion, so well known that it has its own name: the Moon Illusion.

If you measure the angular size of the Full Moon in the sky it varies between 36 arc minutes (0.6 degrees) at perigee, and 30 arc minutes (0.5 degrees) at apogee, but this difference will occur within a number of lunar orbits (months), not over the course of the night as the Moon rises. In fact if you measure the angular size of the Full Moon just after it rises, when it’s near the horizon, and then again hours later once it’s high in the sky, these two numbers are identical: it doesn’t change size at all.

So why does your brain think it has? There’s no clear consensus on this, but the two most reasonable explanations are as follows:

1. When the Moon is low on the horizon there are lots of objects (hills, houses, trees etc) against which you can compare its size. When it’s high in the sky it’s there in isolation. This might create something akin to the Ebbinghaus Illusion, where identically sized objects appear to be different sizes when placed in different surroundings.

Ebbinghaus Illusion 

Ebbinghaus Illusion – the two orange circles are exactly the same size

2. When seen against nearer foreground objects which we know to be far away from us, our brain thinks something like this: “wow, that Moon is even further than those trees, and they’re really far away. And despite how far away it is, it still looks pretty big. That must mean the Moon is huge!”.

These two factors combine to fool our brains into “seeing” a larger Moon when it’s near the horizon compared with when its overhead, even when our eyes – and our instruments – see it as exactly the same size.

Supermoon Nonsense

Originally posted on Dark Sky Diary by Steve Owens @darkskyman

There seems to be a growing excitement about the “Supermoon” that is due to occur on 19 March 2011, when the Moon will be at its closest to Earth in this orbit, and closer than it has been at any time since 1992.


Moon – not Super

The Moon orbits the Earth in an elliptical orbit, i.e. it is not perfectly circular, and so in each orbit there is a closest approach, called “perigee” and a furthest approach, called “apogee”.

At this month’s perigee the Moon will be 356,577km away from Earth, and will indeed be at its closest in almost 20 years. But how close is it compared with other perigees?

Let’s start by comparing it to the Moon’s average distance from the Earth, which is ~385,000km. This perigee will be ~8% closer to the Earth than average. OK, that’s a bit closer, but not significantly so.

What about comparing it to the Moon’s average perigee distance, which is ~364,000km. So this “Supermoon” will be ~2% closer to the Earth than it is most months at perigee. Wow!

So what will this mean to you? Nothing at all. The Moon will be a few percent bigger in the sky, but your eye won’t really be able to tell the difference. It will also be a few percent brighter, but your eye will compensate for this too, so altogether this “Supermoon” will look exactly the same as it always does when it’s full.

As to all of those soothsayers claiming that there will be earthquakes and tidal waves. There very well might be, but they’ll be nothing at all to do with the Moon.

UPDATE: I predict that lots of people will report having seen a huge Moon on or around 19 March

How To See NanoSail-D From Your Own Backyard

Originally Posted on Universe Today by meteorwatch (@VirtualAstro)


Artist concept of Nanosail-D in Earth orbit. Credit: NASA

The night sky has many wonderful objects to look at on a clear evening, including many man-made satellites, and the always impressive International Space Station (ISS). Now there’s a new addition to these artificial delights: the first ever solar sail to orbit the Earth, NASA’s Nanaosail-D Satellite. Want to know how you can see it?

The 10m x 10m reflective sail is designed to act like a brake and gradually create drag in the upper atmosphere, slowly pulling a satellite down and de-orbiting it at the end of its working life. Nanosail-D is testing the potential of this technology to reduce space junk and debris.


NanoSail D. Image credit NASA

The satellite has a huge reflective sail and could potentially be many times brighter than the planet Venus when it catches a glint from the Sun. Unlike the International Space Station (ISS) and other satellites, the sail will not be visible when it is directly above us as we will be looking at it edge on, It will be more visible when closer to the horizon.

The Nanosail-D satellite will be visible from now and for the next few months. To see it you will need to know exactly when it will be visible from your location. To do this, go to heavens-above.com or spaceweather.com where star charts with times and pass details will be displayed after you enter your observing site.

Once you know the time and location in the sky of the pass of the satellite, make sure you are able to get a good view of the horizon, or part of the sky where the satellite due to appear. Give yourself plenty of time, go outside and get ready. I always set a 30 second reminder on my watch or cell phone, so I don’t have to fumble around or guess the time.

Unlike the ISS and most other satellites, Nanosail-D passes may only last a few, or a few tens of seconds, so make sure you are looking in the right place at the right time. You will see an amazingly bright star-like object rise up, get brighter and then suddenly disappear. When it “disappears” it is still passing over, it’s just no longer at the right angle or is no longer being illuminated by the sun. NanoSail-D has few reflective surfaces compared to many on the ISS.

To enjoy the Nanosail-D passes:

• Make sure you know the right place in the sky and the time of the pass, by checking on the web.
• Make sure you will be able to get a clear view of it from your viewing location.
• Set an alarm or get ready for the pass as it only lasts a few seconds.
• NASA expects NanoSail-D to stay in orbit until April or May 2011.
• If you are an astrophotographer, don’t forget, NASA and SpaceWeather.com are having an imaging contest of NanoSail-D. Find out more here.
• Most of all, get your friends and family outside with you to watch Nanosail-D and enjoy!

GLOBE at Night 2011

Originally posted on Dark Sky Diary by Steve Owens @darkskyman

Over two weeks between 21 February and 06 March 2011 you can take part in the international project GLOBE at Night, an annual 2-week campaign. During GLOBE at Night, you can record the brightness of your night sky by matching its appearance against the constellation Orion with star maps of progressively fainter stars. Your measurements are submitted on-line and a few weeks later, organizers will release a map of light-pollution levels worldwide. Over the last five GLOBE at Night campaigns, volunteers from over 100 countries have contributed 35,000 measurements.

So what exactly do you need to do? It’s simple, just visit the GLOBE at Night website and follow their instructions. You’ll need to:

1. print out the Orion maps from the activity pack

2. go outside and find Orion. Make sure you wait till it’s properly dark, which is after about 2015 your time. Hint, Orion will be towards the south, and looks like this:

The constellation of Orion, magnitude 4
The constellation of Orion

3. compare what you see with the seven Orion maps, and note down which one it looks like the most. This will tell you the magnitude (brightness) of the dimmest stars you can see

4. report your observation using the GLOBE at Night web app or through their website including the date and time you made the observation, and your latitude and longitude (don’t worry, there’s an interactive map that’ll help you find these).

Sark: The World’s First Dark Sky Island

Originally posted on Dark Sky Diaries by Steve Owens (@darkskyman on Twitter)

The Channel Island of Sark has been recognised for the quality of its night sky by the International Dark-sky Association (IDA), who have designated it the world’s first dark sky island, the latest in a select group of dark sky places around the world.

Sark has no public street lighting, there are no paved roads and cars, so it does not suffer from the effects light pollution in the same way as towns and cities do. This means that the night sky is very dark, with the Milky Way stretching from horizon to horizon, meteors streaking overhead, and thousand of stars on display.

Caption: “Stargazers on Sark enjoy the wonder of the Milky Way”. Image Credit: Martin Morgan-Taylor

The announcement was hailed as a great success by astronomers. Prof Roger Davies, president of the Royal Astronomical Society, said: “This is a great achievement for Sark. People around the world are become increasingly fascinated by astronomy as we discover more about our universe, and the creation of the world’s first dark sky island in the British Isles can only help to increase that appetite. I hope this leads to many more people experiencing the wonders of a truly dark sky”.

The award follows a long process of community consultation, which included the assessment of the sky darkness and an audit of all the external lights on Sark. A comprehensive lighting management plan was created by lighting Jim Patterson of the Institute of Lighting Engineers, and many local residents and businesses have altered their lighting to make them more dark sky friendly, ensuring that as little light as possible spills upwards where it can drown out the starlight.

Caption: “The Milky Way above the Seigneur’s Mill on Sark”. Image Credit: Martin Morgan-Taylor

The government of Sark, the Chief Pleas, were supportive from the start. Conseilleur Paul Williams, chair of the Agriculture Committee, which oversees environmental matters, said: “Sark becoming the world’s first dark sky island is a tremendous feather in our environmental cap, which can only enhance our appeal. Sark is a wonderful island and this recognition will bring our uniqueness and beauty to a wider audience.”

This designation means that Sark joins the select group of international sites chosen for their dark skies, including Galloway Forest Dark Sky Park, which became Europe’s first International Dark Sky Park in November 2009.

Steve Owens, the dark sky development officer who led Sark’s application to the IDA, recognises the benefits that this might have for the community on Sark: “This is an ideal opportunity to bring stargazers to the island throughout the year, and I think that Sark is about to see a boom in astro-tourism, especially in the winter months. We’ve seen a surge of public interest in astronomy in recent years, with the International Year of Astronomy in 2009 and more recently with the success of BBC Stargazing Live, and it’s great that places like Sark and Galloway Forest Dark Sky Park are allowing people from towns and cities to come and experience a dark sky”.


Sark Tourism: http://sark.info/

International Dark-sky association: http://www.darksky.org/

Campaign for Dark Skies: http://www.britastro.org/dark-skies/

Star Counting

The Campaign to Protect Rural England (CPRE), in conjunction with the British Astronomical Association‘s Campaign for Dark Skies, has recently announced their 2011 Star Count Project.

Star Count Week 2011 (from CPRE website)

Star Count Week (Monday 31 January – Sunday 06 February 2011) aims to get you outside and looking up, specifically to assess how dark – or light – your sky is.

The technique is simple. 1. Find Orion. 2. Count all the stars you can see within the main rectangle formed by Betelgeuse, Bellatrix, Rigel and Saiph, the four stars that make up Orion’s shoulders and feet. (Don’t count the three bright belt stars). 3. Tell the CPRE.

That’s it. By counting how many you can see, astronomers can calculate your sky’s limiting magnitude, or the brightness of the faintest stars you can see. It’s a very simple – and rewarding – project to take part in.

There are other annual star count programmes, such as GLOBE at Night (March 22 – April 4 2011) which I blogged about during their 2010 event. You can also get more involved and conduct a detailed dark sky survey, or take part in local activities such as the Peak District National Park’s Orion in the Peak project

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