Posts Tagged ‘Bortle Scale’

Zodiacal Light

March 26, 2012 Leave a comment

Spring in the northern hemisphere is the best time to view the elusive, faint astronomical phenomenon known as Zodiacal Light.

Daniel López, IAC Zodiacal Light on the left and (false colour) Milky Way on the right

This light – literally “light from the zodiac” – appears only just at the end of evening twilight or just before morning twilight, and is seen as a cone of very faint light stretching up from the horizon, narrowing as it does so, following an imaginary line in the sky known as the zodiac, or to give it its more astronomically correct name, the ecliptic.

The angle which this line makes with the horizon varies throughout the year, and the steeper the angle the more evident the zodiacal light will be. The steepest angle for observers in the northern hemisphere occurs in the evening sky in March and April, or the morning sky in October and November.

How best to see Zodiacal Light

You will need to be as far as possible from any sources of light pollution. In fact the Zodiacal Light is one of the benchmarks of the Bortle Dark Sky Scale, which says that it is only visible in skies with Bortle Class 5 or better, and even in suburban/rural transition sites it is not striking. Under rural skies it is “striking”, while in a truly dark sky site it is bright enough to cast a shadow. Under exceptionally dark skies it might appear as a band stretching from horizon to horizon.

Once you’ve found your dark sky site, you need to find somewhere with as clear a western horizon as possible, and wait until the end of evening astronomical twilight (assuming you’re viewing it in the spring – if it’s the autumn then you need to start observing before the start of morning astronomical twilight). You can find your twilight times using or the excellent Velaclock app for smartphones. As a general rule, for observers in the UK, you need to wait until two hours after sunset before you skies get dark enough to see this elusive light. But wait too long and the bulk of the cone of light may have set, so sunset+2hours is really the perfect time.

What Zodiacal Light looks like

As mentioned above, Zodiacal Light is a faint grey cone of light stretching up from the horizon. The darker your observing site the larger this cone will appear, and the higher into the sky it will stretch. From the very darkest sites on Earth it can stretch overhead and down to the far horizon.

What’s Zodiacal Light made of?

Zodiacal Light is sunlight reflecting off particles of dust and rock orbiting the Sun. This dust is in a lens-shaped cloud with the Sun at the centre, and the cloud lies in the same plane as the planets in the solar system (which is why it’s visible along the ecliptic). The particles in the Zodiacal Light are around 0.15mm in diameter (some smaller, some a little bigger) and probably come from shattered comets and asteroids.

Photographing the Zodiacal Light

As tricky as it is to see with your naked eyes, it’s even harder to catch on camera. Harald Edens has a great page about how best to photograph it.


The Bortle Scale: A Flow Chart

January 19, 2012 1 comment

The Bortle Scale is a useful way of estimating your sky brightness, i.e. to what extent light pollution affects your view of the night sky. By going outside on a clear moonless night and recording what astronomical objects you can see you can assign a Bortle Class rating to your observing site.

I have used the Bortle Scale to assess night sky quality many times, and always felt the lack of a handy flow chart to lead me through it. So I made one. Enjoy. (You can also download the pdf version.)

PS The content of this chart assumes some prior knowledge of astronomy, but any of the terms used are easily google-able.

Dark Sky Bucket List: Part 1

December 8, 2011 3 comments

Astronomy tourism is a burgeoning field – luckily for me, as it’s how I make a living! – with city-folk now starting to make the effort to travel to places with darker skies, in the hope of seeing things that simply cannot be seen from the orange glare of a town sky.

Stargazers at an astronomy tourism event in Sark (image credit: Martin Taylor)

Galloway Forest Dark Sky Park, which I helped establish in 2009, has seen a lot of this dark sky tourism trade, due to its high profile in the media, but many other places around the country – Sark in the Channel Islands, and Exmoor National Park to name but two – have followed suit, hoping to attract the stargazing crowds.

And as more and more people come out of the city for a stargazing weekend under dark skies it’s becoming clear to me that there’s a real appetite for seeing the Milky Way and other elusive dark sky objects. It’s almost become something to put on your “bucket list”: see a wild lion in Africa; fly in a hot air balloon; see the Milky Way.

However there are lots of other amazing astronomical objects visible under a dark sky so here is the first installment of my top-ten “Dark Sky Bucket List”.

1. Milky Way

Our “island universe”, a spiral disk of hundreds of billions of stars, appears in our night sky as a band of grey light arching overhead. From town centres the Milky Way is invisible, lost in the orange glow, but as you move out into suburbia you might start to see traces of it, looking directly overhead. According to the Bortle Dark Sky Scale, under suburban skies (Bortle Class 5) “[the] Milky Way is very weak or invisible near the horizon and looks washed out overhead”, while under rural skies (Bortle Class 3) “[the] Milky Way… appears complex” with structure – dark clumps and bands – set within the grey light. Under perfect conditions (that probably aren’t found anywhere in Europe) of Bortle Class 1, “[the] Scorpius and Sagittarius regions of the Milky Way cast obvious shadows on the ground”. The best we can hope for in the UK – which you might just get in the darkest heart of Galloway Forest Dark Sky Park – is Bortle Class 2 which reveals “[a] highly structured… Milky Way”.

The Milky Way above Natutal Bridges National Monument (Bortle Class 2)

2. Countless stars

Although our Milky Way galaxy contains hundreds of billions of stars, only the closest ones to the Sun are visible as distinct dots in our night sky. From a city sky you would be lucky to see a couple of hundred stars on any clear night – the brightest ones that make up the constellation patterns – but move to somewhere darker and more stars will be visible. Once you get to very dark skies the only limit on how many stars you can see is your eyesight. Assuming you’ve got good eyesight thousands of stars will be visible under a dark sky. The Yale Bright Star Catalogue lists all stars brighter than magnitude 6.5, the approximate limit of human vision, and that list contains over 9000 stars, so if you’re at a dark site (Bortle Class 3 or better) you’ll see around 4500 stars (as you can only see half of the sky at one time).

3. Andromeda Galaxy, M31

The Andromeda Galaxy is our nearest BIG neighbouring galaxy. It lies 2.5 million light years from the Milky Way, and contains even more stars than our galaxy. Even still, it appears as only a small grey smudge in the night sky. From bright city skies (Bortle Class 8) the Andromeda Galaxy is “barely glimpsed by an experienced observer on good nights”, while in bright suburban skies (Bortle Class 6) it is “modestly apparent to the unaided eye”. As you head to darker skies though, Andromeda becomes even more apparent, until under rural skies (Bortle Class 3 or better) you will see it as a very obvious smudge amongst the clamour of stars. Andromeda is relatively easy to find using the constellations Cassiopeia and Pegasus as signposts. Using the W-shaped Cassiopeia, take the right-hand V of the W shape as an arrow that points to the bright star Mirach, or β Andromedae, which sits in the middle of a slightly curved line of three stars, with Alpheratz (α Andromedae) on one side and Almach (γ And) on the other. Go perpendicularly “up” (back towards Cassiopeia) from Mirach to a much fainter star u Andromedae, and then continue the same distance again to find the Andromeda Galaxy.

A Star Chart for finding the Andromeda Galaxy

4. Triangulum Galaxy, M33

Far more elusive than the Andromeda Galaxy is the Triangulum Galaxy, M33. Indeed it is so elusive under anything except near-perfect dark skies that it is used as the benchmark for judging how dark your sky is. It is only visible to the naked eye under Bortle Class 4 or better. At the suburban/rural transition (Bortle Class 4) it is “a difficult averted vision object, only visible when higher than 55° “. Under rural skies (Bortle Class 3) it is “easily visible with averted vision”, but it’s only at truly dark sky sites (Bortle Class 2) that it becomes “easily seen with naked eye”, and in excellent dark sky sites (Bortle Class 1) it is a “direct vision naked-eye object”. It is located near the Andromeda Galaxy, in the opposite direction from Mirach.

A Star Chart for finding the Triangulum Galaxy, M33

5. Orion Nebula, M42

The Orion Nebula is one of the brighter nebulae in the sky, and therefore is seen relatively easily. It lies in the middle of Orion’s sword. Under suburban skies it will be visible to the naked eye, but take binoculars out under rural skies (Bortle class 4 or better) and you’ll be treated to a stunning view of this beautiful object. The Orion Nebula is a region of star birth, located only 1300 light years away.

The Orion Nebula

Coming in part two! Meteor Showers, Northern Lights, Zodiacal Light, Gegenschein, and The Green Flash.

Have you been lucky enough to see any of these objects under a dark sky? What would be one your dark sky bucket list?

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