The key lies in the Earth’s atmosphere. This scatters light, and so, given sufficient depth, is quite capable of revealing the boundary between day and night (technically, the “terminator“). The Earth’s shadow can be seen rising in the east, shortly after sunset — or descending in the west, shortly before sunrise. The sky must be clear, and a low horizon also helps. These conditions are not at all uncommon. Although most noticeable when away from towns and cities (flat countryside, or vantage points high in the mountains are particularly good), you can pick out the Earth’s shadow frequently once you’ve learned to recognise it. It was from the Refuge de la Vallete, in the Vanoise Alps, that I last photographed the shadow:

The Earth's shadow — descending in the west, at sunrise.

The dark line just above the mountains (and dipping behind the twin peak on the left) is the terminator, the division between night and day. The full moon is visible at the top of the photo. The colouring in the sky at this, the start or end of twilight, is quite subtle, beautiful, and subject to surprisingly rapid change.

The geometry of what’s going on here is much more easily explained with a diagram. Fortunately, I don’t have to try bashing out a second-rate illustration with Microsoft Paint, as the wonderful Causes of Color website has done all the hard work already (and way better than I could manage). Be sure to check out the second image on this page, which shows how the Earth’s shadow appears at different times as you move your mouse over the time labels.

Next in this series on atmospheric optics (I’ve got to justify creating a separate category for such articles now!) — supernumerary rainbows…

In addition to landscape photography, I’m particularly interested in meteorology, and within that vast field, atmospheric optics. A well known (though comparatively rare) effect, is the Brocken Spectre, and associated glory. I’ve been lucky enough to see Brocken Spectres on three or four occasions, though have yet to get a half-decent photograph.

The range of optical phenomena visible in the atmosphere over time is huge. From the common (no nonetheless beautiful) fiery sunset, through rainbows and haloes, to rarities such as noctilucent clouds, and the fleeting green flash — there’s a great deal to learn about and subsequently observe.

To anyone with even a passing interest in this area, I can heartily recommend M. Minnaert’s “Light and Color in the Outdoors” (Amazon link). This classic text explaining nature’s multifarious optical displays isn’t always easy going, but is continually interesting, well illustrated throughout with line drawings, and supplemented by some stunning colour photographs.

Online, the best resource I’ve found is Atmospheric Optics at http://www.atoptics.co.uk/. There are many other sites on the subject available, often illustrated with spectacular photography.

To finish this post, here’s a photo of a fairly common phenomenon — a sun dog (also called a “mock sun”, or, technically, a parhelion).

Parhelion or sun dog, South East Essex, UK.

I reckon on seeing five or six of these a year, though they’re undoubtedly visible much more frequently, if only we’d notice!

Related articles by Zemanta

• Better Planning for Landscape Photography. (creativetechs.com)
• Somewhere Over the Moonbow [Image Cache] (gizmodo.com)

My first two trips to this area couldn’t have been more different. Both involved an ascent of Beinn Dorain (with the addition of Beinn an Dòthaidh the second time), using the same route from Bridge of Orchy, but in wildly differing conditions. Here’s a summit photo from each day:

Summit photo, Beinn Dorain, February 1994.

Summit photo, Beinn Dorain, April 2002.

In February ‘94, the final pull up onto the summit ridge was through a full-on blizzard. Careful map-and-compass work and not a little determination led to the cairn in near white-out conditions. Fun, of a sort, but it hardly made for a relaxing day on the hill.

Eight years later, and the same route yielded a pleasant bimble up late spring snow in the sunshine, and expansive views from this singular summit. Reversing this bit of the route and tacking on Beinn an Dòthaidh, we were treated to an extensive display of crepuscular rays, projected through a layer Altocumulus.

Crepuscular rays over Beinn Dorain (from Beinn an Dòthaidh).

And this year’s trip to the Bridge of Orchy Hills? Well, this time the itinerary started a few miles north of Bridge, at Achaladair Farm. Beinn Achaladair and Beinn a’ Chreachain in combination give a very fine day on the hills indeed. Not that it’s easy. And the weather was pretty ropey too. I was all for calling it a day after Beinn Achaladair, and returning to the car at the farm while my companions continued, but they were having none of it.

Fortuitously, shortly after leaving Achaladair’s rocky summit ridge, I got the fillip I needed as the clouds parted to reveal the vast, lochan-studded expanse of Rannoch Moor. The steep profile of The Buachaille, and a distant Loch Ericht were pretty much all I could reliably identify in this expansive vista.

Rannoch Moor from Beinn Achaladair.

Aching legs were easily forgotten now, and, despite a quickly noticed navigational error leaving the top of Beinn a’ Chreachain (the old “180° off” malarkey — must remember; red end of the compass needle points to magnetic north — I was having a bad day!), the remainder of the day was uneventful.

Now, having seen Rannoch Moor from this vantage point for the first time, I really want to get out there and explore more extensively. A brief incursion from the A82 to a cracking little lochan on a fishing trip some years back has been pretty much it so far. Something else to add to the list…