A Wild Vista

The shadow cast by the Earth is vast — a cone over 1 million kilometres in length, which, on occasion, intercepts the moon’s orbit, and reveals itself in the form of a lunar eclipse. It is, however, possible to see part of this shadow cone much more frequently than the rare occasions when Sun, Earth, and Moon happen to line up in just the right way. You just need to know what to look for.

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…