Walking at night with no hand-held light: some observations

Most of my life over the last twenty or thirty years has been divided between a small country town, with street lights, and a bit of land right out in a rural area, away from street lights; both in what is called the Mid North of South Australia.   Approximate light levels from some sources; with no clouds Light source Lux Starlight 0.0001 Full Moon 0.1 Sunlight 60 000 From Wikipedia and elsewhere When I wrote this I was 68 years old and semi-retired. I do quite a bit of walking in order to keep some level of fitness and because I enjoy it. From the table at the right we can see that full sunlight is about 600 million times as bright as starlight. Yet it is quite possible to see sufficiently well to walk along a road or path by starlight. The human eye is remarkably efficient in a huge range of light levels. One of my neighbours (I have spoken to him several times but don't know the bloke's name) walks in the early morning carrying a torch (= USA flashlight). For many years I have believed that one sees less at night while using a torch than one does without. Of course, if you need to see some particular object, it is handy to be able to play some light on it, but if you keep a torch on all the time you see that small part of your surroundings that your light is playing on, and you miss the rest; especially everything beyond the range of your light – including the night sky.

One of the roads I sometimes walk at night
Photo 2007/07/18

Natural sources of light at night I will list these roughly in order of importance...

The Moon Full Moon   'Full', first and last 'quarter' These are rather inconsistent terms. When the Moon is at first- or last-quarter one can see a quarter of the surface; perfectly reasonable. When it is 'Full' one can see half of its surface; wouldn't it be more consistent to call it 'half Moon'? The light of the full Moon is about 500,000 to a million times dimmer than the Sun, but is quite sufficiently bright for a human to be able to see where he is walking easily, and even see some colour. It is so bright that one can see little of the night sky; all but the brighter stars and planets are drowned by scattered Moonlight. Astronomers don't like moonlight, it makes it very hard to see the dimmer objects in the night sky. If the moon is full most astronomers wouldn't bother looking through their telescopes any more than they would in daylight. Any more than a small crescent moon in the sky makes viewing difficult. The full Moon rises at or about sunset and sets at or about sunrise. Roughly speaking, the full Moon is in the part of the zodiac where the Sun is six months later; that is, in the summer the Moon follows the same path through the sky as does the winter Sun; in the winter it follows roughly the path of the summer Sun. Gibbous Moon This is the Moon at a phase between full and quarter; it is so named because it has the shape of a rounded stone, a gibber. Obviously the amount of light from a gibbous Moon will be between that of a full and quarter Moon, and the times of rising and setting will also fit in between those. Quarter Moon   Preferential backscatter reflection Surprisingly I could not find any matches for 'preferential backscatter reflection' on the internet. It is most familiar and obvious from reflective roadside signs; which 'bounce the light back' in whatever direction it came from. The surface of the Moon has similar, but not so pronounced, optical properties. Moonlight is not as simple as you might think. A 'quarter' Moon covers half the angular area that the 'full' Moon covers, yet casts far less than half the light. This effect has been attributed to the very powdery surface of the Moon, and the reason for the difference in apparent surface brightness is due to the Moon showing 'preferential backscatter reflection' and connected to the anti-solar point where there are no shadows. (You might have observed the bright spot surrounding the shadow of the aeroplane that you have flown in; this is the anti-solar point for you at the time.) Enough of that. The important point is that the a quarter Moon is considerably less than half as bright as a full Moon, but it is still quite bright enough to walk by, so long as it is reasonably high in the sky and you are not among trees, especially if they have a dense canopy. The first quarter Moon rises about mid-day and sets about mid-night. The last quarter Moon rises about mid-night and sets about mid-day. Crescent Moon   Top Home Index The crescent Moon casts little light, and since it is never in the sky for long after sunset or long before sunrise, and never high above the horizon at night, the light it gives is not very useful for walking. The waxing (growing larger) crescent Moon always sets within a few hours of sunset, the smaller the crescent the earlier it sets. Similarly the waning (growing smaller) crescent Moon always rises within a few hours of sunrise, the smaller the crescent the closer to sunrise it sets.

Natural sky-glow For an hour or so after the Sun sets light is still scattered and refracted through the atmosphere; this, of course, is the twilight. You may notice a glow in the sky, near where the Sun set, more confined and longer lasting than the twilight glow; this is the zodiacal light and is sunlight that is being scattered by dust in space. Of course both light scattered by the Earth's atmosphere and space dust can also be seen before sunrise. Even when there is no Moon in the sky, the Sun is well below the horizon, and you are a long way from a city, there is some general glow from the sky. The night sky is not black, it is grey.

Venus Venus is the only planet that is likely to be bright enough to provide useful light for walking at night. If you look carefully on an otherwise dark night you can see your shadow cast by Venus. Venus is sometimes called the morning star or the evening star. It is always within 47° of the Sun and usually closer than that.

Starlight If you give your eyes time to adapt to the relative darkness (the level of light in a starlit night is something like a six-hundred-millionth that of a sunlit day) it is quite possible, and enjoyable, to walk along a road or a smooth path without obstacles.