example using a camera with a telephoto lens – a distinct green
edge and detachments can often be seen during clear horizon
sunsets. Photographing the Sun in this manner should be done
using a camera’s review screen and not by direct viewing through
the camera’s viewfinder.Halos
Sometimes, generally when close to full, the Moon appears to be
surrounded by a luminous ring. This is called “a lunar halo”, and
is due to the moonlight being bent or refracted by very thin, high
clouds known as cirrostratus. They are well over 20,000 feet above
the ground, and are composed of ice crystals. They bend light at an
angle of 22 degrees, causing a lunar halo spanning 44 degrees in
the sky. One can also have a solar halo for exactly the same reason.
The actual cloud is so thin that with the naked eye you cannot see it
at all.Sun Dogs and Moon Dogs
A 22 degree solar halo can sometimes show brightenings to the
east and west of the Sun. These are known as Sun dogs (parhelia)
and on close examination show the colours of the rainbow. When[1] Green rim (top) and red rim (bottom), visible at sunset.
[2] A 22 degree lunar halo.
[3] A primary rainbow with a weak secondary just visible to the right
of the image.
[4] A lunar corona.
[5] Delicate colours visible in iridescent clouds.In this chapter we look at a diverse range of phenomena
involving light and particles. Some of these phenomena
are purely astronomical while others are meteorological,
due to atmospheric effects, and some cross the boundaries
between the two. What they have in common is an
extraordinary beauty, making them very desirable both to
look at and to photograph.EARTH AND SKY
Red and Blue Sky
Let us first look at the phenomena which are due to the effects of
the Earth’s atmosphere and come into the realm of meteorology.
Visible light from the Sun is made up of a full spectrum of colours.
Earth’s atmosphere scatters the shorter, bluer wavelengths
more than red. Consequently blue light appears to come from all
directions in the sky. Called Rayleigh Scattering, this is why the sky
is blue.
When the Sun is low down at sunrise or sunset, its light must
pass through a thicker layer of atmosphere. This both scatters away
more blue light and dims the Sun’s intensity. The light that’s left
gives us our red/orange sunrises and sunsets.Green Rim, Red Rim and Green Flash
Another colourful effect sometimes seen with a low Sun is related
to the dispersive properties of the atmosphere. Here, light passing
through the ever-thickening layers of atmosphere as you get
closer to the horizon spreads its component colours out like a giant
prism. This can give the rising or setting Sun a coloured edge which
appears green at the top and red at the bottom. The visibility of
the rims is best picked up when the Sun is about 2 degrees above
the horizon by using the solar projection method (see page 45) or
photography. As usual, it's important not to look directly at the Sun.
Under certain atmospheric conditions, as the last edge of the
Sun sets below the horizon, a green detached portion remains for a
couple of seconds. Known as a green flash, this is best seen against
a sea horizon. Visually, the green flash is difficult to see from Britain
or the northern United States, although under magnification – for(^131)
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Astronomer Book