Daytime astronomy: 6 phenomena to spot before nightfall

Captivating sights are not just on show at night. Lucie Green shows you astronomical phenomena you can see in the daytime sky.

Did you know that some astronomy can be done during daytime? While many celestial sights such as the Moon or even early-rising planets can be seen in daylight, there are a number of astronomical phenomenon known as atmospheric effects that can only be observed – with caution – when the Sun has risen.

Advertisement

Various atmospheric effects cause distortions in the light coming from the Sun – or light reflected from the Moon – to produce an array of beautiful phenomena.

Read more about daytime astronomy:

The rainbow is the most familiar of these, but with a little careful observation under the right conditions, a whole host of atmospheric effects can be seen, such as arcs of rainbow colours at the zenith and circles of light around the Sun.

Interestingly, each phenomenon discloses information about the atmospheric conditions at the time of observation.

Some atmospheric effects are common while some are rare, but they are always spectacular and visually stunning.

Remember: you should never look directly at the Sun without purpose-built equipment such as solar filters or telescopes. Doing so could severely damage your eyesight.

1

Sundogs

Sundogs. Credit: Science Photo Library

Sundogs, also known as Parhelia, are a type of solar halo that are seen as high-intensity spots of light either side of the Sun on a horizontal line.

They’re caused by sunlight Sun being refracted by ice crystals, the majority of which have a horizontal orientation, a bit like a dinner plate.

When the Sun is low to the horizon the sundogs are roughly 22° either side; they move further away when the Sun is higher.

This effect is very easy to see – look out for it when the Sun is close to the horizon during a cold spell in the weather.

2

Circumzenithal arcs

Circumzenithal arc. Credit: Credit: Joseph Thiebes (CC BY-SA 3.0)

These stunning phenomena appear as upside-down rainbows wrapped around the zenith. The curved edge extends down towards the Sun and is red on the outside.

Look out for them when the Sun is fairly low in the sky and particularly if you can see sundogs.

This is because the same ice crystals that produce the sundogs (and also halos) are responsible for producing these arcs.

They only form when the Sun has an altitude of less than 33° and are best seen when the Sun is about 22° high in the sky.

3

Distorted sunsets

Portsmouth’s Spinnaker Tower, silhouetted by the setting Sun – taken from 22km away. Credit: Pete Lawrence

Light from the Sun is always subject to distortion as it passes through the Earth’s atmosphere. The setting Sun is especially susceptible, because the light path through the atmosphere is longer.

Often the setting Sun appears flattened or oval in shape. This occurs when light coming from the lower limb of the Sun is bent upwards, making it appear higher in the sky than it actually is.

Extreme density variations in the atmosphere can cause severe distortions, also called sunset mirages. Under these conditions the setting Sun can appear to be made up of several cigar shapes.

4

Glories

Sun Glory. Credit: Andy W

Glories always appear on the opposite side of the sky to the Sun, at the anti-solar point – the point opposite to the Sun in the sky – and take the form of a series of concentric, coloured rings.

They are thought to be formed when the Sun’s light is scattered back to the observer due to small water droplets in the atmosphere. This also separates the light into its constituent colours, hence the rainbow effect.

On a foggy day at sunrise or sunset, try and see a glory by standing with the Sun behind you. The glory might appear round the shadow of your head. Glories are often seen from aircraft.

5

Green flashes

Green flash. Credit: Science Photo Library
Green flash. Credit: Science Photo Library

The famous but elusive green flash appears as a fleeting change in the Sun’s colour, from orange to green just as it disappears over the horizon. It’s hard to observe because it only lasts for a few seconds.

This effect is caused by layers of air at different densities, which amplify small differences in the separation of the Sun’s light into its constituent colours as it passes through the atmosphere.

A miraged Sun is a good sign that a green flash might happen and a high viewing point is recommended.

6

Anticrepuscular rays

Anticrepuscular rays. Credit: Science Photo Library

These rays are parallel shafts of light coming from the Sun that appear to converge on the anti-solar point. Just like a long, straight road appears to converge in the distance, so these parallel rays appear to converge just over the horizon. Anti-crepuscular rays aren’t particularly rare – look for them when the Sun is setting.

At this time, turn around and look at the opposite part of the sky. However, the cloud structure needs to be right to allow the Sun’s light to be seen as beams.

Advertisement

Lucie Green is a solar scientist and presenter. This article originally appeared in the August 2007 issue of BBC Sky at Night Magazine.