30 June: Sky Day

30 June is Sky Day. Here are 10 things you might not know about the sky:

1. The sky can be defined as everything that lies above the surface of the Earth, including the atmosphere and outer space. It is also sometimes referred to as the celestial dome or celestial sphere.
2. Why is the sky Blue? It's down to a phenomenon called Rayleigh Scattering, in which particles of dust and vapour in the atmosphere scatter the light from the sun. Blue wavelengths of light get scattered the most, making the sky appear blue.
3. If there is a lot of dust, however, other wavelengths get scattered as well, making the sky appear whitish or hazy. When the Sun is low in the sky the light has to pass through a longer path to reach our eyes, which is why sunsets and sunrises can be Yellow or Red. Pollution can turn the sky yellow or Brown because of the presence of different particles which scatter those colours. All colours seen in the sky are the result of scattering caused by molecules or particles in the atmosphere.
4. The term night sky refers to the sky as seen at night, when the Moon and stars are visible (clouds permitting). If there were no moon or stars, it would not be possible to distinguish a silhouette of an object against the sky. There are about 2000-2500 stars visible to the human eye on a clear night. Every one of them is bigger and brighter than our sun. The farthest stars which can be seen are 19 quadrillion miles away.
5. Stars don't twinkle. They only appear to do so because of the Earth's atmosphere. The "scintillation" is caused by the light of the star passing through layers of differing density, which deflects the light. If you went into space and looked at the stars, they wouldn't twinkle at all.
6. The lowest part of the sky is the atmosphere, which has five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere. The troposphere is the lowest of them (0 to 12 km/0 to 7 miles) and is where most of Earth's weather happens. It comprises 80% of the Earth's atmosphere. It gets its name from the Greek word "tropos" meaning to turn. Higher up in the stratosphere and mesosphere, low temperatures mean there is little water vapour and hardly any weather, although there may be polar-mesospheric noctilucent clouds. The aurora borealis and aurora australis occur in the exosphere and thermosphere.
7. The troposphere is the only layer that can be accessed by propeller-driven aircraft, and is in fact where most routine aviation happens. Jet powered planes can reach as high as the stratosphere (12 to 50 km/7 to 31 miles). Any higher than that requires a rocket. There is no aviation in the Mesosphere (50 to 80 km/31 to 50 miles) - it's too high for any conventional plane to reach and too low to be considered as orbit. The International Space Station orbits in the Thermosphere (80 to 700 km/50 to 440 miles). The exosphere (700 to 10,000 km/440 to 6,200 miles) contains most of the satellites orbiting Earth.
8. These five layers are mostly distinguished by temperature. Within them are other layers determined by other things: the Ozone layer, the ionosphere (characterised by ionisation of solar radiation and includes the mesosphere, thermosphere, and parts of the exosphere), homosphere, heterosphere (these two are determined by how well mixed the atmospheric gases are) and planetary boundary layer which is closest to the surface of the Earth and is most affected by it. The depth of the planetary boundary layer ranges from about 100 metres (330 ft) on a clear, calm night to 3,000m (9,800 ft) or more during the afternoon when it is dry.
9. The ozone layer is located in the stratosphere, from about 15–35 km (9.3–21.7 mi; 49,000–115,000 ft). It varies in thickness from place to place and at different times of year. About 90% of the ozone in Earth's atmosphere is contained in the stratosphere.
10. Mythologies usually have deities which are specifically associated with the sky. These include the Egyptian goddess Nut, the Proto-Indo-European god Dyeus, the Greek Zeus, the Roman Jupiter, the Australian Aboriginal Altjira (or Arrernte) and the Iroquois Atahensic.

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