U - Factary

Ultraviolet

The word comes from combining the Latin for beyond (ultra) with the colour of the short-wavelength end of the visible spectrum (violet).

There is no firm definition of which wavelengths are considered ultraviolet (UV), but they are approximately in the region 100 nm - 300 nm.

Suntan and sunburn are caused by ultraviolet radiation from the Sun. Even a suntan is now considered as evidence of damage to the skin, and can increase the risk of skin cancer. Unfortunately, it seems like it will be a while before the connection between having a suntan and ‘looking healthy’ will be broken in many people’s minds.

In addition, the growing 'hole' in the Earth's ozone layer is worrying as it is allowing more UV radiation to reach the Earth's surface and this is adding to the dangers of excessive exposure of our skin to the Sun.

See what the Sun looks like in ultaviolet radiation in images taken from TRACE in the section on loops - tangled Sun

Ultraviolet astronomy

We can get a lot of information about distant astronomical objects by studying the ultraviolet radiation from them. However, since the gases in the Earth's atmosphere absorb much of this radiation we have to send telescopes into space if we want to observe most of the scientifically interesting ultraviolet radiation.

Ulysses

A spacecraft orbiting the Sun which is designed to study the solar wind, particularly above the north and south poles of the Sun.

Umbra

This is the dark, inner region of a fully developed sunspot. It has a temperature of about 4000 °C and has a strong magnetic field. The word is also used to describe the darkest region of the shadow formed during an eclipse.

Universal Time (UT)

This name was adopted to avoid confusions that were arising over other time names such as Greenwich Mean Time (GMT), which itself has two possible meanings, differing by up to 0.9 seconds. For most purposes though UT can be considered the same as that we normally call GMT. Note that the name UT is also more 'international' than GMT and so is less controversial.

UVCS

The UltraViolet Coronagraph Spectrometer on board SOHO. It observes the spectrum of the solar corona out to a distance of 10 solar radii.

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