Earth

mainly oxygen and nitrogen atmosphere;
liquid water (covering about 70% of the surface)

Able to sustain Various kinds of life

Earth's average diameter is 13,000 km and is flattened slightly by 42 km at the poles = oblate spheroid

North and South poles = geographic poles.  The magnetic poles are slightly displaced from these!

Divides the earth into 2 halves =
Northern and Southern HEMISPHERE

Latitude = Lines that run around the globe east to west as a system of reference

Longitude = Lines that run north to south as a point of reference

Earth's polar axis

It orbits the Sun (The Ecliptic Plane)

The sun therefore only lies above the equator on two dates
Spring equinox - 21st March
Autumn equinox - 22/23rd September

Situated at Latitudes 23.5'North and 23.5' South.
Imaginary lines drawn to show the furthest points at which the sun is directly overhead.
Occur on Summer Solstice - 21 June
Winter Solstice - 21 December

Longitude

is a point on the earths surface either east or west of the PRIME MERIDIAN
Expressed as an angle in degrees

PRIME MERIDIAN

Longitude line which runs through Greenwich, London = 0' east/west

Horizon

The line at which the sky meets the land

Main sources
Commercial and sports floodlights
Urban street lights and motorway lights
domestic and industrial security lights
lights above car parks and shopping centes

British Astronomical Association's Campaign for Dark Skies (CfDS)

Set up in 1989
campaign against excessive, inefficient and irresponsible lighting

Eratosthenes = Greek geographer and mathematician in 3BC who first accurately measured the earth's circumference.

A mixture of gasses that we call air
Nitrogen = 78%
Oxygen = 21%
Argon = 1%
Carbon Dioxide = 0.04%
Water Vapour = aprox 1%
traces of neon, helium, and methane

Increasing elevation = thinner atmosphere
merges with outer space at an altitude of 10,000 km

No precise boundary but it has been agreed at 100 km = Karman line

 Atmosphere benefits

absorbs harmful solar ultraviolet radiation
absorbs harmful energetic x-rays and gamma rays
regulates the temperatures so that we rarely experience "extremes"
Provides us with oxygen to breathe partly protects us from meteoroids (they burn up as they pass through = shooting stars)

Atmosphere drawbacks

1= Refraction (bending) of light causes stars to "twinkle"  and restricts the resolution (sharpness, clarity and detail) of an image
2= Selective scattering of shorter (blue) wavelengths makes sky appear blue preventing observations during the day
3= absorption and reflection of of most of the electromagnetic radiation stops most wavelengths from reaching sea level - therefore most ultraviolet, x-ray and gamma ray observatories are  placed on satellites in orbit.

Atmospheric effects on radiation

Longest radio waves are reflected in space by electrons in the ionosphere
Shorter wavelength microwaves are absorbed by water vapour (H2O) and Oxygen (O2)
Most infra-red (IR) radiation is absorbed by H2O, Carbon Dioxide (CO2) and Methane (CH4)
Ultraviolet radiation is absorbed by Ozone (O4)
Shorter wavelengths are absorbed by O2
Xrays and Gamma Rays are absorbed by Oxygen and Nitrogen

Collect as much light as possible and produces a focussed image that can by viewed and analysed.

2 types of telescope

REFRACTOR = glass CONVEX lens collects the light and brings it into focus

REFLECTOR = a curved mirros  (can be of several segments)  collects light

The lens or mirror is called the OBJECTIVE and the 'size' of the telescope refers to its diameter

Advantages of a large telescope

Collection of more light in proportion to the area

Higher (better) resolution in proportion to the diameter

More light enters = observation time is shorter = more objects can be observed

Large mirrors are generally more precise than small lenses.  Professionals tend to use reflectors.

Adaptive Optics
complex technique used to compensate for imperfections in mirrors, absorption by other optical components and atmospheric turbulence

Ideal observation sites considerations

Atmospheric properties = cloud cover, air turbulence, sky brightness, water vapour content
Geographic locations = access, utilities, likelihood of earthquakes, ground stability

Most optical and infra-red observatories are located on the tops of high mountains = Mauna Kea, Hawaii

Advantages of space telescopes (Hubble and Spitzer)
no atmosphere to blur images
no light pollution
no adverse waether
longer observing periods (darker)
ability to detect other wavelengths (gamma, x-ray and far-IR)

Drawbacks to space telescopes

reduced lifetime
difficult or impossible maintenance, repairs and upgrades
more expensive to build and launch into orbit

James Webb Space Telescope (successor to Hubble) is due to launch in 2013 Contains 6.5 m primary mirror and will work mainly in the IR spectrum.
Instruments mist be kept at absolute zero using cryogenic  a cooling system.  Extra sun shield for extra heat protection.

Van Allen Belts

Found above the earth's equatorial region.  These are 2 doughnut shaped rings of spiralling high energy particles held in space by the earth's magnetic field.

Compact inner belt consists of high energy protons formed by collisions between cosmic rays and atoms in the atmosphere

Diffuse outer belt consists mainly of electrons and other charged particles from the Sun