The Earth

Source: NASA Public Domain

• The third planet out from the Sun.
• It revolves once a day.
• It takes a year to orbit the Sun.
• One of 8 planets orbiting the Sun.
• Earth derives from the German name of a goddess who was the mother of Thor in Norse mythology.

Definitions - solar system

• planet: large ball of rock, gas and ice that's not a star
• a moon: a rocky object orbiting a planet
• the Moon: the Earth's moon
• solar system: the Sun and various things that orbit it
• the Sun: the star at the centre of the solar system
• asteroid or minor planet: rocky body, smaller than a planet
• comet: body of ice, rock and dust

Definitions - universe

• star: ball of hot gas powered by fusion
• constellation: a group of stars close to each other in the sky forming a shape
• star cluster: a group of stars close to each other in space
• nebula: a cloud of gas
• galaxy: hundreds of billions of stars
• the Milky Way - our galaxy
• the Universe: everything

Names - constellations

• Mostly taken from greek mythology, e.g.
  • Orion: the hunter
  • Pegasus: the flying horses
• Some in the southern hemisphere come from sea explorers:
  • Carina: keel of a ship
  • Vela: sails of a ship
• The International Astronomical Union specifies 88 constellations.
• Every star is in one constellation.

Names - stars

• The brightest stars have Arabic names, such as
  • Rigel
  • Polaris or the Pole star
• Stars are also named by the constellation they are in
  • α Orionis or Betelgeuse
  • β Orionis or Rigel
  • γ Orionis or Bellatrix
• Mostly, the greek letters (alpha, beta, gamma...) are assigned in order of brightness
• Fainter stars are given our letters, i.e. a, b, c...
• Even fainter stars are given capital letters A, B, C...
• Even fainter stars are given numbers

Magnitude

• Arab astronomers classified the stars in six levels of magnitude
  • Stars of the first magnitude were bright
  • Stars of the sixth magnitude were barely visible to the eye
• The system has been put on a scientific basis now
• A small or negative number means bright:
  • Sirius (brightest star in night sky) is magnitude -1
  • Vega is magnitude 0
  • Venus is magnitude -4
  • the Moon is magnitude -10
  • the Sun is magnitude -27
• Faint objects are larger numbers:
  • the Pole Ztar is magnitude 2
  • typical binoculars can let you see down to magnitude 8
  • large telescopes can show magnitude 20 stars

Our view from the Earth

• The constellations would look the same from anywhere on Earth
• And from anywhere in the solar system
• But the constellations would look different viewed from another star
• The constellations don't change much over hundreds of years
• But they will change over ten of thousands of years
• We're moving
  • the Earth is rotating, giving us day and night
  • the Earth is orbiting the Sun, giving us the year

Longitude and latitude

Source: geographyworldonline.com

• Any place on Earth can be specified by a longitude and latitude
• Longitude is measured east or west from the Greenwich meridian
• the Greenwich meridian runs from the north to south poles, through Greenwich
• if you move west in longitude the Sun rises and sets later

Where on Earth?

• The length of day, the stars we can see and the seasons all depend on latitude:
  • a latitude of 0° is on the equator
  • a latitude of 90° is at the pole, north or south
• At the poles:
  • the stars never rise or set
  • day lasts 6 months, so does night
  • if it's day at the north pole, it's night at the south pole
• At the equator:
  • day and night are of similar length
  • seasons aren't much different
  • the Sun rises near east and sets near west
  • the Sun can pass overhead

Celestial sphere - alt/az

Source: Wikipedia CC-BY SA 3.0

• Imagine the stars are painted on the inside of an imaginary sphere.

Altitude and azimuth

• Azimuth is the compass bearing: measured from 0° to 360°:
  • 0° is North
  • 90° is East
  • 180° is South
  • 270° is West
• Altitude measures how "high" the star is, from 0° to 90°:
  • the zenith is at 90°
  • the horizon is at 0°

Star maps and celestial co-ordinates

• The azimuth and altitude of a star change with time.
• So we cannot make star maps using azimuth and altitude.
• How do we make an atlas of the stars?
• We can project the equator and poles onto the sky.

Celestial sphere - equatorial

Source: Wikipedia CC-BY SA 3.0

Right Ascension and Declination

• Declination is like latitude:
  • 0° is on the celestial equator; this star passes through the zenith if you're stood on the equator
  • 90° is at the celestial pole; the pole star is almost at 90° N
  • declination is measured north or south from the equator
• Right Ascension (RA) is like longitude
  • but there's no Greenwich meridian; its zero is the Vernal Equinox
  • the Vernal Equinox is where the Sun is on 21 of March (also known as the First point of Aries)
  • the first point of Aries is where the Sun is on March 21st
  • like longitude, RA is related to time
  • so it's measured in hours, minutes and seconds, from 0 to 23

The Zodiac and Ecliptic

• The Sun, Moon and planets all move through 12 constellations.
• These constellations are called the Zodiac.
• They form a circle all round the sky.
• The path of the Sun is called the ecliptic.
• The Sun takes 1 year to move round the ecliptic.
• The ecliptic is the plane of our solar system.

The Sun's motion over the year

Source: scistand.com