Space is Big

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Space is Big!

• Space, it says, is big. Really big. You just
  wont believe how vastly, hugely, mind bogglingly
  big space is. I mean, you think it is a long way
  down the road to the Chemist, but that is peanuts
  to space. Listen. and it goes on.

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Charting the Universe
Mapping and Modelling
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The Universe is the Totality of Space, Time,
Matter and Energy!

• We are used to the idea that the universe is all
  of space..but physicists believe that it is the
  totality of time as well.

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Birth and Death of Stars
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Space is Big!

• Length Scale
• Light Years

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Light Years.

• 1 Light year distance travelled by light
  in 1 year.
• Distance speed x time.
• 9,500,000,000,000 km/year.
• 1 L.Y. 10 trillion km or 6 trillion miles!

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Perspective ?!

• The Earth has a diameter of 13,000km
• this corresponds to 1/20 of a light second.
• If you counted at a rate of 1 number/ second
• 1000 16 minutes
• 1 Million 2 weeks (counting for 16 hrs/day)
• 1 Billion 50 years!
• We need to become comfortable with billions of
  light years, trillions of stars, billions of
  years.

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Galaxy 100 Billion Stars
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Spiral GalaxyGalaxies Have Different Geometries
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Galaxy Cluster
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The Obvious View

• We see the Universe from Earth!
• The Sun appears to move around the Earth, as do
  the 3000 stars we can see with the unaided eye
  during the night.
• The Stars are so far away that we can perceive no
  relative movement between them.
• The Fixed Heavens

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Constellations

• As the heavens appear fixed or constant
• .ancient cultures have grouped stars into
  Constellations, representing gods in the sky,
  mythical beings, or heroes.
• Different (Babylon, Greek, Chinese) cultures gave
  different names to the constellations, but
  interestingly some symbols are the same!

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Constellation Orion
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Orion !!

• The amorous pursuit of the Pleiades (the 7
  daughters of Atlas). To protect the Pleiades from
  Orion, Greek gods placed them among the stars.
• Orion nightly stalks them across the sky!

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More on Constellations

• Aid to navigation, along with the pole star.
  (Polaris is part of the Little Dipper).
• Ancient Calendars for religious festivals and
  agriculture. (Geocentric model)
• There are 88 constellations. (Most are seen in
  Windsor at some part of the year.)
• Still useful for depicting regions of the sky.
• Note the stars are not close to each otherthey
  just appear to be!

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Orion in 3-Dimensions

• Distances between the stars were determined by
  Hipparcos satellite in the 1990s

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Orion on a Backdrop of Stars!
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Constellations near Orion
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The Celestial Sphere

• Simplest Model of the fixed stars on a sphere.
• The heavens rotate around the Earth.

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More on the Celestial Sphere

• It is a crude unphysical model!
• We now realise that the Earth is spinning on its
  axis.
• Note the position of the celestial poles and
  equator, respect to that of the Earth.
• As stars are all deemed to be the same distance
  from the Earth we only need determine their
  angular separations!

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Celestial Sphere View from 35 º North

• Stars rise in the east and set in the west.

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Circumpolar Stars

• These stars never set (within a cone defined by
  the latitude angle, e.g. 35 º North).
• They orbit the pole star.

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Angular Measure

• It is often more convenient measure the angular
  separation (a) between the stars or (b) across an
  object.
• 360 Degrees
• Full Circle
• 60 Arc Minutes
• 1 Degree
• 60 Arc Seconds
• 1 Arc Minute

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Angular Measure Continued

• Note the angular size is not enough to know the
  actual diameter of an object the distance to
  the object must also be known.
• Both the Sun and Moon subtend and angle of 30
  arc minutes in the sky.
• (Compare with thumb at arms length)
• 1 dime at a distance of 2km has an angular size
  of 1 arc second! Small unit!

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Solar and Sidereal Days

• A solar day is the time from one noon to the
  next.
• A Solar day is longer than Sidereal day by
  4mins.

In a solar day the earth has moved 1º in the
solar orbit. From points A to A in successive
days, the Earth has actually rotated 361º
Angle exaggerated!
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More Definitions and Observations I

• Sidereal means with respect to the Stars.
• The difference between the sidereal and solar
  days results in the stars appearing at a slightly
  different position in the sky each night.
• This eventually results in the stars we see at
  night change with the seasons.

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Seasonal Variation

• The Earths axis of rotation is inclined at a
  constant 23.5º from the perpendicular to the
  plane of the Earths orbit around the Sun. The
  annual variation in solar illumination creates
  the seasons.

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Typical Night Sky - Southern Horizon
Summer Winter
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The Zodiac
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Cancer and Capricorn

• are winter and summer constellations,
  respectively, for the northern hemisphere..
• However, from the perspective of earth, the Sun
  rises (with respect to the background stars) in
  the constellations of Cancer in the summer and
  Capricorn in the winter.
• These highest and lowest elevations along the
  ecliptic define the tropics

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Astrology

• Traced back to the Babylonians 2000BC.
• Belief that the heavens can influence our (a)
  behaviour and (b) destiny.
• Initially were warning for the king / Ruler,
  later adapted to horoscopes for individuals.
• Words having astrological origins
• lunatic, jovial, martial, disaster, influenza

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• Ignoring gravity (!) there is no possible
  physical interaction between stars and us.
• Consider If there were such a predetermined
  force (or fate) , what would that imply for free
  will, choice and responsibility?
• Astrology was eventually forbidden by Roman
  Catholic Church in 1585.
• Even so, it was - and still is - widely
  practised.
• Scientific conclusion? Forget it get a life!

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The Ecliptic, Equinoxes, and Solstices.

• The Ecliptic is the apparent annual path of the
  Sun, as projected onto the celestial sphere.

Note at equinoxes, ecliptic and celestial
equator coincide
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Solar Definitions Solstice

• Solstice (sun - sol make stand) occurs twice
  a year when the sun reaches the highest and
  lowest points in the sky at noon, resulting in
  the longest and shortest day.
• The summer and winter solstices are on 21 June
  and 21 December, respectively.

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Solar Definitions Equinox

• Equinox (equal nights) occurs when days and
  nights are of equal duration (12 hours).
• The Autumnal Equinox is on 21 September. The
  Vernal Equinox is on 21 March.
• The time for one vernal equinox to the next is 1
  tropical year 365.242 mean solar days.

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Long Term Changes

• Earth spins on its own axis, orbits around the
  Sun, which in turn, moves around the center of
  the Milky Way galaxy.
• Complex motion!
• The earths axis also precesses
• .or wobbles, like a spinning top.
• Cause? Gravitational forces (torques) of the Sun
  and Moon.
• 23.5º tilt angle remains fixed.

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Precession of the Vernal Equinox

• Earths axis wobbles, or precesses. One cycle
  26,000 yrs.
• Pole star changes with time!
• Now it is Polaris it was Thuban for the
  Egyptian Pharaohs.

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Consequences?

• Sidereal year 365.256 mean solar days.
• Tropical year 365.242 mean solar days.
• Why is there a 20 minutes difference?
• Tropical year related to vernal equinox not
  the fixed stars. As the equinoxes move due to
  precession the next time it occurs it is
  shorter than expected with respect to the fixed
  stars.

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• Our calendars are based on Tropical years.
• If it were on the Sidereal year, then summer
  would be in February 13,000 years from now!
• Therefore, we keep summer fixedand let the
  constellations move! In 13,000 years Orion will
  be a summer constellation.
• Gregorian Calendar (1582AD) maintains seasons
  takes precession into account.
• Previous Julian Calendar formalised
• the use of 1 in 4 leap years.