Space is Big!

“Space”, it says, “is big. Really big. You just won’t 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.

Charting the Universe

Mapping and Modelling

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.

Birth and Death of Stars

Space is Big!

Length Scale:

Light Years

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!

year

day

day

s

s

kmYL 25.365400,86000,300.1

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 minutes1 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.

Galaxy: ~100 Billion Stars

Spiral Galaxy

Galaxies Have

Different Geometries

Galaxy Cluster

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”

ConstellationsAs 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!

Constellation Orion

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!

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 other…they just appear to be!

Orion in 3-Dimensions

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

Orion on a ‘Backdrop’ of Stars!’

Constellations near Orion

The Celestial Sphere

• Simplest Model of the ‘fixed stars’ on a sphere.

• The ‘heavens’ rotate around the Earth.

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!

Celestial Sphere:

View from 35 º North

Stars ‘rise in the east

and set in the west.’

Circumpolar Stars

• These stars never ‘set’ (within a cone defined by the latitude angle, e.g. 35 º North).

• They orbit the pole star.

Angular MeasureIt 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

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!

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!

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.

Seasonal Variation

The Earth’s axis of rotation is inclined at a constant 23.5º from the perpendicular to the plane of the

Earth’s orbit around the Sun. The annual variation in solar illumination creates the seasons.

Typical Night Sky - Southern Horizon

Summer Winter

The Zodiac

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”

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

• 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!

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

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.

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.

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 earth’s axis also precesses

….or wobbles, like a spinning top.• Cause? Gravitational forces (torques) of

the Sun and Moon.• 23.5º tilt angle remains fixed.

Precession of the Vernal Equinox

• Earth’s 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.

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.

• 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 fixed…and 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.