Transcript Charting The Universe - University of Windsor

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.
km
s
day
1 L.Y  300,000
 86,400
 365.25
s
day
year
= 9,500,000,000,000 km/year.
1 L.Y. ~ 10 trillion km or 6 trillion miles!
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.
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”
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!
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.
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
Angular Measure
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.