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Day
and
Night 
At
any point in time, half of a planet faces the star it orbits and
is bathed in its light (day light) and half of it is turned away
from the light source and is in darkness (night time). The time
taken for it to turn once on its own axis is called the length
of the planet's day (even though that actually includes the night
too!)
Year
Length
The
time taken for the planet to orbit the star once is called the
year. The further the planet is from the star the longer the year
length will be. The time taken to orbit by another planet is called
'its year' and is often expressed in Earth years so that we can
compare the time easily.
How
the apparent daily and annual movement of the Sun and other stars
is caused by the movement of the earth: Seasons and Day Length
Variation.
The sun appears
to rise in the east and to set in the west. This is because as
the earth rotates on its own axis it changes our view of the Sun
in the sky.
The maximum
height of the Sun in the sky is lower in winter because we are
tilted away from the Sun as we rotate. It also spends less time
in our view as it is lower on the horizon. Therefore the daylight
hours are shorter in the winter.
This
difference in daylight time is because the Earth is tilted on
its axis as it orbits the Sun. In the winter the northern hemisphere
is tilted away from the Sun, therefore countries such as ours
spend more that half of the time facing away from the Sun (longer
nights!). In the summer the northern hemisphere is tilted towards
the Sun, therefore countries such as ours spend more that half
of the time facing the Sun (longer days!). In September
and March the equinox occurs and daylight hours equal night time
hours.
Click here
for an animated link
The
Sun's rays are parallel to each other as they hit the Earth's
surface
The
Earth is such a long way from the Sun that it only intercepts
a tiny fragment of the Sun's energy. Most of it goes out into
Space where out Sun is seen shining as a star.
The
Sun gives out energy in rays.... it radiates from the Sun in all
directions. Let us look at some rays coming from a point an consider
how they travel relative to one another.
These
are obviously going further away from each other... radiating
from the point, just as the Sun's rays radiate from its surface.
The greater
the distance this effect is observed over, the more parallel the
rays are to each other. As they hit Earth the rays are travelling
so parallel to each other you would need very special equipment
to show that this wasn't the case.
Our
Earth is a long way from the Sun. The rays it intercepts are travelling
parallel to each other when they hit Earth...
Consider
beams of energy from the Sun of equal width (therefore equal energy).
Beams
A and C hit the Earth near in the polar region... therefore the
energy is spread over a much larger area than the rays that hit
in the equatorial region. This is one reason why the Equator is
warmer than the Poles.... the Sun's enegy is more concentrated
at the Equator because it is spread out over a smaller area of
the Earth's surface. That is why you need a stronger protection-factor
sunscreen when you take your vacation in the tropics rather than
in Britain!
In
this diagram the Earth is tilted AWAY from the Sun....
Why
do we have Seasons?
The
Earth is tilted on its axis. Therefore the length of the daylight
time varies throughout the year. In December the UK has short
days because the Northern Hemisphere is tilted away from the Sun.
In June the opposite is true and in March and September we get
equinox (equal day and night - 12h each).
The
tilt makes the concentration of suns rays hitting a spot on the
Earth vary too. In the summer a point in the Northern hemisphere
recieves more concentrated rays that in wintertime. The affects
the average temperature at that location.
These
variations have an effect on the flora and fauna (plants and animals).
They have to develop ways of coping with the change. Hibernating
animals and deciduous trees have evolved to cope with the variations
in daylight and temperature during the year. Also there is the
way that animals moult and grow a summer/winter coat or migrate....
if we had no tilt they would not be needed and our planet would
be very different. Each spot on the Earth's surface would have
very little variation in average temperature from month to month.
A planet with no tilt would have diffenet life cycles that were
not linked with orbital periods.
What
causes an eclipse? - jump here
to find out!
You
should be able to explain why the eclipse occurs and what you
would observe when one occured.
The
relative position of the Earth, Sun and planets in the solar system
must be known.
(image
from Nine planets site)
This can be remembered
using the following mnemonic:
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My
Very
Easy
Method
Just
Speeds
Up
Naming
Planets
|
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
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Try
the Matching
Exercise to see how well you know it!
- The further a
planet is away from the star it orbits, the longer is its year
and the cooler is its average surface temperature (although if
it has a thick atmosphere this can act as a blanket and it can
be warmer than you would expect because it is able to hold onto
energy from the star it orbits more efficiently..
- A planet's year
length is the time it takes to orbit (journey once around) the
star. This increases as the planet gets further from the star.
kepler worked out some equations that realet the period of a star
(the orbit time or year) with the radius, but the maths is too
difficult for you at this stage!
- The length of
a planet's day is the time it takes to rotate on its own axis.
Sometimes a planet can have a day that is longer than its year!!!
Find
out more about the planets by visiting 'Nine planets' on the WWW
at http://www.dkrz.de/mirror/tnp/nineplanets.html
The
dominant force that acts upon heavenly bodies
Gravitational forces
govern the movements of planets around the Sun and the movement
of stars within galaxies. The factors affecting gravitational
force are the masses of the objects involved and the distances
between them. The bigger the masses the bigger the force but the
further they are from each other the smaller the force.
The
Sun and other stars are light sources
(nuclear reactions (fusion) within the star gives out light energy)
but the planets and other bodies (e.g.. satellites) are seen only
by reflected light.
Artificial
satellites are used to observe the Earth and to explore the solar
system. E.g.. weather satellites, spy satellites, satellites that
look out into space to monitor and observe the universe (like
the Hubble telescope - image from NASA site)
and communication
satellites (used by telephone and television companies) - image
from NASA site.
(image
from: http://www.geocities.com/CapeCanaveral/7639/INDEX.HTM)
Relative
sizes of planets, stars and galaxies (and that the Milky Way is
our galaxy) should be known
The Universe is
full matter
and dark matter. Matter is anything that takes up space and has
mass - there are three state of matter - solid, liquid and gas.
We are used to matter which we will call visible matter. Visible
matter can be seen because it gives off light or reflects light
given off by another object. Dark matter cannot be seen. It does
not give off light or reflect light. It
is believed that over ninety-percent of the matter in the Universe
is dark matter. By studying dark matter it is hoped we will gain
new information about the Universe - its size, shape and possible
future and [processes such as how galaxies formed. We cannot see
dark matter, so we study it by looking at how it affects visible
matter by using computers and satellites - The Hubble Space Telescope
has taken pictures that have helped scientists discover where
dark matter can be found. Read
more about dark matter here
and here.
We know that the
known (matter) part consists of clusters of galaxies (they are
not evenly spread out but gravity pulls them into 'clumps').
Galaxies are full
of clusters of stars (our galaxy is called the Milky Way) . Globular
clusters are groups of several hundred thousand stars all bound
together by gravity. There are globular clusters above the disk
of the Milky Way, and in the early twentieth century they were
used to determine our position in the Milky Way.
A galaxy is a massive cluster of stars, bound
together by gravity. Galaxies come in different types: spiral,
elliptical and irregular. The Milky Way is the spiral galaxy in
which we live, containing about 10 billion stars.
In the centre of each galaxy is a black hole.
The stars in the galaxy are swirling around this black hole and
scientists think that the black hole might give birth to the stars
in the galaxy. The size of the black hole is proportional to the
speed of the outermost stars in that galaxy. A galaxy is a massive
cluster of stars, bound together by gravity. Galaxies come in
different types: spiral, elliptical and irregular. The Milky Way
is the spiral galaxy in which we live, containing about 10 billion
stars.
Some
stars have planets orbiting them (and sometimes comets and asteroids
too). These are then called star systems (ours is called the Solar
System because our Sun is called Sol)
Some planets have
moons (or natural satellites) orbiting them.
 You
need to be aware of why we see the moon as a different shape in
the sky at night. See phases
of the moon
When
we look at the stars we can use our imagination to see patterns.
We link the stars into constellations. Since ancient times mankind
has done this.
(Jump
to constellations to find out more!)
Distances
in space are so great we use light years to measure them.
(Jump
to the light year page to find out more)
How
can we tell if we are looking at a star (giving out its own light)
or a planet in our system (simply reflecting light from the Sun
- but looking like it is a star)?
If you are travelling on a train,
things that are a long way away seem to be hardly moving at all,
while close up objects can be seen whizzing past! It is like this
when we observe the sky at night. The very distant objects (stars)
are virtually still, it takes many months of plotting their position
to see that they have changed position in the sky and even then,
relative to each other, they have not moved! But, the planets
and the moon can be observed moving relative to us very easily.
This is because they are close to us.
How gravitational forces affect the movement
of moons, comets and satellites
Gravity pulls the
moons into elliptical orbits around planets and planets into elliptical
orbits around stars. An ellipse is an oval shape. (See Open University
article for
A level discussion of gravity)
How
stars evolve over a long time scale.
A nebula is usually
a vast cloud of inter-stellar gas (mainly hydrogen) and dust.
Some nebulae are remnants of a supernova explosions, others are
stellar maternity units, where young stars are born. They are
seen as fuzzy patches seen in the sky.
It shrinks under gravity to form a star. When
the mass of the gas is great enough nuclear fusion occurs. This
makes the star shine and give out light and heat energy. This
is the stable state of the star. After a very long time the star
begins to run out of fuel (in fusion the hydrogen gas atoms merge
to form helium). The star then swells to be a red giant.. Then,
as it runs out of fuel even more, it shrinks to become a white
dwarf and then 'goes out' - ceases to shine and becomes a black
dwarf.
If it is a really
big star it doesn't become a white dwarf. instead it explodes
into a supernova and then becomes a neutron star or a black hole
!
See
the poem 'The Star' by Hannah Wood
The differences
between big stars and little stars in the night sky
If we look at two
stars in the sky at night that look the same size and/or brightness,
that is not necessarily because they are the same. One might be
close and reasonably dim, the other very bright but a long way
off!
An
explanation of how the Universe originated and evolved into its
present state
The
'Big
Bang Theory' is based on the belief that all of the matter
in the Universe came into being a one instant and then rapidly
expanded into the Universe we know now. We know that the Universe
is still expanding. Click here
for an interactive journey back in time to the birth of time itself!
See
Red Shift for an explanation of
how we know that the Universe is expanding.
If
you click on the links below you can find out what other ideas
we have had on the Universe's form...
This
information comes from an excellent
site based on the ideas of Stephen Hawking
Mrs.
Jones (October 2000 amended March 2003 and February 2005)
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