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Earth and Space
Key Facts

Solar system formed about 500 million years ago from stellar dust collapsing under
gravity.

Eight known planets orbit our Sun, all in the same counterclockwise direction.
Gravitational force (the attractive force between masses) maintains orbits.

The further away, the weaker the gravitational attraction.

In order, nearest first: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
and Pluto. First four are rocky, remainder gaseous with small rocky core.
Distances sometimes measured in Astronomical Units (AU’s)
1 AU = distance from Earth to Sun
Sun
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Distance
(AU)
Radius Mass
(Earth's) (Earth's)
0
0.39
0.72
1.0
1.5
5.2
9.5
19.2
30.1
39.5
109
0.38
0.95
1.00
0.53
11
9
4
4
0.18
332,800
0.05
0.89
1.00
0.11
318
95
17
17
0.002
Number
Rotation
of
(Earth's)
Moons
25-36* 9
58.8
0
244
0
1.00
1
1.029
2
0.411
16
0.428
18
0.748
15
0.802
8
0.267
1
Orbital
Inclination
Density
(g/cm3)
--7
3.394
0.000
1.850
1.308
2.488
0.774
1.774
17.15
1.410
5.43
5.25
5.52
3.95
1.33
0.69
1.29
1.64
2.03

Planets spin on their own axes, all clockwise except Venus in virtually circular orbits,
except Pluto which has been disallowed as a planet now.

Further planets have moons, sometimes called natural satellites, which orbit the
planet itself. Our moon has a period of 27.3 days, and always faces in the same
direction towards the earth.

Our Sun is 150 million km from us; its light takes about 8.75 minutes to reach us. We
say that the Sun is therefore 8.75 light-minutes away.

Planets are much closer to us than the nearest stars. A star is a collapsed cloud of
hydrogen and helium undergoing nuclear fusion at very high temperatures, hence
producing vast quantities of energy in the form of heat and light.

Nuclear fusion happens when hydrogen nuclei are so energetic that they fuse together
to form helium nuclei, resulting in a loss of mass. Einstein predicted that this loss in
mass was equivalent to an energy according to E = mc2. The heat produced in the
star tends to inflate it. Gravitational collapse is balanced by thermal inflation.
J MacArthur
2001
-1-
Earth and Space

Our Sun will eventually use up all its hydrogen, its core will collapse and its outer
layers will expand massively, engulfing the Earth, and will become a Red Giant. Later
still, when it uses up all its helium, it will collapse to form a White Dwarf star.

Our closest star is Proxima Centauri, at a distance of 4.3 light-years. This means that
the light from this star takes over four years to reach us at a speed of 300 million
m/s. We see the star as it was 4.3 years ago. Because of the great distances away
from us, the stars appear to be stationary in the sky. A telescope shows the planets
moving with respect to these stars, since they are relatively much closer.

Our Sun is a relatively small star in our galaxy, called the Milky Way, which is a spiral
galaxy consisting of billions of stars. Our nearest large galactic neighbour is
Andromeda, 2 million light years away. Billions of other galaxies make up the known
Universe.
In the hot Big Bang Theory, the observable universe began with an instantaneously
expanding point, roughly ten to twenty billion years ago. Since then, the universe has
continued to expand, gradually increasing the distance between our Galaxy and external
galaxies. The expansion of the universe "stretches" light rays converting blue light into
red light and red light into infrared light. Thus, distant galaxies, which are rapidly moving
away from us, appear redder. This expansion also cools the microwave background
radiation. Thus, the cosmic microwave background radiation, which today has a
temperature of 2.728 Kelvin, was hotter in the early universe. Gravity slows the
expansion of the universe. If the universe is dense enough, the expansion of the universe
will eventually reverse and the universe will collapse. If the density is not high enough,
then the expansion will continue forever. Thus, the density of the universe will determine
its ultimate fate.
Evidence in support:

the observed expansion of the universe

the observed abundances of helium, deuterium and lithium, three elements
thought to be synthesized primarily in the first three minutes of the universe

the thermal spectrum of the cosmic microwave background radiation

the cosmic microwave background radiation appears hotter in distant clouds of
gas. Since light travels at a finite speed, we see these distant clouds at an early
time in the history of the universe, when it was more dense and thus hotter.
J MacArthur
2001
-2-