Summary
Summary Unit 44 - Astronomy
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The moon:The moon has mares, these are large and dark areas that are filled with lava. The mare formed as a result of
the crater forming. The crater was formed when large asteroid impacts were filled by lava. The craters are
rough circular depression in the surface and were formed high speed meteoroids struck the moon. The craters
vary in size and shape. There are rays that radiate from some of the large craters these are streaks of debris.
The rays can be as long as 3000 km. The moon’s surface moves which causes cracks in the surface known as
rills and its similar to the faulting on earth.
The lunar cycle:
The moon is visible to us because sunlight
reflects back to us from the its surface. Over a
one-month period the moon circles around
the earth once. If we were to view the moon
from the solar system, we would see that half
of it is always lit as shown in the diagram. But
when viewing the moon from earth not always
the lit side faces earth meaning that as the
moon orbits around earth the amount of lit
side, we see changes. The changes that we see
are known as the phases of the moon and this
repeats in a certain way over and over.
The first phase of the lunar cycle is the new
moon. This is where the moon is lined up between the earth and the sun. We don’t see a moon because the
side that is facing earth is not lit by the sun and we experience an eclipse. We start to see more sunlit side of
the moon each night as the moon moves eastwards away from the sun. A few days after the new moon we can
see a thin crescent in the western evening sky. Crescent appear to become larger each night. When half of the
Moon's disc is illuminated, it is known as the first quarter moon. When the moon has more than half of the
disc illuminated it would have a gibbous shape. The moon would appear to grow fatter and then we will start
to see a full sunlit face of the moon and this phase is known as the full moon. At the this phase the moon rises
almost exactly as the Sun sets and sets just as the Sun rises the next day. Now the first half of the lunar cycle is
complete. The second half of the lunar cycle the moon gets thinner each night and this is known as waning.
The shape of the moon is still gibbous at this point but it grows thinner every night. As it reaches the three-
quarter point in its month cycle, the Moon once again shows us one side of its disc illuminated and the other
, side in darkness. However, the side that we saw dark at the first quarter phase is now the lit side. As it
completes its journey and approaches new moon again, the Moon is a waning crescent.
Mercury:
Mercury is the closest planet to the sun as its distance from the sun is 57,909,227 km and it's also the smallest
planet in the solar system. Mercury takes 88 days to orbit around the sun so a year there is only 88 days. The
closer the planet to the sun the less time it takes to orbit around it so the planetary year of each planet varies
depending on the distance from the sun. Mercury is only 4,879 Kilometres across its equator whereas the
earth is 12,742 kilometres across its equator. Mercury is very dense it has a density of 5.4 grams for each cubic
centimetre this is due to the fact that its mainly composed of heavy metals and rock. Only earth has a higher
density than mercury. Mercury has Lobate scarps which were formed as the iron core of the planet cooled and
contracted and this caused the surface to the planet to wrinkle. The scarps on the planet can be a mile high
and hundreds of miles long. Mercury has the highest number of craters in the solar system which are caused
by encounters with asteroids and comets. Gravity in mercury is only 38% of earths meaning that the gravity is
too weak to hold on to an atmosphere so its atmosphere gets blown away by solar winds. However, while
gases escape into space they are constantly being replenished at the same time by the same solar winds,
radioactive decay and dust caused by micrometeorites.
Venus:
Venus is the second planet in the solar system. It’s 108,209,475 km away from the sun. Venus takes 225 days
to orbit around the sun and 243 days for it to orbit around its access which means that a day on Venus is
longer than a year on Venus. Venus is very bright in fact the only brighter thing in the solar system than Venus
is the moon. Venus has a magnitude of between -3.8 and -4.6. On Venus the atmospheric pressure is much
greater than the atmospheric pressure on earth. Venus is very similar to earth in terms of size and mass and
Venus is also known as the earths sister planet this is because Venus only has a 638 km difference in diameter.
Also, Venus like earth has a central core, a molten mantle and a crust. Venus has an average surface
temperature of 462 0C. On Venus there is no seasonal variation because it doesn’t tilt on its axis. Greenhouse
effect is found on Venus because of the dense atmosphere which is 96.5% carbon dioxide.
Mars:
Mars is the second smallest planet in the solar system and is the third planet away from the sun with a
distance of 227,943,824 km. Mars has a thin atmosphere that is mainly composed of carbon dioxide. Surface
temperature on mars can be from –153 to 20 0C. One year on mars is 687 days that is 1.9 years on earth.
Olympus mons is a mountain on mars and it’s the tallest mountain in the entire solar system this is a shield
volcano and there is a possibility that it's still active. Mars has two moons one called Phobos and the other
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