tugas

1. Why Seawater is Salty?
Most of our planet’s surface is covered in water – salt water. The oceans that support so
much of Earth’s life are around 3.5% sodium chloride – 50 million billion tonnes of salt.
But where does it come from? While some of it comes from volcanic vents or rocks on
the seabed, most of it is actually from the land around us. Every time it rains, tiny
amounts of mineral salts are washed into rivers, which eventually flow into the sea.
The salt in rivers is less than 1/200th the amount usually found in seawater. It becomes
more concentrated in the ocean, as the Sun’s heat causes water from the surface to
evaporate, leaving the salt behind. Extra salt added every year from rivers is balanced by
salt which returns to the sea floor.
But salinity isn’t the same everywhere. Towards the poles, water is not as salty because
it’s diluted by melting ice, while the extra heat in the tropics makes water there saltier –
and denser.

2. How Tidal Currents Happen
Tidal currents, as their name suggests, are generated by tides. Tides are essentially long,
slow waves created by the gravitational pull of the moon, and to a lesser degree, the sun,
on the earth's surface. Since the moon is so much closer to the earth than the sun, its
pull has more influence on the tides.
The moon's gravitational pull forces the ocean to bulge outwards on opposite sides of
the earth, which causes a rise in the water level in places that are aligned with the moon
and a decrease in water levels other places. This rise in water level is accompanied by a
horizontal movement of water called the tidal current.
Although tides and tidal currents don't have much impact in the open oceans, they can
create a rapid current of up to 15.5 miles (25 kilometers) per hour when they flow in and
out of narrower areas like bays, estuaries and harbors. Fast tidal currents toss sediment
around and affect plant and animal life. Currents also may transfer a fish's eggs from an
estuary out into the open sea or carry nutrients that the fish needs from the sea into the
estuary.
When the tide is rising and the flow of the current is directed towards the shore, the
tidal current is called the flood current. When the tide is receding and the current is
directed back out to sea, it is called the ebb current. Because the relative positions of the
moon, sun and earth change at a known rate, tidal currents are predictable.

3. Tsunami
The term of “tsunami” comes from the Japanese which means harbor ("tsu") and wave
("nami"). A tsunami is a series of waves generated when water in a lake or the sea is
rapidly displaced on a massive scale.
A tsunami can be generated when the sea floor abruptly deforms and vertically displaces
the overlying water. Such large vertical movements of the earth's crust can occur at plate
boundaries. Subduction of earthquakes are particularly effective in generating tsunamis,
and occur where denser oceanic plates slip under continental plates.
Tsunami always bring great damage. Most of the damage is caused by the huge mass of
water behind the initial wave front, as the height of the sea keeps rising fast and floods
powerfully into the coastal area.

4. How Venus Eclipse Happens
On May 16, 2010, people in most regions in the world have seen a very rare natural
phenomenon. It was Venus eclipse. It was very rare amazing natural event. It was reported that
the Venus eclipse will seen again in the future in 2050. Do you know how this rare Venus eclipse
happens?
Well, actually Venus eclipse is like Sun eclipse. Venus eclipses occurs when the position of the
earth, moon and Venus is parallel. Venus planet will slowly disappear for a moment because it is
covered the surface of the Moon. Venus planet seems to move to the back side of the Moon.
The moon and planets are sharing a similar apparent path in the sky. That is why, it is not
unusual for the moon to appear to pass close to Venus. In fact, the moon appears somewhere
near it about once a month. However, most people don’t see these events because they are visible
in the evening sky only half the time, and then only for a short period after sunset. The apparent
closeness varies from month to month as well.