The Universe – Tides Notes & Worksheet
Tides are the regular rising and
falling of the sea. You have seen this if you have been to the ocean.
When it is HIGH tide, the water has come a long way up the beach and
at LOW tide you will see lots of the shore because the sea has gone a
long way out. There are about TWO high tides and TWO low tides a day.
Tides are due to the gravity
of the Moon trying to pull the part of the sea closest to it
towards it. In the (exaggerated) diagram below, there will be a
high tide at point A, called the near tide, because of this
pulling effect. There will also be a high tide on the opposite side of
the Earth at point C, called the opposite tide. At points B and
D there will be a low tide.
The Earth rotates once every 24 hours
which means that the places on the Earth where HIGH and LOW
occur tides are always changing. The diagram below shows where
HIGH and LOW tides will be 6 hours after the diagram above.
The Sun also has a gravitational effect on
the sea. Although the Sun is larger than the Moon, it is further away
from the Earth, which means that it has less effect on our tides.
Twice a month, during the new moon and the full moon, the Moon and the
Sun are in line with the Earth and so they pull together. This causes
very high high tides and very low low tides called SPRING tides.
Twice a month, during the first and third (or last)
quarters, the Sun and Moon are at right angles to each other, and so
their pulls sort of cancel each other out, and are not as great. This
causes much smaller tides. These are called NEAP tides.
QUESTIONS ON TIDES
- If it is LOW tide at a harbor at 9:00 am, at about what time
would you expect it to be HIGH tide in the afternoon?
- If it is HIGH tide at 8:00 pm, when would you expect the next
HIGH tide to be?
- The following two diagrams show the position of the Sun, Moon
and Earth. Which would produce the HIGHEST tides? Give reasons for