EARTH’S TIDESSECTION 3

WHAT CAUSES TIDES?

• Tides: daily changes in the level of ocean water

• Tides are caused by the differences in the gravitational force of the sun and the moon across earth

• This difference in gravitational force is called tidal force

• The tidal force exerted by the moon is stronger than the tidal force exerted by the sun• This is because the moon is closer to earth than the sun

• This means the moon is mainly (but not completely) responsible for earth’s tides

•How often tides occur and how tidal levels vary depend on the position of the moon as it revolved around earth

• The gravity of the moon pulls on every particle of earth• Liquids move more easily than solids, so the pull on liquids (oceans)

is much more noticeable

• The moon’s gravitational pull on earth decreases with the moon’s distance from earth• This happens with all gravity – the farther away, the less gravity’s

affects

• The part of earth facing the moon is pulled towards the moon with the greatest force• Water on this side of the earth bulges toward the moon

• The solid earth is pulled more strongly toward the moon than the water on the opposite side of the earth from the moon (because the earth is closer to the moon than the water on the opposite side) • Water on this side of earth is not pulled as much as earth, and so “lags”

behind earth’s movement towards the moon

• This creates another bulge on the side of earth opposite of the moon

HIGH TIDES VS. LOW TIDES

• The bulges that form in earth’s oceans are high tides• High tide means the water level is above average sea level

• Low tides form in the areas between the high tides• Low tide means the water level is below average sea level

•As the moon moves around earth and earth rotates, tidal bulges move around earth

• The tidal bulges follow the motion of the moon

•Many places on earth have two high tides and two low tides each day as a result

WHAT LOW TIDES VS. HIGH TIDES LOOK LIKE:

WHAT ARE TWO KINDS OF TIDAL RANGES?

• The moon’s effect on tides is twice as strong as the sun’s effect

• The combined gravitational effects of the sun and the moon on earth result in different tidal ranges

• Tidal range: difference between the levels of ocean water at high tide and low tide

• Tidal range depends on the position of the sun and the moon relative to earth

SPRING TIDES: THE LARGEST TIDAL RANGE

• Tides that have the largest daily tidal range are spring tides

•Spring tides happen when the sun, the moon, and earth form a straight line

•Spring tides happen when the moon is between the sun and earth and when the moon is on the opposite side of earth • AKA during new and full moons (or every 14 days)

NEAP TIDES: THE SMALLEST TIDAL RANGE

• Tides that have the smallest daily tidal range are neap tides

• Neap tides happen when the sun, earth, and moon form a 90° angle

• During neap tides, the gravitational effects of the sun and the moon on earth do not add together as they do during spring tides

• Neap tides occur halfway between spring tides• AKA first and third quarter moons

• This causes two smaller tidal bulges

WHAT CAUSES TIDAL CYCLES?

• The rotation of earth and the moon’s revolution around earth determine what tides occur

• The moon revolves around the earth much more slowly than earth rotates

•A place on earth that is facing the moon takes 24 hours and 50 minutes to rotate to face the moon again

• The cycle of high tides and low tides at that place happens 50 minutes later each day• So if high tide occurs on Monday at 12:00pm, the next day it would

happen again at 12:50pm

WHAT IT LOOKS LIKE:

• In many places there are two high tides and two low tides each day

•Because the tide cycle occurs in 24 hours and 50 minute intervals, it takes about 6.25 hours for water in an area to go from high tide to low tide