Answer
An observer can experience two unequal high tides during a single day due to the gravitational forces exerted by the Moon and the Sun on Earth's oceans. The Moon's gravitational pull causes the ocean waters to bulge, resulting in a high tide on the side facing the Moon and on the opposite side due to the centrifugal force. When the Moon is directly overhead or on the opposite side of the Earth (at the new moon or full moon phases), the gravitational force is strongest, leading to higher high tides, known as spring tides.
However, when the Moon is at a right angle to the Earth and Sun (during the first quarter and third quarter moon phases), the gravitational forces partially cancel each other out, leading to lower high tides, known as neap tides. As a result, during the first and third quarter moon phases, observers experience two high tides that are noticeably unequal in height.
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