In oceanography, tides observed in the sea are primarily caused by which of the following influences acting on the Earth oceans?

Introduction / Context:
Tides are regular rising and falling movements of sea level that occur daily along coastlines. They are an important topic in physical geography and oceanography because they affect navigation, fishing, coastal erosion, and marine ecosystems. This question asks what main forces cause tides, testing your understanding of gravitational interactions between the Earth, the Moon, and the Sun.

Given Data / Assumptions:

• The phenomenon under discussion is tides in the seas and oceans.
• We must choose which celestial bodies influence tides.
• Options mention the Sun only, the Moon only, the combined effect of Moon and Sun, and gravitational force of Earth and Sun only.
• We assume the basic tidal theory taught in school geography.

Concept / Approach:
Tides are produced mainly by the gravitational pull of the Moon and, to a lesser extent, the Sun on the Earth oceans. The Moon effect is stronger because it is much closer to Earth, while the Sun, though more massive, is also much farther away. The interaction between lunar and solar tidal forces creates spring tides (higher than normal) and neap tides (lower than normal). The Earth gravity holds the oceans in place overall, but variations in the gravitational attraction of Moon and Sun across the Earth generate tidal bulges. Therefore, the primary cause is the combined gravitational effect of the Moon and the Sun.

Step-by-Step Solution:
Step 1: Recall that the Moon gravitational pull is the main contributor to tides because of its proximity to Earth. Step 2: Recognise that the Sun also exerts a gravitational pull, and when the Sun, Moon, and Earth are aligned, their tidal effects reinforce each other to produce spring tides. Step 3: During quarter phases, the Sun and Moon pull at right angles and partially cancel each other, producing neap tides, which shows that both bodies affect tides. Step 4: The Earth gravity is essential for keeping the oceans attached, but the tidal bulges and daily rise and fall are due to differential gravitational forces of Moon and Sun. Step 5: Because both the Moon and the Sun contribute, though unequally, to the tidal pattern, the best description is that tides are caused by the combined gravitational effect of Moon and Sun. Step 6: Select the option that explicitly mentions this combined effect.

Verification / Alternative check:
Standard tidal diagrams in textbooks show two tidal bulges on Earth roughly aligned with the Moon direction, with modifications due to the Sun position. Explanations of spring and neap tides are always framed in terms of how lunar and solar gravitational forces combine or oppose each other. This repeated emphasis confirms that the correct conceptual description of tidal causation is the combined effect of Moon and Sun, not one body acting alone.

Why Other Options Are Wrong:

• Effect of the Sun only: The Sun does influence tides, but it is not the only body responsible; lunar tides are stronger, and both act together.
• Effect of the Moon only: The Moon is the dominant tidal force, but solar effects are also important and are necessary to explain spring and neap tides.
• Gravitational force of the Earth and the Sun only: Earth gravity is basic, but without the Moon presence, the observed tidal pattern would not match reality.

Common Pitfalls:
Students sometimes remember only that the Moon causes tides and forget the role of the Sun. Others may think the Sun must dominate because it is more massive, overlooking the crucial impact of distance. To answer correctly, keep in mind that the Moon is the primary tidal generator, but the Sun adds a secondary tidal effect, and together they create the complete pattern of spring and neap tides, making their combined gravitational effect the best description.

Final Answer:
Tides are caused by the combined gravitational effect of the Moon and the Sun on the Earth oceans.