They occur when the sun, moon, and earth are aligned.
Explanation
Tides are the periodic rise and fall of sea levels caused principally by the gravitational forces exerted by the Moon and the Sun, combined with the rotation of the Earth. The relative positions of these celestial bodies determine the magnitude of the tide.
Option Analysis:
- They occur when the moon is in the first quarter. is Incorrect. When the moon is in the first or third quarter, the Sun and the Moon are at right angles to each other relative to the Earth. The gravitational forces counteract each other, resulting in Neap Tides, which have a lower tidal range.
- They are the lowest tides. is Incorrect. Spring tides are characterized by the highest high tides and the lowest low tides (maximum tidal range), not merely "the lowest tides." The term "lowest tides" is ambiguous but often refers to the low amplitude seen in Neap Tides.
- They occur when the sun, moon, and earth are aligned. is Correct. Spring Tides occur when the Sun, the Moon, and the Earth are in a straight line (a configuration known as Syzygy). This alignment happens during the Full Moon and New Moon phases. The combined gravitational pull of the Sun and the Moon reinforces each other, creating the highest tidal bulges.
- They are caused only by the sun. is Incorrect. Tides are caused by the gravitational attraction of both the Sun and the Moon. While the Sun is much more massive, the Moon plays a greater role in generating tides due to its closer proximity to Earth.
Key Takeaway:
Spring Tides occur during the New Moon and Full Moon when the Sun, Earth, and Moon are aligned (Syzygy), resulting in the maximum tidal range. Neap Tides occur during the quarter moons when the Sun and Moon are at right angles (Quadrature).