The time gap between two successive high tides at a given place is about:

At most places, two high tides and two low tides occur in about 24 hours and 52 minutes, so the interval between two successive high tides is about 12 hours 26 minutes.

Why the interval is 12 hours 26 minutes, not 12 hours:

• Tides are caused mainly by the gravitational pull of the Moon.
• The Earth rotates once in 24 hours, but during that time the Moon also moves forward in its orbit around the Earth, in the same direction.

So, a place on the Earth needs about 52 extra minutes to come back directly under the Moon. This period of about 24 hours 52 minutes is called a lunar day.

Since there are two tidal bulges, one facing the Moon and one on the opposite side, each place passes through two high tides in a lunar day, about 12 hours 26 minutes apart.

Similarly, the gap between a high tide and the next low tide is about 6 hours 13 minutes.

Why there are two tidal bulges:

• On the side facing the Moon, the Moon's pull is strongest, so water piles up towards it.
• On the opposite side, the Moon's pull is weakest, so the water there is "left behind", forming a second bulge.
• Types of tides: semi-diurnal (two equal high tides a day, as along most of India's coast), diurnal (one high tide a day) and mixed (two unequal high tides).

Tidal range:

• The difference between high tide and low tide is called the tidal range.
• The Bay of Fundy in Canada has the highest tidal range in the world, up to about 16 metres.

Why not the other options?

• ✗6 hours is roughly the interval between a high tide and a low tide.
• ✗24 hours and exactly 12 hours ignore the Moon's movement in its orbit.