The moon revolves around the earth in its elliptical orbit. Therefore sometimes it comes closer to the earth, and at some other point of time it is at the farthest distance.
When the distance between the earth and the moon is minimum, the moon is said to be in perigee. On the contrary, when the moon is at the farthest distance from the earth, it is said to be in apogee (Figure 51.9). The moon moves from perigee of 375, 200 km to apogee of 405, 800 km.
Greater tidal ranges are experienced during perigean tides. Since the tide- generating forces are inversely proportional to the cube of the distance from the center of the earth to the center of the tide-generating object, which in this case is the moon, the perigean tides are about 20% higher than the average tides.
Conversely, the apogean tides are 20% lower than the average. If by chance the apogean tides coincide with the spring tides, there is an unusual increase in the height of these tides. Likewise when the apogean tides and neap tides occur at the same time, the tidal range is abnormally low.
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Such types of tides occur in the Gulf of Fundy. In this gulf the tidal range of the perigean tides exceeds 15.4 m, while the tidal range of the apogean tides is only 12.3 meters. Thus, there is a difference of about 3 meters in the heights of the apogean and perigean tides.