Much of the United Kingdom's coast will experience the highest tides in nearly two decades -- the result of a periodic combination of astronomical phenomena.

The unusually high tides are the result of an aligning Earth, moon and sun, as well as a the positioning of the sun and moon over the equator. Additionally, the gravitational tug on tidal ranges is stronger during the moon's perigee.

Especially low and high tides, called spring tides, happen during new and full moons, when Earth, moon and sun are directly aligned. Spring tides, following the lunar cycle, happen regularly. Occasionally, spring tides happen while the moon's orbit around Earth is at its most intimate. Less occasionally, these two scenarios coincide on or around the spring and autumnal equinoxes and the moon's so-called nodal cycle.

The moon's path around the Earth isn't fixed. Instead, its orbital trajectory moves around the plane created by the Earth's path around the sun -- the ecliptic plane. In other words, the moon's orbit sees it moving periodically farther away and closer to the Earth's equator. At its nodal points, movement about the equator is least dramatic.

"In 2015 the moon's orbital excursion above or below the equator takes the minimum value of 18 degrees," researchers at the National Oceanography Center explained in a recent press release. "This slightly increases the chances of the moon being directly overhead at the equator coinciding with the other factors that contribute to extreme tidal forces."

Researchers say the unique combination of astronomical positions will result in spring tides rising as much as one-and-a-half feet higher in some places.

"NOC scientists continue to lead the world in the study of tides and all factors contributing to sea level change," said Kevin Horsburgh, a professor and researcher at NOC. "The 18.6 year cycle is a fascinating result of heavenly motions. Whilst many features of tides have been known for centuries we are still making new discoveries -- for instance we recently showed how slow changes in global sea level can affect the ocean tides."