What really happens when a cruise ship crosses the equator at zero degrees

Crossing the equator is a rite of passage for cruise passengers, moving between the northern and southern hemispheres. Aside from the lack of signage at this imaginary border, something else strange happens at the middle of the globe. The sea is peculiarly calm.

Today, I'm sailing from Indonesia to the Philippines, gazing at the smoothest possible Pacific Ocean. The water's surface is as glossy as a freshly polished sports car, flowing like mercury around the ship. Every other time I have crossed the equator, it's had the same lacquered look, and nowhere else compares.

The reason is scientific. This location is known as the Intertropical Convergence Zone, where the north-east and south-east trade winds converge and cancel each other out. At this narrow, low-pressure belt, the intense heat forces the air upward, rather than blowing horizontally, which prevents the formation of waves.

"Where north and south meet is where the sea becomes a mirror," says expedition guide Jacqueline Windh, who presented last night's lecture on Swan Hellenic's Minerva.

Although not guaranteed, these conditions are common enough to have shaped maritime history. Sailing ships could be stranded for days in this windless area, which seafarers called "the doldrums", likely derived from combining the Old English word "dol" (meaning dull or slow) and "humdrum". Over time, the expression became more widely used to describe feeling low or stagnant.

On cruise ships, the mood is lifted by a silly ceremony led by King Neptune, often played by the captain, accompanied by his consort and a judge, doctor, barber, pirates and mermaids. The light-hearted event honours a centuries-old naval tradition for novice sailors.