Crossing the line

A couple of weeks into the cruise, we crossed into the southern hemisphere to take measurements and samples at the southern boundary of the Panama Basin. This was the first time crossing the line at sea (or indeed at all) for many on board, and as is tradition, King Neptune conducted a ceremony to mark the occasion. Pollywogs had to pay their respects to the ruler of the oceans and after a small penance had officially joined the ranks of shellbacks. 

The new recruits after crossing the line

By far the most widely believed falsehood about crossing the equator is that water in your bathtub or sink will drain rotating in the opposite direction. This common misconception is due to a misunderstanding of what is known as the Coriolis Force (not actually a real force, but a ‘pseudo-force’)- an effect caused by the rotation of the Earth on objects (e.g. the ocean and atmosphere) not fixed to the Earth’s surface. You can see the effect in the rotation direction of hurricanes and other weather systems, and in the movement of large bodies of water. 

Hurricane rotating counter-clockwise in the Northern Hemisphere

A simplified explanation for the effect is because the Earth is spherical, it rotates more quickly at the equator than at the poles, or the ‘linear velocity’ of the Earth’s rotation is larger at the equator than the poles. A point at the equator such as Quito in Ecuador must travel 40,000 km in one day to return to the same position, but Durham at 54 degrees N, for example, must only travel 23,000 km. One outcome of this effect is that if you fired a cannonball north from Quito, it would land to the right of immediately north even though the object was traveling on a straight course, as the cannonball would retain the faster linear velocity of Quito. Likewise, if you were at the North Pole, and fired a cannonball towards Durham, it would land to the right, as Durham has a faster linear velocity and would have moved further around. You can see the effect of this in the formation of a hurricane. Air moves from high to low pressure, so air moving from nearer the equator towards low pressure in the north would also look like it was moving to the right, thus storm systems tend to spin anti-clockwise in the northern hemisphere, and as a mirror-image, they spin clockwise in the southern hemisphere. 

Cannonball fired north from the equator has a faster linear velocity than the ground beneath it, so lands to the right of directly north. A cannonball fired south lands to the left of directly south, looking from the equator.

An analogy closer to home would be if you and a friend were standing on a merry-go-round, throwing a ball to each other. The ball would appear to have a curved path to you, as your friend would have already been moved round by the movement of the merry-go-round, but in fact the ball is travelling in a straight line to an outside observer. 

Winds appear to bend to the right in the Northern Hemisphere, creating counter-clockwise rotation about a low pressure zone

The myth about the water draining in your sink rotating in the opposite direction in each hemisphere is scientific misconception; the effect is real but the scale is too small to show a noticeable impact in this scenario. Other factors have a much greater effect on the direction of spinning: the geometry of the bowl, or the direction of input of water are much more important. A study done in Boston used a tank filled with over 1000 litres of water to investigate the Coriolis Effect. If left to stand after filling the tank, to let any movement disperse, and then emptied with a plug designed not to effect the motion too much, the average direction of rotation around the plug after many repeats of the experiment was indeed counter-clockwise.

 (For more information on this study, please see Ascher Shapiro's letter to Nature journal, December 1962, or for the equivalent experiment in the southern hemisphere, Lloyd Trefethen et al's letter to Nature, September 1965.)

Please feel free to ask any questions you may have in the comments section below.