Madrid to Rio de Janeiro

An Equator museum in Mitad del Mundo, Ecuador. In the foreground there is a sink to test the myth that you can see water flow down the drain in different directions either side of the line.

A monument marking the location of the Equator as it crosses Pontianak in Indonesia, one of 14 countries along the Equatorial line.

A monument marking the location of the Equator as it crosses Ilhéu das Rolas, in São Tomé and Príncipe, a small island nation off the west coast of Africa.

A world map marked with the line traced by the Equator, which crosses 14 different countries and 12 separate sections of sea and ocean as it passes around the world.

A road sign in Nanyuki, Kenya, marks the line of the Equator as it runs through the country. The line traced by the Equator crosses through 14 different countries as it passes around the Earth.

Mean average temperatures show how much higher Equatorial temperatures are (red, >30 degrees C) compared with those at higher latitudes (blue) where the Sun is less intense.

This diagram shows how the Earth's sunlight varies by equinox (period of neutral tilt) and solstice (period of maximum tilt) relative to the Sun - showing a substantial difference.

The tilt of the Earth means that the Sun's energy hitting the Equator is more concentrated than at the poles, where it is dissipated through a thicker band of atmosphere and covers a larger area.

This diagram shows how the tilt of the Earth and the distance from the Equator affects the seasons: here, the Southern hemisphere experiences summer as it is closer to the Sun.

The rotational axis of the Earth is tilted, so the line of the actual Equator (dark green) is very different from the line that would be plotted on the Earth without the tilt (light green).

A monument marking the location of the Equator as it crosses Bonjol in Sumatra, Indonesia. The Equator crosses Indonesian land masses and islands in five separate places.

The French explorer and geographer Charles Marie de La Condamine is commemorated at the Equatorial marker near Quito in Ecuador.

A monument marking the crossing point of the Equator in Queen Elizabeth National Park in Uganda.

An NOAA rainfall map shows an active April ITCZ off the coast of central America, with the heaviest areas of rain and storms marked in bright yellow and red.

A diagram showing how the ITCZ affects monsoon rain patterns over India, demonstrating that its effects are much wider ranging than immediately over the Equator.

The location of the ICTZ can vary quite substantially throughout the year, by as much as 20 degrees of latitude. This has significant effects on local weather conditions on the ground.

This diagram maps rising air in blue, and sinking air in red. The blue line across the Equator demonstrates the strong area of convection (as heat rises) along the ITCZ.

Sailing ships relied totally on the wind for any controlled movement, therefore days on end stuck in "the Doldrums" could move them dangerously off-course or cause critical delays.

A diagram showing the global prevailing wind systems, with wind north of the Equator tending to blow southwest-northeast, and south of the Equator from northwest-southeast.

An aerial view of heavy flooding caused by prolonged monsoon rains in Punjab Province, Pakistan in 2010, which was influenced by the prolonged action of the ITCZ.

1984 - Drought-ravaged Senegal was severely affected by lack of rainfall, failure of crops and widespread famine after repeated years of failed rains, influenced by the ITCZ.

Women rush to a food centre set up by the UN in Somalia in July 2011, after widespread hunger and famine caused by the worst drought experienced this century.

2011 - This image shows the impact of the on-going drought, caused in part by the location of the ITCZ, in East Africa. Dark red shaded areas indicate crops that are weak or failing.

This thermal image (where black is hottest and white is coolest) shows the thick belt of ITCZ cloud and an active hurricane, Bill, in the Atlantic Ocean just to the north of the ITCZ.

The Intertropical Convergence Zone, or ITCZ, is clearly marked here by a thick belt of rain clouds, caused by the intense heat and humidity of conditions over the Equator.

A huge thunderstorm within the cloud belt of the ITCZ as viewed from an aircraft at 13,000m. The intense convection over the Equator results in powerful storm clouds.

US Marines, pollywog sailors, are brought before the court of "King Neptune" who will decide if they are worthy of becoming shellbacks, all part of "crossing the line".

Sailors of the Brazilian Navy take part in initiating pollywog sailors who have not crossed the Equator into the "Kingdom of Neptune". Ceremonies have taken place since the 18th century.

1830 - A print depicting 19th century rituals and celebrations of the ship Ioâo crossing the Equator. Many sailors were beaten or flogged as part of the initiation ceremony.

Two newly initiated shellback sailors celebrate having "crossed the line" as their ship crosses the Equator, taking part in traditions which have taken place for hundreds of years.

US Sailors and Marines participate in a crossing the line ceremony aboard USS Blue Ridge as it crosses the Equator, as the pollywog sailors are initiated into "Neptune's kingdom".

A 3rd century Roman mosaic depicting King Neptune, thought to be god of the seas: his legend takes a central role in "crossing the line" ceremonies even today.

1908 - A certificate issued to a sailor aboard the USS Virginia, celebrating that they became a "trusty shellback" as they crossed the line, signed by "Neptunus Rex".

Some airlines also issued passengers with certificates or mementos to confirm that they had flown over the Equator, such as this 1965 certificate from QANTAS.

1979 - The ceremonies and rituals involved in "crossing the Line" often involve the whole crew, and are often a welcome break from the monotony of long passages at sea.