Crossing the Tropic of Capricorn
We are now officially in the “tropics”. We've just crossed the Tropic of Capricorn aka the Southern Tropic, the southernmost latitude at which the Sun can be directly overhead at the December (southern) solstice. Its northern equivalent is the Tropic of Cancer and we'll cross it in the future as we head north to the USA and out of the tropics. We crossed the Tropic of Capricorn on land when we visited Namibia in 2007 and again just recently … at least there was a sign!
There seems to be some confusion as to when these imaginary lines were named and first used. Some references say “2000 years ago”; some quote the first known usage in the mid 16th century. Sources agree, however that at the time they were named, the Sun was positioned in the Cancer constellation during the northern summer solstice in June and in the Capricorn constellation during the northern winter solstice in December. Due to the Earth's axis and orbit variation, the Sun is no longer in these constellations today during the solstices, but the names have remained. The word “tropic”, by the way, derives from the Greek “trope” and means turn or change of direction, referring to the change of the sun's path at the solstices.
Interestingly enough, the Tropic of Capricorn is a moving target. It's gradually moving northward at a rate of 0.47 arcseconds, i.e. 48.75' (15m) per year. According to Wiki, by definition, the positions of the tropics are “dependent on the tilt of Earth's axis relative to the plane of its orbit around the sun (the obliquity of the ecliptic)”. On our older paper charts, the Tropic of Capricorn is fixed at around 23.5º south, but in actuality it's further north. This means that the area of the “tropics” is decreasing by ~425 mi²/year (1100 km²) Find out exactly where the Tropic is today by clicking here. It updates the location daily.
So … another milestone for Nine of Cups on its trip across the Atlantic. One more reason to celebrate.
The Tropic of Capricorn is one of the five major imaginary circles of latitude that are used on maps and globes of the Earth. Can you name the other four?