Northern hemisphere spring comes to an end and its summer begins at 6:07 pm CDT on 20 June as the sun ‘reaches’ the celestial coordinates of 23.5oN and 6 hours right ascension. With respect to the Earth’s surface the sun is described as over the Tropic of Cancer, 23.5oN of the Earth’s equator. At this same time the sun is still within the boundaries of the constellation Taurus the Bull - but just barely. Interestingly a few hours later the sun ‘will move’ into the region of Gemini as it crosses the boundary between it and Taurus. This is interesting in that the Sun is still in Cancer according to astrology, and we still use the Tropic of Cancer despite the fact that the Sun, while still in Taurus, is actually nearly in Gemini. So why not the Tropic of Taurus?
   We know that it is the Earth’s orbital motion around the sun giving rise to the sun’s apparent eastward motion amongst the stars in the background. This is how the sun ‘reaches’ a celestial coordinate, how it ‘crosses’ the boundaries between constellations, or how it is ‘in‘ a constellation. What is perhaps not well known is about a very slow regular motion of the Earth, precession, a motion of the Earth's axes of rotation that in a way resembles the wobbly motion a spinning top has as its spinning slows down.
   The Earth, however, is not slowing down like the top! It is,however, this spinning motion of precession that has shifted the location of the solstice from Cancer through Gemini into Taurus over the course of the last few millennium. The name Tropic of Cancer was derived from the time period when at this date the sun was within the boundaries of Cancer the Crab. The date is referred to as the summer solstice (sol = sun; stice = standing over) and it was a reference to the point where the sun would get no higher in the sky. It was standing, in a sense, over a certain latitude on the Earth’s surface - The Tropic of Cancer, as well as within the constellation Cancer