Northern hemisphere spring comes to an end and its summer begins at 12:04 CDT on 21 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
9 hours later the sun ‘will move’ into the region of Gemini as it crosses the boundary between it and 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.
Precession and its effects are perhaps not well known - a very slow regular motion of the Earth's axes of rotation that in a way
resembles the wobbly motion a spinning top has as the spinning slows down. The Earth, however, is not slowing down like the top!
Yet it is this spinning motion of precession that has shifted the location of the solstice from Cancer through Gemini now nearly 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.
With respect to the southern hemisphere this is the end of their summer and start of their fall season so my preference has been
to use the name of the month to designate the season change. Hence the use of the term June Solstice rather than the limited to northern hemisphere term summer solostice.
Click here to read a little more about precession at my blog.
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
9 hours later the sun ‘will move’ into the region of Gemini as it crosses the boundary between it and Taurus.
Yet it is this spinning motion of precession that has shifted the location of the solstice from Cancer through Gemini now nearly into Taurus
over the course of the last few millennium.
