A Noob's question on declination
I don't think this particularly useful way to think about it. The equator is the equator, Cancer is 23.4 North, and Capricorn is 23.4 South. The equator is a great circle on the sphere of the sky, but the Tropic of Cancer is not a great circle.  These are fixed.  

Now (in Northern Hemisphere summer) the sun is directly above a point on (or near) the tropic of cancer.  This doesn't make the tropic of cancer a "virtual equator". It just makes it the tropic of cancer in summer.

It makes a difference because the path of the sun seen from the equator on midsummer is not the same as the path of the sun seen from the tropic of Capricorn on the equinox.  On the equinox, the sun traces a great circle in the sky (from wherever you view it) On midsummer's day the sun doesn't trace a great circle (anywhere).