Questions about equinox of March 20th,2011 day to night ratio?

A question was brought up in my high school physics class and I was wondering if anybody will be able to help me. What is the correlation between the start of spring and this equinox? It could be suspected that on this day that the hours in the day would be equal to the hours of the night, but as my teacher had stated, this is not true. The times are not exactly half and half, so my question is, what are the reasons behind this? What are the minor differences between the hours? What time does the day end and night begin? References to astronomical terms, and any responses will be greatly appreciated.