Today at 11:44 UT (= 7:44 AM Eastern Time) the sun appeared to cross the celestial equator in its northward movement. (See Movement of Our Star for the real explanation.)
Most people in the Northern Hemisphere think of this as the first day of Spring. I prefer to think of it as the end of Winter. Springlike days are still a few weeks off in Toronto.
Here's a brief explanation from Wikipedia.
An equinox in astronomy is the moment in time (not a whole day) when the centre of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year.
More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e., declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point.
[Photo Credit: The photograph shows the position of the sun at various times throughout the year at 12h (UT) over the Temple of Aphaia (490-480 BC), Athens (Anthony AYIOMAMITIS). The image is called the solar analemma. See here for an explanation. The celestial equator is perpendicular to the long axis of the image and half way between the top and bottom. The crossover point of the figure eight depends on your distance from the equator. I'd love to see one taken on the equator or in the southern hemisphere.]
[Image Credit: eSky]