When do the seasons start?

There are four astronomical seasons on Earth, defined by the movement of the sun in the sky. For the northern hemisphere:

• Spring starts at the moment when the sun is directly over the equator, going from south to north: the "vernal equinox"'.
• Summer starts at the moment when the sun is farthest north: the "summer solstice".
• Fall starts at the moment when the sun is directly over the equator, going from north to south: the "autumnal equinox".
• Winter starts at the moment when the sun is farthest south: the "winter solstice".