Stray Dogs That Amble In at the Spring Equinox
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From Bad Astronomy:
The Earth's spin axis is tilted with respect to its orbital plane. This is what causes the seasons. When the Earth's axis points towards the Sun, it is summer for that hemisphere. When the Earth's axis points away, you get winter (from the diagram above you can see that the north end of the Earth's axis never points directly at the Sun, but on the summer solstice it points as close as it can, and on the winter solstice as far as it can. That diagram is taken from Nick Strobel's excellent Astronomy Notes website). Midway between these two times, in spring and autumn, the spin axis of the Earth points 90 degrees away from the Sun. Note that this happens twice a year, in spring and autumn. ...
So on the first day of spring, the Earth's axis happens to be pointing perpendicularly to the direction of the Sun. Although it might seem like a special event, all it really means is that day and night have about the same length: 12 hours each, more or less.