It has begun to feel like fall, with schools in session and a chill in the air, but the season actually arrives at the moment of autumnal equinox at 10:29 p.m. Monday.
At that instant, the Earth is facing the sun in such a way that the northern and southern hemispheres receive equal sunlight, and the length of day and night are roughly equal. In Baltimore, the sun will stay up for 12 hours or more until Friday.
It's the tilt of the Earth's axis that gives us seasons -- at the winter solstice, the axis is tilted 23.5 degrees away from the sun, and at the summer solstice, it's tilted 23.5 degrees away from the sun.
Meteorological fall started Sept. 1, however, and it will end Nov. 30, three weeks before astronomical fall ends.
An earlier version of this post inaccurately described the orientation of Earth and its axis at the equinox. The Sun regrets the error.