Earth reaches the autumnal equinox at 4:44 p.m. Sunday, marking the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere.
At the equinox, the Earth is spinning upright relative to the sun -- this only happens twice a year because the Earth's axis is tilted by 23.5 degrees. During our fall and winter, the axis is tilted away from the sun, while during spring and summer the North Pole is tilted toward the sun, giving us more direct sunlight and warmth. (The axis always points the same direction, as the above graphic explains; its orientation relative to the sun just changes as the planet revolves around the sun.)
This means that the sun rises from due east and sets due west and that the length of day and night are equal, at least at the equator. In Baltimore, day and night aren't equal until Wednesday, a lag that occurs because of our distance north of the equator.
The equinox marks the beginning of the march toward the winter solstice, as the northern hemisphere gradually tilts further and further from the sun until the shortest day of the year, Dec. 21. The days have been growing shorter since the summer solstice June 21, but once the autumnal equinox passes, the length of night hours surpasses the daylight hours.