We think of the winter solstice as the "shortest" day of the year, but the solstice actually occurs at an instant, when the sun is the furthest south in the sky because of the Earth's tilted axis.
This year, that happens Saturday at 17:11 Coordinated Universal Time, which is 12:11 p.m. Eastern Standard Time. There will be 9 hours, 24 minutes of daylight in Baltimore.
The Earth's axis tilts at a 23.5-degree angle, which is what brings the seasons, and at the point of the winter solstice, the North Pole is tilted furthest from the sun. Starting Saturday afternoon, the tilt will begin shifting upright until the Vernal Equinox.
The solstice marks the start of winter in the Northern Hemisphere, of course. Although the hemisphere reaches its furthest from the sun Saturday, the coldest weather lags a month or two, with January and February, on average, colder than December here.
At the solstice, the Arctic circle is in 24-hour darkness, while it Antarctica is in full sunlight.