The 2019 winter solstice is at 11:19 p.m. ET on Dec. 21, marking the official start of the astronomical winter and the shortest day of the year for the Northern Hemisphere. In the Southern Hemisphere, that day is the summer solstice and the longest day of the year.
The 23.5-degree tilt in Earth's axis of rotation creates a rise and fall appearance of the sun over the course of a year. During the winter solstice, the Northern Hemisphere is tilted at its farthest distance from the sun, bringing less light and colder temperatures.
The tilt of the Earth - not our distance from the sun - is what causes winter and summer. In fact, Earth is closer to the sun during winter than it is in summer months.
The solstice is not the only major astronomy event this month.
On Dec. 26, an annular solar eclipse will be visible in the Middle East, southern India and parts of Indonesia. An annular solar eclipse occurs when the moon is too far away to block the Sun entirely, leaving behind a ring of fire in the sky.
Winter is here: Solstice marks shortest day of year