Today’s Wonder of the Day was inspired by Mrs. Jacob's Rockstars from MO. Mrs. Jacob's Rockstars Wonders, “Why do the days get shorter in Winter?” Thanks for WONDERing with us, Mrs. Jacob's Rockstars!

Brr! Do you feel that chill in the air? Does it feel like winter where you live? Even though many of us equate dropping temperatures with the start of winter, the season actually kicks off at a slightly different time each year.

For most of us, we look around us for signs of the seasons changing. Frigid temperatures mean winter is here. Warmer days and the appearance of flowers mean spring has arrived. Sunny, hot days signal the start of summer. Cooler days and falling leaves tell us when autumn is near.

For scientists, however, there are specific scientific phenomena that they use to determine the official start of each season. When it comes to winter, that season begins in the Northern Hemisphere on the winter solstice, which is also known as the December solstice. In the Southern Hemisphere, the December solstice is called the summer solstice, since it marks the beginning of summer there.

The winter solstice marks the point in time when the North Pole is tilted at an angle (23.5 degrees) farthest away from the Sun. Because the exact time when that event occurs can vary depending upon the rotation of the Earth on its axis, the revolution of the Earth around the Sun, and the insertion of leap days into the calendar, the winter solstice occurs sometime between December 20 and December 23 each year.

In general, December 21 or 22 solstices happen more frequently than December 20 or 23 solstices. The next December 20 solstice won't roll around until 2052, and you'll have to live until 2303 to see the next December 23 solstice!

In 2019, the winter solstice will occur on December 22 at 4:49 universal time. If you live in the United States, that means the winter solstice will occur on Monday, December 21 at 11:49pm Eastern Standard Time.

In addition to marking the beginning of winter, the winter solstice marks the shortest day of the year. On the day of the winter solstice, the Northern Hemisphere receives the fewest hours of daylight of any day of the year. On the plus side, each day moving forward will have a little more sunlight. On the negative side, the coldest days of the year are ahead.

After the winter solstice, the amount of daylight each day will gradually increase as the Sun follows a higher and longer path through the southern sky. This occurs for about six months until the summer solstice occurs, at which point the days begin to get shorter again.

Despite increasing daylight each day, it takes a while for oceans to warm up. In addition, the Northern Hemisphere continues to lose more heat than it gains for several more weeks. This is why the coldest temperatures are yet to come after the winter solstice before things begin to heat up toward spring.

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