for anyone that wants to know why dec 21st is the shortest day of the year

In the Northern Hemisphere, the winter solstice always occurs on or about December 21 and marks the beginning of the winter season. As many people notice, it's the shortest day of the year, featuring the least amount of daylight between sunrise and sunset.

In the Southern Hemisphere, this is the time of the summer solstice and the longest day of the year. From now on, as the northern days grow longer so do the southern days get shorter.

The term solstice means "sun stands still." On the year's two solstices (winter and summer) the sun appears to halt in its incremental journey across the sky and change little in position during this time. Of course, contrary to appearances from Earth, the sun's "changing position" throughout the year is actually caused by the rotation of the Earth on its tilted axis as it circles the sun each year.

The solstice occurs twice a year (around December 22nd and June 21st) when the sun is farthest from the tilting planet's celestial equator.

For half of each year the North Pole is tilted toward the sun, and for half of the year the South Pole enjoys that privilege. This phenomenon creates our changing seasons, because the hemisphere facing the sun receives longer and more powerful exposure to sunlight.

In the Northern Hemisphere, the winter solstice occurs in December when the tilting of the earth makes the sun appear to be furthest to the south and furthest away. In the Southern Hemisphere, both the seasons and solstices are reversed.

contrary to popular ignorance about anything related to astronomy, the changing seasons on earth are caused by the earth's 23 1/2 degree tilt on it's axis, and not because the earth is any closer or farther from the sun.

by the way, the diagram is not to scale. the sun is ENORMOUS! it has a volume 1.3 million times that of the earth...

to give you some sense of how much bigger that is, imagine you had to construct 2 boxes, one representing the earth, the other the sun. both boxes must be to scale.
to make things easy, let's say we make the first box equal to 1ft long x 1 ft wide x 1ft deep.
now, that's the planet earth. in order to make our second box to scale, were going to need a much bigger box.

this box will have sides of 246 miles long x 246 miles wide x 246 miles deep...