First day of winter, otherwise known as the » winter solstice. The word solstice means 'sun stopping,' because the sun appears to cease its southerly trek for a few days around this time each year, before heading back north.
Today is the longest night of the year (most hours of darkness). Starting tomorrow, the days begin to lengthen. The seasons are caused by a tilt (23-degrees) in the earth's axis .. as we make our annual trip around the sun.
Of course, if you live down in the southern hemisphere, everything is reversed. Today is your first day of summer, with the longest day of the year. Summer solstice.
Consider this » the earth is 92 million miles from the sun (give or take a few mil). Let's call that distance » the 'radius'. We can calculate the distance of our annual orbit by using the formula for the circumference of a circle » pi x diameter .. which = 3.14 x 184 million, which means we travel » 578 million miles every year (around the sun) ..
.. at a speed of 578-MM / 365 days per year = 1.58 million miles per day, which = .066 million miles/hour, which equals » 66,000 mph (if my math is correct). That's almost 100 times faster than the speed of sound. We're bookin'. Better fasten your seatbelt. Imagine hitting something at that speed. Ouch.