Winter is finally here, almost. Our weather has been so strange, one week cold and the next week so warm that it's hard to believe winter will soon arrive. The December solstice comes on Dec. 21 at 9:48 p.m., which is the longest night of the year. That's when winter arrives.
This is when the Sun reaches its southern most point for the year, which is why the southern hemisphere is entering summer, while we enter winter. It also gives us our longest period of darkness and shortest period of daylight for the year. Great for sky watching if you don't mind the cold!
What's interesting is that the longest days of the year come each December for the entire planet. These days are measured from one solar noon to the next. Earth takes about half a minute longer to complete a rotation in December. That doesn't seem like much for one day, but it does add up over three months. We can't measure this with our clocks, but a sundial could.
Earth's perihelion, our closest point to the Sun, comes early in January. When we're closest to the Sun, Earth moves a little faster in its orbit around the Sun. This causes us to travel through space a little farther than average each day, and Earth has to rotate a little farther on its axis for the Sun to return to its noontime position. This creates the longer solar day.
Because of this, the northern hemisphere's earliest sunset precedes the winter solstice, and our latest sunrise of the year comes after the winter solstice. The southern hemisphere is just the opposite. The actual time depends on how far north or south we live, but can be one or two weeks before or after the solstice.
Because we're closest to the Sun in early January, our winter is the shortest of all of our 4 seasons, and the opposite happens with the summer solstice. Since Earth is farthest from the Sun in early July, it moves more slowly in its orbit. So our summer, and the southern hemisphere's winter are the longest seasons. Winter is 88.99 days, spring is 92.76 days, summer is 93.65 days and fall is 89.84 days.
That's enough seasonal education. The Ursid meteor shower always peaks around the solstice. This year it's the night of Dec 22. Obviously, the best time is early in the morning of the 23rd. Sometimes they have a burst of 100 meteors per hour, but usually only 5-10. So if you're up early, look in the north near the Little Dipper. They'll be visible for several nights.
This is the last night for the Gemenid Meteor shower. Hopefully, you got to see it. The full moon is on the 19th, but it occurs late on the 18th. So that's an early morning viewing after 3 a.m. Venus is still shining brightly in the SW after the sun sets, and soon Mercury will appear.