Happy Solstice! Today at 23:09 UT (4:09pm Pacific, 7:09 Eastern) the Earth’s axis points as much toward the Sun as it possibly can. This marks the changing seasons. That’s a kind of complex statement, so let me try and explain it.
The planet Earth rotates around an axis and orbits around the sun. If you think of the Earth as moving on a plane (like the surface of the table), it could be rotating in a lot of different ways, which would lead to very different seasonal patterns. If it rotated like a perfect top, with the rotational axis pointed straight up, then we’d have no seasons because the Sun would illuminate from south pole to north pole every day of the year.
If instead, the Earth rolled around the Sun, with its rotational axis parallel to the surface of the table, we’d have extreme seasons as we went from the Sun only illuminating the Northern Hemisphere to the Sun only illuminating the Southern Hemisphere. The point when the North Pole directly points at the Sun would be the Northern Summer, and the time when the Southern Pole directly points at the Sun would be the Southern Summer. In between these times will be points when the Sun hits the Earth equally on both hemispheres, with the light spanning from pole to pole and the Sun appearing directly overhead on the equator. This is very close to what happens with the planet Uranus, which has its pole tilted just 3 degrees off of this parallel situation.
The planet Earth is intermediate to these two situations. Like a top ready to fall over, it is tilted about 23 degrees off of straight up.
Today is that 1 day of the year when the North pole of the Earth is as tilted toward Sun as it will ever get. If you define a plane that includes the rotational axis of the Earth and is perpendicular to the Earth’s orbit, today that plane also cuts through the Sun. The Sun hits the Earth directly over the Tropic of Cancer, and is as far North in the sky as it will ever get. In the Northern Hemisphere, today is the longest day of the year, and in the South (where sunlight hits at its steepest angle of the year) today is the shortest day. From here on out, the changing of the days reverse direction. Here where I am in the North, all the heat trapped in the warm earth and sea continues to build us toward the hottest days of the year.
For most of human history, calendars were based on special days like today. The number of days in a year isn’t an integer – it isn’t a whole number. You can’t just count the days to get from one solstice to the next; a complex pattern of leap days is required to keep the calendar days in sync with the Sun. It took a long time to sort out how to insert leap days into the passing years, and until that could be done, …. we had the Sun. It is relatively easy to define a place to stand and a rock to watch the Sun set over and say “Today, because the Sun set there, I know it is the Solstice.” Rocks and the Sun; that is all you need to build an accurate calendar if you are just patient enough to make sure all the rocks are in the right place.
And now you know.