You won’t notice it, but from now on, each day in 2021 will have a little bit less light. You might notice that sunrise and sunset, which have been happening a little farther north on the horizon each day, now start moving the other way, a little bit farther south. When ancient people saw this, sometimes they built monuments to mark the spot where the sun rose and set at its extremes. We call these monuments names like Stonehenge, pyramids, and mounds today. Here in Teton Valley, we don’t need to build monuments, just notice which mountain the sun rises and sets behind when seen from your home, and there you go.
But why does this happen? If the Earth always turns at the same speed, why are some days “longer” than others, with more sunlight? Well, the Earth does turn at the same speed all the time, but sometimes it leans toward the sun, and sometimes it leans away. When the North Pole is tipped toward the sun, we get more sunlight and more heat, and we call that summer. Last Sunday night, the North Pole was tilted more toward the sun than any other time, so we had what some people call “the longest day,” some call “the first day of summer,” and some call the “summer solstice.”
In half a year, the North Pole will be pointed away from the sun, and we will have “the shortest day,” “the first day of winter,” and the “winter solstice.” This week marks the beginning of the annual journey toward that point.
Even though there was more sunlight last Sunday than any other day of the year, it wasn’t the hottest. The hottest day of the year here in Teton Valley tends to be about five weeks after the summer solstice. There is a lag. This lag is because it takes the sun time to heat things up to the max, sort of thermal inertia of the atmosphere. Think of it as putting a coffee pot on a campfire; the coffee takes a while to get hot, but not five weeks, thank goodness. The atmosphere is a much bigger pot.
So here we go toward winter in terms of sunlight, but not quite yet in terms of temperature. It’s gonna get hotter. Watch summer unfold, and let us know how hot it gets for you on the Teton Valley Weather Facebook page and what changes you notice in sunrise and sunset times and location here in our own little natural Stonehenge.