Eric – is there a shorthand rationale as to why the sunsets are getting later a week before the actual shortest day and why is it that after the summer solstice the sunsets remain at about 8:30 until they finally set earlier about July 4 and why it seems a fast ride down to earlier sunsets especially losing about 45 minutes alone in August? – Bill Graff
It’s true! You’re enjoying a whopping 4 extra minutes of light at the end of the day on the solstice than you were over a week ago. That being said, sunrises are still getting later in the morning, so we’ve still seen a net loss in daylight leading up to the solstice. Sunrises continue to get later until early January. So why is there this unexpected offset?
There are a few layers to this question. First off, it depends where on the planet you are. The lower your latitude, the more pronounced the effect will be. Above is a handy map from Brian Brettschneider, which shows how far apart your earliest sunset and latest sunrise are. Unless you live above the Arctic Circle, there’s going to be a discrepancy!
Now, we have to think about the concept of solar noon. In other words, when the sun reaches its peak in the sky. Think of ‘high noon’ duels in the old American west. But this doesn’t actually happen at noon all the time. There are seasonal shifts to solar noon, as the sun reaches the midpoint between sunrise and sunset at various times throughout the year. And here comes the next important element – a solar day isn’t actually 24 hours long. If you measured solar noon to solar noon, you’d find that it lasts longer. Inherently you know this – it’s why we have Leap Years! There is an extra 30 seconds in a solar day around the time of the solstice. So while solar noon may be 11:33am a few weeks before the solstice, it actually ends up being 11:43 on the solstice itself. The later solar noon ends up giving us a later sunset.
Okay, there’s one more layer to this. The earliest sunset may be ~2 weeks earlier than the WINTER solstice, but it’s not the same for the SUMMER solstice. For this answer, we go to orbital physics. I often find that many are surprised to hear we’re *closer* to the sun during the winter than during the summer (in the northern hemisphere). Perihelion, or our closest point in orbit to the sun, occurs in January (there’s another whole tangent we could go on here, about how our orbit actually changes on a cycle of tens of thousands of years, and that we’re not ALWAYS closer in January, but we’ll save that for another blog).
Since we are near perihelion during the winter solstice, we’re also moving faster in our orbit during the winter solstice. Gravitational forces from the sun are stronger at this time. During the summer solstice, we’re close to aphelion – our farthest point in orbit. This means we’re moving slower, and it makes the length of a solar day shorter (by about 17 seconds). So the sunset/sunrise offset from solstice is only one week instead of two.
Here’s one thing we can all agree on – it’ll be nice to get back on the daylight train!