The Summer Solstice happened earlier this year than others I've checked before. The exact minute of this years summer solstice was 4:24 a.m. EDT (0824 GMT) . It's now officially summer, and today is the longest day of the year. I used to wonder if anyone has ever measured that with the extremely accurate clocks we have these days. There's always some wobble to the Earth such that times can move around with days milliseconds longer or shorter than predicted. Today is supposed to be 0.3664 milliseconds (or 366.4 microseconds, if you prefer) longer than 24 hours.
Screen capture from TimeandDate.com
The sunrise/sunset app I have on my phone (really old - I think it's not supported anymore) says sunrise this morning was in the minute it calls 6:26 AM. The earliest sunrise of the year was called 6:25 AM and the phone app said sunrise was 6:25 from June 2 until June 19. Sunrises will get later every morning from here until the new year. Sunset, meanwhile is 8:21 and doesn't reach the latest sunset of the year, 8:23, until June 25 and stays there until July 8th. After that, sunsets get earlier until around the end of November.
You were expecting symmetry?
It's there, but not what most people seem to expect. Sunrise and sunset don't both move smoothly getting earlier and later reaching their min/max on the solstice. In the summer, the earliest sunrise is before the solstice, latest sunset is after. In the winter, the earliest sunset is before the solstice, late November/early December and latest sunrise is after it - the first week of January. That seems like mirror image symmetry. I should emphasize this in the northern hemisphere. I've never checked this pattern in the southern hemisphere.
I went looking through the blog history searching for articles on this and found that I've only written about the summer solstice a couple of times before, but have written about the winter solstice more times. That's probably at least partly because I greatly prefer our winter over our summer, so I look forward to the winter solstice more.
If you've ever spent time with a good globe, you'll know about an analemma, that shows the motion of the sun - usually shown as seen from the equator. I've posted one pretty picture an amateur took of the sun that recorded the phenomenon for a full year. That was in a winter solstice post.
You've thrown me here. Because it seems that you're conflating rotational day with daylight "day". I won't claim that it's wrong to state that the length of a rotational day is longest at the summer solstice, but I've never before encountered that claim.
ReplyDeleteUnfortunately, there's no web searching to sort that out, as even attempts to use terms such as "astronomical", "synodic", or "sidereal" don't filter out results talking about "daylight" day.
It surely makes me wonder about how that could be true. I can only imagine a gravitational effect, due to being farther away from the sun. But I remain dubious.
- jed
I should let things percolate a bit before commenting. Still...
DeleteI'm thinking that due to the elliptic nature of Earth's orbit, the longer radii at the equinoxes make for less difference between the sidereal period and the synodic.
Or, it's even more weird than that.
- jed
The best explanations I know of are at that timeanddate.com and the second link in the post is to them. Especially go to A Day is Not Exactly 24 hours. (Yeah you can get there by going to the link in the post and just doing Page Up until you run out). It's a combination of our elliptic orbit, the ~23 degree inclination of the Earth's axis, and the effects of the nearest handful of planets - primarily the moon, Venus and Mars due to proximity and out to Jupiter which has an effect primarily because of how big it is.
DeleteNope, there's no assertion there, or correlation between the summer solstice and the longest solar day. In fact, if you look at the history of solar day length for the past 53 years, June doesn't appear at all. Just glancing, Mar-May look like the most common months. And there's no consistent pattern of a 6month (appx) between longest and shortest days. June appears 8 times in the shortest days, which appear most frequently in July and August.
Delete- jed
The tilt of the earth was in the USAF SATCOM maintenance tech lecture on the differences in the sunrise and sunsets. That was in the first hour of instruction then the math kicked in.
ReplyDeleteI still have my mil issue pocket transit. Once the Sat Finder app came out the transit has had little use.