Tag Archive: Equinox


View out the hospital widow when I'm walking for exercise.

View out the hospital widow when I’m walking for exercise.

The sun will rise today in Fairbanks at 7:34 in the morning, and set 12 hours 16 minutes later at 7:51 pm. Yes, it’s the equinox today – at 6:29 pm, to be exact. But equinox does not mean 12 hour days

Why?

In the first place, sunset and sunrise are defined by when the top edge of the sun touches the horizon. Twelve hour days would be true if the center of the sun was the defining point.

Second, where you see the sun is not where it is actually located, especially when it is near the horizon. Even seen a road mirage where it looks as if a hot road is covered with water? What you are really seeing is the light of the sky, bent by the strong temperature gradient near the road. The opposite kind of bending occurs when the air near the ground is denser that that higher up, which is the normal case in the atmosphere. In this case, the sun looks higher in the sky than is really the case. The result is that days at the equinox are really a little bit longer than the nights.

It didn’t rain much in Fairbanks between the first few days in September and the last weekend, though we did get a good dollop last Saturday. Anchorage (I’m still in the hospital there) has been making up for it. Well over 4″ so far, and raining most of the time I’ve been in the hospital here.

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Alaskan Trees, 9/21/13The sun rose this morning at 7:38, and will set 12 hours, 8 minutes and 33 seconds later at 7:46 this evening. We’re still losing 6 minutes and 37 seconds a day.

Wait a minute. Yesterday at 12:44 pm was the Autumnal Equinox, the time when day and night are supposedly equal at 12 hours each. What are we doing with days still longer than 12 hours?

Actually, the 12 hour days on the equinox would be true if (a) the sun were a point source of light and (b) the earth had no atmosphere. Since neither is true, the days are still longer than 12 hours today and tomorrow. Why?

The sun actually has an apparent diameter of half a degree, and sunrise and sunset are defined as when the upper edge of the sun is just on the horizon. Further, the air is densest near the ground, which means that the light rays are to a certain extent bent around the Earth’s curvature. This last means that there is actually a day-to-day variation in the difference between the geometrical sunrise and the observed sunrise, so the times I give are only approximate. Specifically, they are taken from a website that calculates them based on a standard atmosphere (which is certainly not the case here in the winter!) To quote from the website I use:

Sunrise and Sunset

My yard, 9/20/13The times for sunrise and sunset are based on the ideal situation, where no hills or mountains obscure the view and the flat horizon is at the same altitude as the observer. Sunrise is the time when the upper part of the Sun is visible, and sunset is when the last part of the Sun is about to disappear below the horizon (in clear weather conditions).

If the horizon in the direction of sunrise or sunset is at a higher altitude than that of the observer, the sunrise will be later and sunset earlier than listed (and the reverse: on a high mountain with the horizon below the observer, the sunrise will be earlier and sunset later than listed).

The Earth’s atmosphere refracts the incoming light in such a way that the Sun is visible longer than it would be without an atmosphere. The refraction depends on the atmospheric pressure and temperature. These calculations use the standard atmospheric pressure of 101.325 kilopascal and temperature of 15°C or 59°F. A higher atmospheric pressure or lower temperature than the standard means more refraction, and the sunrise will be earlier and sunset later. In most cases, however, this would affect the rising and setting times by less than a minute. Near the North and South Poles it could have greater impact because of low temperatures and the slow rate of the Sun’s rising and setting.

For locations north of 66°34′ N or south of 66°34′ S latitude, the Sun is above the horizon all day on some days during the summer and below the horizon all day on some days during the winter.

Technically, sunrise and sunset are calculated based on the true geocentric position of the Sun at 90°50′ from the zenith position (directly above the observer).

In case you’re wondering, my house is at 64° 33′ North latitude.

The photos above, by the way, were taken Friday. Saturday night and yesterday we had mixed snow and rain all day. I’ll add a current photo below as soon as it’s light enough to take one, but I fully expect an inch or so of snow.

9:23:13 Nyard

This morning, 7:50 am

No, that's not Alaska! My sister took this photo this past weekend in California. I couldn't resist sharing the snow-covered palms.

The sun rose (past tense!) this morning at 7:53 and will set this evening at 8:06, for 12 hr 13 min of daylight. What, you say, more than 12 hours when the equinox isn’t until tomorrow? The combination of atmospheric refraction and the finite diameter of the sun always ensures that the day is a little longer than geometry would have it, and as far as I’m concerned, it’s spring!

Not in temperature. That’s been above zero most days and below zero most nights, and there’s been virtually no melting of the snow so far. It has, however, settled. A week ago we had close to 2 ½’ on the ground; today it’s just a little over 2’. The sun, when it’s out, is high enough that a fair amount of energy is absorbed within the snowpack, and the bonds between crystals are slowly collapsing. I don’t think we’re losing any mass yet, though, except by evaporation. We can get 40 below this late,  so I’m not complaining about temperatures in the teens.

Besides, look what my sister ran into day before yesterday in California!

It’s light long enough in the evening now I can get out and do things. Yesterday afternoon I attended a matinee performance of The Music Man put on by FLOT, and was actually able to drive home in daylight afterwards.

#WriteMotivation:

One thing that wasn’t on the list: I made transfers from my book cover, with the award sticker, and applied them to a tote bag and T-shirt. I also revamped my bookmarks.

1. Learn to use at least one legal method of getting images other than photos I’ve taken on my blog. DONE, though I’m still looking for an African brush fire for next week.

2. Continue to blog at least 5 days a week. (I’m doing 7 now, but I’ve signed up for a number of adult classes in March.)  On Track. I’ve posted every day so far this month, and I have 7 written and scheduled for the 12 days remaining. The other 5 are still in the idea stage, but I do have ideas. And I’ve made my OLLI classes so far, even the day I managed to high-center the car in a whiteout and had to get it pulled out by a tow truck.

3. Edit Chs 2 and 9 of my WIP to give more showing, less telling. (Ch 2 has been on Six Sentence Sunday; Ch 9 is the next section from Tod’s POV.)  I haven’t even looked at Chapter 9 yet.

4. Participate in at least one Platform-building challenge. I did both.

Fairbanks Weather 9/26/11

It’s fall—and to prove it we’ll have only 11 hours and 52 minutes of daylight today. The sun rose at 7:45 this morning and it will set at 7:37 this evening – no more attending things that start at 7 pm, unless I can be sure of a ride back. At its highest the sun will be not quite 24° above the horizon, and days are now longer than those everywhere to the south of us..

Officially, we started fall last Friday at 1:05 in the morning, but it wasn’t until Sunday that we got down below 12 hours of daylight. Why? Because sunrise and sunset are defined according to when the top of the sun, not the middle, is just visible on the horizon. To be exact, you actually have to take into account also the fact that the atmosphere curves the path of the light rays slightly, so that the actual position of the sun is always a little lower in the sky than what our eyes tell us. This is only important when the sun is very near the horizon, of course, but at high latitudes, where the sun rises and sets at a very shallow angle, it can make several minutes difference in the time of sunrise and sunset. This also changes the apparent direction of sunset and sunrise – on the day of the equinox the sun actually rose 2° N of due east, and set 1° N of due west.

The weather has, sad to say, caught up with the season. We had a frost Saturday night, and only the hardiest plants are still going strong. I pulled the rest of the beets yesterday, and picked the few beans that were ready, as well as removing the hoses and laying them out to drain. I’m glad I brought in the potted plants last week. Next step? The potato bag.

The native deciduous trees have lost most of their leaves, with the exception of a few golden holdouts, and even exotics like my Amur maple are close to dropping their foliage. The world has changed form green to shades of tan. Even the evergreens are darkening. Good-bye, summer. See you next year.

We broke 12 hours of daylight Friday, 2 days before the equinox. Why? And is this just due to my being close to 65 degrees North, or is it a more general anomaly?

There are two parts to this peculiarity. One is latitude combined with the finite diameter of the sun, which can be calculated. The other is the refraction of the atmosphere, which varies from day to day and can only be estimated.

Let’s take latitude first. Sunrise and sunset are defined as the time that the upper edge of the sun is just visible above a flat horizon. “Equal days and nights” (which is what equinox means) assumes the dividing line between day and night is the time when the center of the sun is on the horizon, assuming light moves in straight lines. If the sun rose vertically, as it does at the equator, it would rise at a rate of about 1 solar diameter a minute, and the calculated sunrise time based on the center of the sun would be only half a minute after the time the upper edge first showed.

At higher latitudes, however, the sun appears to rise at an angle and sunrise and sunset appear slower. At 65 degrees latitude the sun’s path at the equinox is 65 degrees from the vertical, and a little trigonometry stretches that half minute to about 1 minute 10 seconds, or twice that in day length. Latitude alone is still not enough to allow our days to be 12 hours 15 minutes long at the equinox. For that, the refraction of the atmosphere becomes important.

The apparent break in the spoon handle is due to refraction.

Everyone is familiar with refraction, though you may not know it by that name. The optical illusion of a broken spoon in water is caused by the fact that the speed of light in water is less than that in air. Yes, the speed of light in vacuum is constant, but in any other transparent medium it moves a little slower. When it crosses a boundary between two transparent media with different speeds of light, any light rays not moving at a right angle to the boundary are bent. Air is one of those transparent media, and while the speed of light in air is not a great deal slower than that in vacuum, there is enough of a difference that the bending affects what we can see.

The actual difference in speed depends on the density and moisture content of the air, which in turn depend on pressure, temperature and relative humidity. Air near the ground is almost always denser than that above it, and this is particularly true at sunrise. The change with height is gradual, and thus the light rays are not bent sharply, as in the water-air interface, but curved along the earth’s surface. Objects far away appear higher than they are, and this certainly applies to the sun at sunrise. The amount by which the sun appears higher in the sky than it really is will depend the atmospheric density and how it changes with height.

For practical purposes the time of sunrise is calculated assuming that the upper edge of the sun is visible when the center of the sun is 50 minutes of angle—almost a degree—below the horizon. This also means that the sun at the equinox will rise not quite due east, as it “rises” while it is still physically below the horizon and slightly north (in the northern hemisphere) of east. The difference, however, is slight.

Refraction is also responsible for the fact that the sun appears to flatten as it approaches the horizon when setting or just after rising. The part of the sun closest to the horizon is more strongly affected by atmospheric refraction than is the upper part of the sun, so the two appear pushed together and the sun appears flattened, rather than round. I’ve probably overused this in Tourist Trap.

Equinoxes and Daylight Saving

#daylightSavings As long as I’m on the subject of the #equinoxes, I’d like to point out that day length (and the times of sunset and sunrise, and even the sun’s path across the sky) is symmetrical around the summer solstice. If the purpose of daylight savings is really to encourage people to take advantage of the fact that sunrise is earlier in the summer, its starting and ending times should also be symmetrical around the solstice. They aren’t. Apparently politicians don’t realize how day length works. They have decreed that daylight time starts shortly before the spring equinox (day and night equal), but ends more than 6 weeks after the fall equinox. Here in Fairbanks, we’re getting up before sunrise in November regardless, and shifting off daylight savings only moves the 5 pm traffic from dusk to full dark. Daylight savings should start about the same length of time before the spring equinox as it ends after the fall equinox, as that would have sunrise and sunset times about the same at the start and end of daylight time.