Category: Astronomy


The sun will rise this morning at 8:02, and set 11 hours 57 minutes later at 7:52 this evening. Since we’re now gaining 6 min 43 seconds a day, by tomorrow we’ll have more day than night.

But the equinox won’t be until 8:57 am Thursday the 20th! Why do we have equal day and night lengths before the equinox?

At the risk of sounding like a broken record, I’ll explain again. The equinox is defined by true sunrise and sunset, when the center of the sun would line up exactly with the horizon if light rays traveled in straight lines and your eyes were right on the ground. None of these assumptions are true.

In the first place, sunrise and sunset are defined by the first (and last) appearance of the top edge of the sun. At the equator, this differs by about a minute from the time as defined by the center of the sun. Where I live it is closer to 2 minutes.

The position of your eye doesn’t matter much normally, but I’ve seen the midnight sun (not normally visible at Fairbanks) from a small plane.

Finally, the atmosphere bends light, especially near sunrise and sunset. Exactly how much depends on the density structure of the atmosphere, which up here tends to produce a lot of bending. The numbers above are based on a standard atmosphere, so it is possible that the day length today is even longer than calculated.

At any rate, it is beginning to feel like spring. We had several thawing days last week, and I am sorry to say that some of the ice sculptures are beginning to melt, especially those with delicate parts. I went out again Friday to get some photos with the sun in a better position, and found a group of school buses unloading children! Luckily most were far more interested in the slides than the sculptures, so I was able to get some good shots. Some still looked pristine; some were slightly melted; some were partly collapsed. Here’s a pair of photos showing some of the damage wrought by the warm weather. Both photos are of “Guardian of the Deep” sculpted by Chris Foltz, Dean Murray, Jillian Howell and Amela Rombach, all of the USA. If you compare the two, you can see that Neptune has lost his trident, and the seahorse has lost a hoof and part of his mane.

Guardian of the Deep coldGuardian of the Deep 3:14:14Final comment: the warm weather seems to have ended. The coming week is forecast to be about normal: partly cloudy, no precipitation, highs in the 20′s and lows zero or a little below.

Snostk 3-8-14Daylight savings, and we’re back to sunrise at 8:27 am. We’re actually on double daylight savings based on longitude, as Alaska Standard Time is already an hour farther east than our true longitude, and 2 hours east for Nome, also in the same time zone. This far north, true time zones are close together.

At any rate the sun will set tonight at 7:37, after a day almost 11 hours 10 minutes long. The Equinox is coming*; only about a week and a half to go, now!

Temperatures this month have been slightly above normal until the weekend, with highs mostly in the 20’s and lows sub-zero – ideal for the ice park, where it’s warm enough that water can be used as glue to weld ice blocks together without being so warm that the ice melts. We did have a couple of inches of snow midweek, and the backyard snow stake now reads very close to two feet. We’re also getting into a cold spell over the weekend (highs not always above zero) but it’s forecast to be pretty short.

*At 8:57 on March 20 ADT, to be precise.

Winter Soltice today!

The days are getting longer!

This post is scheduled to go live at 8:11 Saturday morning, Alaska time, at which time the earth reaches the point in its orbit where the south pole points as closely as it ever does directly at the sun. The north pole, of more interest to those of us in the northern hemisphere, is pointing as far as possible from the sun, so that the sun is never visible from points north of the Arctic circle.

In Fairbanks the sun will rise. It will poke its upper edge above the horizon, 25° east of south, at 10:58 in the morning, and will set at 2:40 this afternoon, 25° west of south. At its highest, at 12:49 pm, its center will be a mere 2°, four times its own diameter, above the horizon.

But from now on the days will be getting longer.

(This was last year, when it was clear on the solstice. I’m pretty sure it was taken out of an office window at the Geophysical Institute, probably from right next to my old office, if not from it. The slight jaggedness on the horizon is the Alaska Range, seventy to a hundred fifty miles away. The mist is ice fog in the Tanana Valley — the temperature last year when this was taken was between -38° F and – 45° F.)

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

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White ice breakupI remember a year when the first faint traces of tree-leaf green were visible by the beginning of May. Not this year! The snow stake still shows a foot and a half of last winter’s snow. We’ve had just enough thawing that the white ice on my road is getting rutted 6” deep on warm days, and then freezing solid at night. I tried to shovel a path to the plant trays Saturday, and the snow was like concrete. At least the paved roads are dry.

There are signs of spring. The birch tree visible out the north window of my office now has bare ground at its base, thanks to the dropped seeds and wild rose bushes absorbing the sunlight. The snow is dripping off the roof. I’m generally driving with shoes on, rather than boots (thought the boots are still in the car.) And the sun is definitely back

birch treeSunrise this morning was at 5:23 and the sun will set 16 hours and 52 minutes later at 10:15 this evening. It’s now almost 40° above the horizon at noon. But we no longer have nautical night (the sun never goes more that 12° below the horizon) and civil twilight lasts until half an hour before midnight. Star gazing? It barely gets dark enough for that.

I don’t know if we’ll set a record low temperature for the month (probably not, though I’ll be checking) but I can say that temperatures have not been at or above normal since the first three days of May. Where’s spring?

P.s. added 3:50 pm: Probably the third coldest April, we set a new record low of 2 below last night, and it was snowing shortly after noon today.

We’re well south of Barrow, but when the sun rises there we know the Earth is really tilting more toward the sun. A video was taken by the FAA last Sunday, and to quote from the Weather Service Facebook page:

“Residents of Barrow, Alaska watched the sun climb above the horizon for the first time in 65 days, after it set on November 18, 2012. The sun skirted along the southern horizon for about 43 minutes today. Tomorrow it will remain above the horizon for 1 hour and 27 minutes. The amount of sunlight will rapidly increase in Barrow until May 10th, at which point the sun will remain above the horizon for 24 hours a day for nearly 3 months.”

I can’t seem to get the video to show on the page, but click here and you can see the Barrow sunrise.

I had to share this video. For years I worked in an office with a south window just a block down the street from the museum from which this was taken, and I have seen the low arc of the sun over the Alaska Range. This video was on the Alaska Dispatch as a time-lapse of the Mayan Apocalypse (which just happened to be the Winter Solstice) with comments from the photographers.

The museum (and my old office) are on a ridge north of the Tanana Valley, with the main part of Fairbanks to the southeast, and part of the residential portion of College directly to the south. The bright patch below the Alaska Range on the horizon is the sun reflecting off the top of the ice fog; the discrete streamers are exhaust from chimneys.

I went to an Alaska Writers Guild meeting Tuesday night, and mentioned Friday’s post on the effect of orbital tilt. This led to a discussion of day length, and I realized that while I knew some planets had really weird day lengths, I wasn’t sure which ones. (I thought it was the inner planets, which turned out to be right.) So as long as I was looking the information up, I thought I’d share it.

Mercury

Mercury (Wikimedia)
Mercury turns out to be the planet whose days are longer than its years. For many years the planet was thought to keep the same side facing the sun all the time: one rotation about its axis relative to the stars for each revolution around the sun. We now know it rotates three times for each two revolutions around the sun, making its days a year and a half long. Luckily it’s a short year (88 Earth days.) Its tilt, by the way, is so near zero it is hard to measure. (Its closeness to the sun doesn’t help.)

Venus

Venus, Hubble photo
Venus is the really weird one. Its rotation is in the opposite direction from its revolution around the sun, so from the surface of Venus, the sun would appear to rise in the west! At perihelion the sun may actually appear to stand still or go backward in the sky. That is, it would if you could see the sun through the sulfuric acid clouds. A Venusian day is long, however: 116.75 Earth days. A Venusian year is 1.92 Venusian days or 224.65 Earth days long. The tilt of its axis is only about 3.4°.

Mars

Mars (Hubble)Mars is easily the least different from Earth when it comes to day length: 24 hours 39 minutes and slightly more than 35 seconds. This is more precise than is generally stated for the other planets, quite simply because Mars is the planet with human-piloted rovers on its surface, and to have daylight, these pilots must work on Martian days (or sols) even though they are located on Earth. (Pilot may not be quite the right word, given that radio communications take 4 to 20 minutes to get to Mars.) Its axial tilt is also similar to Earth’s: 25.2°. A Mars year is 1.8809 Earth years.

Jupiter

Jupiter (Hubble)Jupiter has the fastest rotation rate, and thus the shortest day, of any of the planets: slightly less than 10 hours. Why the vagueness? All we can see of Jupiter is the cloud tops, and those rotate at slightly different speeds at different latitudes. It is clear, however that Jupiter’s days are very short, especially compared with its year length of 11.86 Earth years. Its axial tilt is small, only 3.13°.

Saturn

Saturn (Hubble)Saturn, like Jupiter, rotates fast and the rotation seems to vary with latitude but is slightly more than 10 hours. The year, however, is over twice the length of Jupiter’s – 29.46 years. The axial tilt is relatively large: 26.73°, which is why the visibility of Saturn’s rings from Earth varies so much. Seasonality is probably weakened by internal heating and the large distance from the sun.

Uranus

Uranus (Hubble)Uranus rotates slower than the gas giants but still faster than earth, with a day length of 17 hours, 14 minutes. Its year is 84 Earth years long. It is a few years past an equinox (2007) and won’t reach another solstice until 2028. There is some question as to which is the north pole, since its axis is either tilted at 97.77° with normal rotation or 82.14° with retrograde rotation.

Neptune

Neptune (Hubble)Neptune has a day length of roughly 16.11 hours. Very roughly – Neptune has even more variation in rotation rate of the cloud tops with latitude than does Jupiter, with apparent rotation periods varying from 12 hours at the poles to 18 hours at the equator. Its tilt is a little larger than earth’s, about 28.32°, which should give it pronounced seasons, though not as pronounced as those of Uranus! It year is roughly 164.8 years.

All of this variation is just in our own solar system. What else may be out there?

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What point at the top of the atmosphere gets the most solar radiation on the day of the summer solstice?  Would you believe the North Pole?

Yes, that’s right. If the Earth’s pole of axial rotation were perpendicular to its orbital plane, the North Pole wouldn’t get any incoming radiation, and summer solstice would not even be defined. But with an axial tilt of only 23.5°, the pole still gets more radiation over 24 hours on the date of the summer solstice than any other point of the northern hemisphere on any date. Only the South Pole gets more, on the day of the winter solstice.

It doesn’t show up in temperature, first because much of the incoming solar radiation is scattered away during its long path through the atmosphere, and second because the ice and snow at the North Pole reflect much of the radiation back to space. (The second factor may be changing, and this is one of the reasons the Arctic is such a sensitive region.)

But suppose the axial tilt were 90°?

Uranus (Hubboe)

Uranus, as viewed by Hubble.

We do have one planet in our Solar System that approaches this: Uranus, with a tilt of 82.14°. But let’s stick with the Earth and assume it has a tilt of 90°. What would the seasons be like?

Summer solstice at the pole would be unbearable. Imagine the sun directly overhead at noon. Now stretch that noon out in time, so that the sun stays overhead for 24 hours. Hot? No place on Earth has that much incoming solar radiation today. Granted there would probably be clouds. In fact, there would probably be hurricane-like monsoonal storms unknown on our planet today. But it would still be hot.

By contrast, the South Pole would be in the middle of a six-month long night. It would have some stored heat left from the intense summer, probably enough to keep maritime climates above freezing. But it would still be dark except for the stars, the moon, and the southern lights.

The equator? At summer solstice, the equator would be pretty chilly. The sun would never rise or set, but just appear to sit at the northern horizon. As time moves toward the autumnal equinox, the sun gradually begins to rise in the north-northeast at 6 am, ride to its maximum height in the northern sky, and then set in the north-northwest at 6 pm. By the equinox, the sun would rise in the east, rise to directly overhead and then set in the west. But at the north pole, the sun has been spiraling gradually down the sky from overhead, until it finally just glides along the horizon at both poles on the day of the equinox, which begins a 6-month night for the North Pole and a 6-month day at the South Pole.

What happens if you add up all of the incoming solar energy over the course of a year? Not too surprisingly, the poles are the winners, with the equatorial regions being relatively cool. Given that water is much better at storing heat than land, the oceans would be warmer at the poles than the equator. Land areas are far more likely to follow a strong annual cycle. High-latitude continental climates would have tremendous seasonal variation, while maritime climates would be much more uniform. Monsoons, which are driven by these land-sea differences, would be extreme. And equatorial climates, which on our earth are primarily wet or dry, would be intensely cold near the solstices and as warm as they get on the equinoxes.

I haven’t actually tried this as a science fiction world—I want my planets to be habitable! But I do have a planet with zero axial tilt—Eversummer—in Tourist Trap. To quote Marna, the planet’s name must have been picked out by a publicity agent!

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The Transit of Venus

Telescopes at Transit

The reflector I used to see Venus was the large tube at the far right. The tan circle with the orange tube just to its left is the one that produced the shadow image shown below.

Tuesday was the last opportunity I’ll ever have to see the transit of Venus with my own eyes, and Alaska is one of the places where it was (theoretically) visible from beginning to end. Local astronomers with properly shielded telescopes were set up by the Noel Wein Library in Fairbanks, so since the sun was actually shining around 2, I took off to see the fun.

I said theoretically because while the sun was up for the duration of the transit, and the transit was visible (unlike a solar eclipse) from anywhere that the sun was visible, it’s been cloudy most afternoons. I set out with more hope than expectation, as towering clouds were visible in all directions. (It had hailed the day before.)

Crowd for the transit

Sunlight came and went.

I’m not going to repeat in detail the reason why transits of Venus are rare—the Wikipedia article I’ve linked to does a good job of that. Basically, the orbit of Venus is inclined to the orbit of Earth by 3.4°, which means that Venus appears actually to cross the sun only when both planets are very near the line of nodes, the line defined by the crossing of the two orbits, at the time Venus comes closest to Earth. Last Tuesday was the last time this century that this will occur.

Sun's image, Venus at lower right.

Shadow image of the sum. Venus is the small dot at the lower right. (Click on any photo to enlarge.)

By the time I made it to the library, the lawn sprinkled with telescopes was sunlit – most of the time. Clouds were scudding back and forth over the sun, and a thunderhead was towering to the east and headed our way. (Yes, thunderstorms often move from east to west up here.) I got a look at the sun through a properly filtered reflector during a break in the clouds, and later managed a photograph of a setup where a small telescope was focused on a mirror that produced an image on a white card. Literally minutes later the sun was covered with dark clouds.

Clouds just after they hid the sun

This was taken minutes after the shadow image. Note there are no shadows–the viewing was over for the moment.

I’m glad I had a chance to see this. I’m not a big observer of astronomical events, but I got to watch the total solar eclipse in 1963, any number of lunar eclipses, the partial eclipse last month (via a pinhole camera) and now the Venus transit of 2012. Wish I could find my solar eclipse photo – it was spectacular.

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