Category: Astronomy


UPrior to the invention of the telescope, five planets were known: Mercury, Venus, Mars, Jupiter and Saturn. Earth, of course, is a planet also, but this was not recognized until the acceptance of the Copernican model of the Solar System. A seventh planet, however, is visible without a telescope: Uranus.

Hubble false-color infrared image of Uranus (Source)

Hubble false-color infrared image of Uranus (Source)

Uranus was not recognized as a planet until telescopes became available because it is so dim relative to the classical planets. In fact, at magnitude between 5 and 6 it is not visible to most people today, simply because artificial light has made truly dark skies hard to find.

The oddest thing about Uranus is that its pole is almost in the plane of the ecliptic. In fact, which is the North Pole depends on how north is defined. On earth, the sun, and every other planet, the right-had rule reigns. If the fingers of the right hand are wrapped around the equator with the finger pointing in the direction of rotation, the thumb points north. On that basis, the North Pole of Uranus is on the wrong side of the ecliptic. On the other hand if the astronomical definition is used, that the North Pole is the pole on the same side of the ecliptic as the Earth’s North Pole, the planet is rotating backward, with the sun rising in the west.

Like the gas giants, Uranus has rings, which being equatorial are nearly at right angles to the Ecliptic*. Its weather is not well understood, and its seasons must be extreme. After all, its Arctic and Antarctic circles are almost at its equator, while its tropics of Cancer and Capricorn are very close to its poles.

Wouldn’t working that weather into a science fiction story be fun?

(If the word Ecliptic is new to you, it is the plane of the earth’s orbit.)

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SI used to teach basic astronomy, including the life cycle of stars. I ordered the Life and Death of Stars from the Great Courses as a refresher and to see what was new in the last 20 years.

The little "eggs" have baby stars in them. Photo Source

The little “eggs” have baby stars in them. Photo Source

A lot.

The Hubble space telescope has been in orbit for almost 25 years, and some of the information it was revealing was available back then. But newer space telescopes, and some on the ground, have yielded far more information. For one thing, the newer telescopes cover a far larger fraction of the electromagnetic spectrum than Hubble, which is essentially a visible-light scope. Newer instruments take pictures in wavelengths from radio waves to x-rays, giving a far better picture of the life cycle of stars than was available when I was teaching.

A planetary system forming. (The black center is a mask over the star.) Photo Source

A planetary system forming. (The black center is a mask over the star.) Photo Source

Some things have stayed the same. A star’s mass is still its DNA, controlling its life cycle, its color, its luminosity, and what elements it is able to produce. It is still true that nuclear fusion within stars, and the violent explosions that mark their deaths, produce virtually all of the elements except the hydrogen and helium we inherited from the big bang. But improvements in both observations and computer simulations have taught us far more than we knew when I was teaching.

The course is designed for non-scientists, and there were times I was bothered by the anthropomorphism applied to stars. But at the same time, I learned a lot. We knew that certain gaseous nebulae were cradles of star birth, but now we can peer into those nurseries with infrared and see individual infant stars. We know that stellar birth is often, perhaps most of the time, associated with the production of a family of planets. We are beginning to understand much more about the details of the stellar deaths that lead to planetary nebulae, and the more violent ones that produce supernovae and black holes.

If you are interested in the stars, and want to know more of what we have learned in the last fifteen years, this course is worth watching.

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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

AtoZ 13 logoIf you’re looking for the A to Z posts, click on the logo to the left or scroll down.

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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