Day and Year Lengths of the Planets

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 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 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 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 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, 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 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 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?

The Transit of Venus

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.)

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.

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.

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.

Six Sentence Sunday

Welcome back, Sixers. I’m going to post something today that is completely different from what I’ve done before, though still in the same universe. This is from the third book of the trilogy, working title War’s End — though I hope I can come up with a better title before publication. This is roughly halfway through the book, and a moment before this scene, Coralie is on a spaceship.

She had to protect the baby.  Coralie tried to make a rigid shield of her body and arms as she rolled through wet, foul-smelling greenery, punctuated by harder masses that might have been tree trunks or rocks — she was too confused to tell.  Around her, familiar voices cried out in shock, and somewhere a dog yelped.  What had happened?  This wasn’t the ship!  The uncontrolled tumble ended with a blow that drove the breath from her lungs, and for a moment she could see nothing but colored flashes as she struggled for air.

If you like this excerpt, or want to play yourself, check out the other fine authors at Six Sentence Sunday.

Quotes from Anne McCaffrey

All of this past week’s quotes but the last were taken from Dragonsdawn, by Anne McCafffrey. This is the first of the Pern novels in terms of internal (Pernese) time, but nineth in terms of when it was written.

“Things were going far too well.” Kenjo, on the first landing approach to Pern.

“History was something one read about in books.” Sorka, then elementary age, is unimpressed by the idea that she is making Pernese history.

“Horses, always. We were promised horses.” Sean, talking with Sorka about the need of the traveling folk for draft animals.

“Alaskans had a reputation for never throwing anything away.” Sallah Telgar, commenting on the way the quartermaster (who is Alaskan) has stripped the colony ship, sending everything remotely usable down to the new colony.

“It’s one thing to see, and another to know.” Sorka, when she is taken along on an exploration trip.

“Mankind prove[s] in many ways that greed is universal.” Admiral Benden at a meeting of the colony’s leaders while they are discussing a legal framework for the new colony.

“If they drink that much beer, they wouldn’t be drinking that much water.” Tourist Trap, by Sue Ann Bowling. Roi’s observation on the Eversummer plague situation, which sends Marna looking for a water-borne source.

Note that all of these quotes were tweeted on @sueannbowling. Follow to get more Context? Quotes, and challenge yourself to identify them before these weekly roundups!

Six Sentence Sunday

Today’s snippet is from near the end of the first chapter of Rescue Operation, my current WIP. Zhaim has been arguing that he’s done the right thing in imposing slaving on Horizon, a recently colonized planet, as they refuse to pay their dues and are breeding people faster than their economy is growing.

Right if he wanted to make the Confederation into a military dictatorship rather than something that allowed over a hundred human-occupied planets to live in peace, if not harmony, Roi thought as he returned home. Not that there weren’t times he would have liked more power over individual planets, especially those that abused their own people. For that matter, he’d like more power over Central, to eliminate slavery there, but not at the cost of turning the Confederation into something people feared, instead of a protection.

Mark and Ginger, the latest of the slaves he’d rescued, adopted and educated for freedom, found him sitting in his office with his face in his hands. “Audi told me,” the young man said awkwardly. “Were you able to do anything?”

Be sure to visit the other Six Sentence Sunday authors.

Skywatching: DVD Course Review

Every now and then I order a course on DVDs from The Great Courses. Most recently, I’ve been viewing Skywatching, a course by Alex Fippenkio on the sky, day and night: what can be seen in it and the physics of why it looks the way it does.

Roughly the first third of the course deals with what we can see in the daytime sky. Dr. Filippenko discusses sky color in midday and when the sun is rising or setting, clouds, lightning, and the interaction of sunlight with water and ice (giving rainbows and halos.) This is closely related to what I researched and taught, so I didn’t really lean anything new. The presentation, however, was generally good. I did catch an error in one diagram, but I suspect that was the graphic designer. (The diagram is the one used to explain polarization in reflected light, and the error is that the angle of reflection and the angle of incidence are not shown as equal.) I was also rather disappointed that Dr Filippenko did not point out that frozen droplets are initially near-spherical, and develop their hexagonal prism shape (and the optical effects this produces) only later, by vapor-phase growth. But I suppose I shouldn’t expect everyone to be familiar with ice fog.

This section of the course should be of particular interest to writers needing information on sky and cloud cover, storms, and less common phenomena such as rainbows or sundogs. If you are going to describe an evening sky, you’d better have some idea of what’s happening.

Roughly half the course deals with the constellations and observing the bodies of the solar system. Most of this I was familiar with as an amateur, and I’ve used some of it — lunar phases and seasons, for instance — in my writing. Every writer who wants to put a moon in the sky should watch the section on lunar phases. Rising crescent moon in the evening? Nope. Just doesn’t happen. Neither does a narrow crescent high in the sky.

The lecture on solar eclipses brought back the one I saw, shortly after I moved to Alaska in 1963. I didn’t have a car yet, but two other graduate students gave me a ride down to Sourdough, Alaska to see the total solar eclipse of July 20, 1963. There were scattered high clouds, and while they added suspense –would the sky be clear during totality? – they wound up adding to the experience. Every bright spot of Bailey’s Beads had its own rainbow (technically iridescence.) I know I took a picture; I remember taking photos both before and after the eclipse, the ones after being a series with the exposure set at a constant value to capture the change in the light. I found that series, but so far the ones before and during totality are missing. They may have been separate from the others and lost during the fire twelve years ago.

Overall I’d give the course an A. Dr. Filippenko is a wonderful teacher, and with few exceptions the graphics are excellent. The course takes 3 DVDs and consists of 12 45-minute lectures.

Rain Clouds

One of our assignments at Summer Arts Festival this year was to look at several paintings from one of the water color classes and use them as inspiration for something to write. One that appealed to me had heavy clouds over a mountain valley, and inspired this.

Beyond the clouds heavy with rain,
Beyond the blue we call the sky,
What galaxies! What nebulae!
What other worlds
Where clouds may float
Heavy with rain.

Plate Tectonics: Part I

The important thing about science is that it has a built-in mechanism for corrections. It doesn’t always work as well as it should, because scientists are people and resist changing their beliefs. The fact remains that assumptions are always open to challenge.

I was reminded of this in watching a DVD on How the Earth was Made, which I’ll review soon. The point I want to make here is that the DVD presents examples as if the scientists involved were searching for pieces of a puzzle that they knew had missing pieces. More often the major breakthroughs – such as plate tectonics – are made when a gradually increasing number of people realize that the accepted theory just doesn’t explain something. Or many somethings. Essentially, that the puzzle pieces available have been put together wrongly, and the picture is in fact quite different.

This happened with plate tectonics.

I wasn’t involved directly, but I was at the Geophysical Institute when it happened, and had a chance to read many of the papers as they came out. And I was interested enough to do just that.

Even as early as grade school, I was unsatisfied with the encyclopedia’s explanation of mountain-building and geosynclines. What the encyclopedia said was that mountains were formed by the cooling and shrinking of the Earth, much as wrinkles are formed on the skin of a drying apple. Erosion wore the mountains down, depositing the sediments offshore, and the weight of those sediments pushed the ocean crust down so the mountains grew higher. It did not make sense to me, even then. These processes would have resulted in filling the oceans and leveling the mountains, not building them!

When I was a little older – high school age – I was given a book that gave some of the results of the International Geophysical Year – the IGY. The one that stuck in my mind as an unsolved mystery was the discovery of major east-west trending faults in the Pacific Ocean. Based on the offset of newly discovered magnetic stripes, these faults had large displacements – tens to hundreds of miles. But the displacements totally disappeared when the faults reached land! Not only could the east-west displacements not be found, in places such as the California coast there were well-known faults such as the San Andreas tending more nearly north-south.

At Harvard I took a basic geophysics class, hoping it would help me make sense of what seemed to be an increasingly frustrating puzzle. What I learned there – and it was the cutting-edge science of the early 60’s – left me as puzzled as ever.

Gravity measurements had proved that continents stood higher than oceans because continental rock was less dense than ocean rock. This was known as isostacy – the height of terrain essentially depended on how high it floated on the mantle.

Continental drift was nonsense – there was no way continents could plow through oceanic crust, and there were no traces of any such plowing through on the sea floor. The matching of rock formations on the opposite sides of the Atlantic was sheer coincidence.

Exchanges of plants and animals over  geological time were via land bridges.

The elephant in the room, from my point of view, was that isostacy did not allow sea floor to rise and form land bridges.

I went to the Geophysical Institute as a graduate student partly because of these mysteries, but I was sidetracked into atmospheric science and ice fog. Nevertheless, I stayed interested, and since many of the seminal papers in plate tectonics were published in the Journal of Geophysical Research (JGR) I watched the plate tectonics revolution happen. Next week I’ll talk about some of the breakthroughs that eventually led to the new paradigm of plate tectonics.

Inside Planet Earth: DVD Review

This is an excellent DVD for getting across the idea that the inner workings of the earth, while at times disastrous, are essential for life.

The DVD actually has two programs, both originally shown on the Discovery channel: Inside Planet Earth and Amazing Earth. The graphics are intriguing, though some are repeated a bit too often. The actual camera work is excellent.

My only objection was that at times the narration could be misleading. True, we have been in an ice age for the last 40 million years. But most of the evolution of mammals – and certainly of humans – has taken place during that period. We are adapted to an ice age in the broad sense. My concern is that many people will take “ice age” to mean the periods like 20 thousand years ago, when ice sheets covered much of North America and Europe.

Over all, I found this a good program if a bit sensationalist – and this is my field, so I am aware of shortcomings.

Before Computers

There was a time when digital data recorders did not exist. Data was recorded on strips of paper with grids on them, generally wound around a slowly turning drum while a pen marked them. Trying to do anything with data of this sort required digitizing it.

My first job as a research assistant at the Geophysical Institute involved doing just that.

The process was called scaling, and involved a device that was moved along the paper, lined up with the ink trace at specified intervals, and a button pushed. The eventual result was a string of numbers for one component of the magnetic field. This was done for both horizontal components.

I then had to plot these numbers on an x-y graph, connecting the dots in time order for a number of stations and events. Plotting in those days used millimeter graph paper, with points entered and connected by hand.

Today, it would take five minutes on a computer — but this was 1963. It took a small army of graduate students (SAGS was actually used as an acronym) just to get the data in a form in which it could be analyzed. (SAGS are still used, but these days it is generally in collecting the data, not in doing things a computer can do better.)

All of this was carried out in the basement of what is now the Chapman Building, which looked then very much as it does today, except that it had a dome on the roof. Eventually, we found that the disturbance in the magnetic field during a sudden impulse was elliptically polarized at high latitudes, and my first paper was actually written on the results of that study.

It may sound like a silly thing to do, but that discovery provided a small boost toward our understanding of the effect of the solar wind on the magnetic field of the earth — a subject not to be ignored in the design of long-distance power lines. But I’m very glad for computers!