As for what will happen to Phobos, the Wikipedia article rests in part on a NASA Web Archive. As stated above: it could break up or hit the surface. We will just have to wait and see.
Friday, December 28, 2018
Similar to my critique of Steven Hawking’s A Brief History of Time, the source of the many problems may be that the editors at Sky & Telescope are trying to condense complicated truths into a few lines of common English.
February 22: “Small meteorites aren’t hot when they hit the ground. Earth’s atmosphere heats and removes a very thing surface layer, but the rest of the meteorite is still ice-cold from its time in space.”
August 13: “Occasionally, meteor-watchers hear a hiss or crackle accompany particularly bright fireballs. Astronomers still debate the source of the sound.”
Those two resulted in an interesting search: “are meteorites hot or cold when they hit earth?” Apparently the answer may be “yes.” The consensus is that they are cold. They quickly lose their outer layers by ablation, the same phenomenon used to protect spacecraft: the outer layers burn away carrying the heat. That said, accounts of meteorites being at least warm are not totally discounted. At the same time, also accepted are reports of newly-fallen objects being covered with frost. That meteorites are hot when they fall was commonly accepted until recently even by educated people. An article in Popular Astronomy for February 1934 took the Smithsonian to task for perpetuating the error. (See Astrophysics Data Site archive of Harvard online here.)
December 27: “In 50 million years or so, Phobos will spiral into Mars, crashing on its surface or breaking up in pieces. In the meantime, though, the little moon has quite a view: Mars fills much of its sky because of the close orbit.” The spiral is not a possible orbit. That was a teaching point from a Heinlein juvenile novel. Living in a spacefaring culture, our young hero takes an aptitude test to be trained as a pilot. “What would you do if you suddenly found that you were spiraling in to a planet?” It is a trick question because the spiral is not a possible orbit. Considering the problem again, the easiest general statement is that successive elliptical orbits decay by atmospheric drag.
Mathematical Association of America
Re-entry of Orbital Debris
Video Tutorials on Mechanics and Orbital Motion
Physics Department at the University of New South Wales
October 31: “As it sails beyond the solar system, Voyager I hears the lonely radio whistles of plasma waves passing through interstellar space. Listen here: . This was interesting, but it is a transduction. Note that the video (YouTube here: ) also presents color blobs representing those waves. We can make them sounds in any octave, lines or shapes in any color, depending on our choices of coordinates. And, as we all know, you really cannot hear radio waves traveling through interstellar space.
December 5: “You can cry in space, but your tears won’t fall—due to water tension (and a lack of gravity) they form a floating, liquid sphere.” “There is no gravity in space” is one of those many easy sayings spoken by people who watch television shows about science, but who never take a formal class in astronomy or physics. If you lived in a spacefaring culture you might be told that if you were trapped in a gravity well, your tears would be pulled away from your face as the tremendous inertial acceleration overpowers the molecular adhesion that holds them to your face. If you were “in space” as a passenger of an accelerating vehicle, you would experience an inertial force indistinguishable from “gravity.”
(Some tests could reveal the larger context of your condition. You might be on a rocket accelerating; you might be in a spinning torus like the classic space station; you might be on a large body such as a moon or planet in space. But within the reach of a human—a fathom—it would be difficult to find any differences. The differences were explained to me by a friend of mine from high school who went to MIT. I am not smart: I learn well.)
March 28: “Thanks to its solar wind, the Sun is losing roughly an Earth’s worth of mass every 150 million years.”
March 29: “Because the Sun is (slowly) losing mass, Earth’s orbit gets about an inch bigger every other year.”
April 15: “The Moon is drifting away from Earth at a rate of 1.5 inches per year.”
July 7: The expansion of space means that the solar system is expanding, too, but only at an infinitesimal amount: one part in septillion over its lifespan.
1. A so-called “solar wind” is an essential characteristic of every star. The phrase is just a way that we conveniently think of the energy of the star. The loss of mass is integral to the nature of the star and we know how stars age.
2. What is interesting is that Earth is losing mass, also. That must mean that all of the planets and, in fact, all other material bodies do so as well. Net loss to Earth is 50,000 tonnes per year (BBC News Magazine online here). That is the difference between the meteoric dust which falls in and the hydrogen and helium gasses which escape into space. Other effects come from the core’s heat being lost, volcanoes and other relatively minor events. (See this homework problem from Weber State University.)
3. Finally, as for the expansion of “space” (or space-time, or the stuff “in” it or “in” which space-time exists), while not quite lying with statistics, casually tossing out large numbers obfuscates the question. If the universe is 13.8 billion years old and expanding one part in a septillion over 10 billion years, the numbers just do not explain the facts we believe from observation and measurement – unless the expansion rate is highly variable, greatly slowing down or expected to greatly increase. Deceleration seems to be the accepted theory of the day. But it is more complicated than can be explained in five lines of a calendar page.
· Universe's Expansion Rate Is Different Depending on Where You Look By Elizabeth Howell, Space.com Contributor July 13, 2018 04:03 pm ET
· The Expanding Universe: From Slowdown to Speed Up; Distant supernovae are revealing the crucial time when the expansion of the universe changed from decelerating to accelerating By Adam G. Riess, Michael S. Turner on September 23, 2008
4. Just to note about the actual age of our planet and its sisters: “The inner edge of the Sun’s habitable zone is moving outwards at a rate of about 1 metre per year. The latest model predicts a total habitable zone lifetime for Earth of 6.3 billion–7.8 billion years, suggesting that life on the planet is already about 70% of the way through its run. Other planets — especially those that form near the outer boundary of a star’s habitable zone or orbit long-lived, low-mass stars — may have habitable-zone lifetimes of 42 billion years or longer.” (Daily news blurb from Nature online here.)
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Monday, December 24, 2018
Last year, I bought myself the Page-a-Day Incredible Cosmos desk calendar from Shop at Sky dot com, the retail arm of the leading magazine for amateur and hobby astronomers. Most days offered interesting snips of knowledge to consider and even wonder about. About 40 pages made questionable assertions. A few were laughable, the bad science being a consequence of poor philosophy.
First of all, it is important to note that astronomy is one of the few remaining studies—numismatics being another—where professionals work with and learn from amateurs. All of the world’s institutional observatories combined are not enough to watch and record all of the sky all of the time. Competing research schedules force narrow windows on professional staffs. On the other hand, amateurs can devote all the time they want to their own passions. Amateurs routinely discover comets and asteroids. NASA’s public webpages on meteorites cites the International Meteor Collectors Association. Cornell University’s page on the subject points to the American Meteor Society. Put “discovery by amateur astronomer” in your search engine. Some projects, such as the search for planets orbiting stars outside our solar system (“exo-planets”), involve amateurs being enrolled to access and review thousands of images from space-borne telescopes such as NASA’s Kepler. In most cases, amateurs work entirely on their own. So, we amateurs and hobbyists expect good science from Sky & Telescope.
August 19: “Cosmic rays aren't rays--most of them are protons shooting through space at a significant fraction of the speed of light.” So, what is a “ray”? A so-called “gamma ray” or “x-ray” is quantum packet, right? Isn't everything? Alpha rays are helium nuclei. Beta rays are electrons. I had a freshman physics homework problem to calculate the wavelength of a fast ball using Planck's equation.
HyperPhysics from Georgia State University here
July 9: “The temperature of the Sun is almost 10,000°F (5,778 K)—hotter than burning rocket fuel.” In fact, the temperature of the Sun varies from the core to the corona; and actually the surface (photosphere) is cooler than either of those.
- The center of the Sun: about 15 million degrees Kelvin (often stated as “Kelvins”).
- Radiative Zone: Temperature falls from about 7 million to about 2 million K across this zone.
- Convection Zone: drops from 2 million K to 5800K in this zone.
- Photosphere: about 5800K, although sunspots are about 3800K – that’s why they are dark.
- Chromosphere: 4300 to 8300 K from inside edge to outside edge
- Corona: about 1 million degrees
August 5: “A third of all planets discovered by the space-based Kepler mission have a super-Earth or mini-Neptune. (Most of these planets are not at all habitable, though.)” Well, now they are not. Neither were the Arctic and Sahara habitable until someone figured out how. If on principle the Moon is not considered uninhabitable why would Neptune be?
February 7: “When you look up tonight, you see stars because the universe is transparent. Between 380,000 and a few hundred million years after the Big Bang, that wasn’t the case.”
August 15: “A fog of neutral hydrogen kept the universe dark in the so-called ‘dark ages’ during its adolescence between 380,000 years and a few hundred million years.”
August 16: “The first galaxies began forming a few hundred million years after the Big Bang.” In the first place, the universe is not transparent. If it were, we could never detect anything, not galaxies, and not our keyboards. It is the problem of Superman’s X-Ray Vision: if he sees through everything, then he sees nothing. A better statement is that interstellar space is mostly not opaque. (See, also, Olber's Paradox.) The cosmology implied by these three claims is that the hydrogen “fog” began to coalesce as (diatomic) hydrogen molecules were attracted to each other. But it is also true that we believe that most of the “stuff” of the universe is as-yet-undetectable “Dark Matter.” So, we may still be in the Dark Ages …
February 4: “When two galaxies merge, the space between the stars is so vast that they almost never collide. Instead, the stars are swept into different orbits around the new galactic center.” Galactic mergers are complex events. One resource is the GalMer Project of the Paris Observatory () where you can try your hand at any of the 1000+ simulated interactions between 15 Hubble Types of galaxies. Easiest here is that it seems typical of elliptical galaxies that the stars do not orbit the galactic center, but move along seemingly random paths of Brownian motion on a galactic scale. However, that stage is supposed to be the last in a complicated interaction. While galaxies are merging, whatever were their centers are arbitrarily the new foci, two “centers” that add (by vectors) to a new barycenter. The same phenomenon applies to our own solar system: the planets do not revolve around the center point of the Sun; their orbits are ellipses with the Sun’s (shifting) barycenter as one focus.
February 5: “Intracluster light is the name for the ghostly glow in the space between galaxies, emanating from the lost stars tossed out of their hosts during gravitational interactions.” Given the complexities (see above), it remains more likely that the stars were pulled out their host galaxies, not tossed out by them.
Previously on Necessary Facts
Friday, December 21, 2018
“‘Tis the season to be jolly…” It was easy enough to note the shortest day as the longest night. But the earliest sunset and latest sunrise are several days different from the solstice. In fact, it makes sense to celebrate the Winter Solstice Season (“Christmas”) from December 8 (“St. Nicholas Day”) to January 4 (no best feast day there). On the other side of the year, Summer Solstice would be from June 10 to July 1, call it Flag Day to Independence Day on the civil calendar.
Our New Year comes from Rome during the dictatorship of Gaius Julius Caesar. As farmers, the old Romans had started their year in March.The two winter months had no names. They were dead times. It is why we still call the ninth month by the name of the seventh: “September.” Julius Caesar courted Cleopatra Ptolemy and sought to unite Rome and Egypt. He moved New Year’s Day to January 1.
|Dates and Times of Earliest Sunsets (EarthSky dot com here.)|
“In this paper the question of prediction of the Nile flood based on the first morning visibility of Sirius is considered. It is shown that the only text that describes this event is formulated very vaguely. It makes impossible to derive a reliable astronomical dating. Modern interpretations of this text are based on the free interpretation of the original source, and often do not match. According to historical evidence of Greek authors and later Egyptian texts, flooding of the Nile based on heliacal rising of Sirius could be predicted at the beginning of [the first] millennium AD. This fact is confirmed by astronomical calculations.” (From “Heliacal rising of Sirius and flooding of the Nile” by Nickiforov, M. G. and Petrova, A. A. in Bulgarian Astronomical Journal, Vol. 18, No. 3, p. 53 archived at the Astrophysics Database at Harvard here.)
For the Celts, New Year’s was harvest time, Samhain, what we call Halloween. In some places in medieval Europe, New Year's Day was May Day. During a warming period, New Year's Day became the Vernal Equinox: the first day of spring. Easter is still tied to Passover: the first Sunday after the first full moon after the first day of Spring. Those who kept the old calendar were “April Fools.” Correlating the lunar and solar calendars and marking those over centuries gave rise to modern astronomy. In any case, it was Julius Caesar who made January 1 New Year’s Day. It marked the close of the Saturnalia solstice celebrations.
Earliest sunrise latest sunset near solstice (Washington Post here)
A few years ago, I was working as a security guard and it was late March or early April. Three of us were walking a shift change about 6:00 PM. One guy was an Army brat who had lived around the world, a lot of it in Scotland. The other guy grew up here in Austin. We stepped outside. After about ten days of rain, the sky was clear. “Hey, look,” I said, “the sun is still up.” The guy from Austin was taken aback. “What do you mean?” he said. “The sun goes down the same time every night.” Well, in Austin, it mostly does, but not where I’m from and surely not in Scotland.
Analemma 2011 from The Washington Post here.
The apparent position of the Sun depends on the tilt
of the Earth and the eccentricity of our orbit.
|More than you ever wanted to know |
at the US Naval Observatory web site here.
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Thursday, December 20, 2018
Obvious though it seems, how and why solar eclipses became important to our civilization is not at all clear. The path of totality is narrow and fleeting, gone in about six or seven minutes. I never experienced a total eclipse, but I have seen three partials: September 20, 1960, May 10, 1994, and August 21, 2017. In every case, had it not been announced long in advance, I would not have known from common experience just what—if anything—had happened.
It is often repeated, citing Herodotus, that Thales of Miletus predicted the solar eclipse of May 28, 585 BCE. How he did that is not recorded. We take for granted our Arabic numerals and positional notation. Absent them, calculation is even more painfully laborious than most of us experience it to be. And the Greeks used geometry, not arithmetic.
|Just one construction from|
"On the Sizes and Distances of the Sun and Moon" by
Aristarchus of Samos from Aristarchus of Samos:
The Ancient Copernicus by Sir Thomas Heath
(Oxford University Press; 1977, 1913).
Having long kept records, the Babylonians were aware of the 18-year cycle of eclipses. (18 years 11 days plus a third of a day. See the explanation of the Saros Cycle at Wikipedia. )
The Chinese also kept records of eclipses, at least back as far as 1302 BCE, the first perhaps in 2159 BCE. (“The eclipse in China”, F. Crawford Brown, Popular Astronomy, Vol. 39, p.567 at The Digital Library for Physics and Astronomy at Harvard and “Examination of early Chinese records of solar eclipses,” Liu, C., Liu, X., & Ma, L., Journal of Astronomical History and Heritage (ISSN 1440-2807), Vol. 6, No. 1, p. 53 - 63 (2003) at the same archive. )
That being as it may, explanations for the physical events—that the Moon and Earth cast shadows on each other—were lacking until the Greeks, again, beginning perhaps as early as 600 BCE, but certainly known to Aristotle c. 300 BCE. Nonetheless, that knowledge was not widespread.
“The majority of people didn’t really understand what eclipses or shooting stars were until at least the 17th Century,” says Edwin Krupp, director of the Griffith Observatory in California. The ancient Greeks weren’t alone, either.
While a handful of astronomical scholars, from the 8th Century BC onwards, successfully understood the celestial mechanisms behind an eclipse, for another 2,000 years most of the world’s population clung steadfastly to the ancient belief that astronomical events, and particularly solar and lunar eclipses, were the work of the gods.” (BBC here.)
My best eclipse was May 10, 1994. The sky did grow visibly darker. The air turned cool. The birds were silenced. But it was impossible to look directly at the sun. For that, I built a viewing box to project the image on a sheet of paper. The first partial eclipse I witnessed on September 20, 1960, was an annular, a ring, because the Moon was too close to the Earth to completely block the Sun. What I saw was a partial annular, a geometric “lune” the shape of part of one circle over another. Although I had exposed film to view it through, the cloud cover was just right to view the sun directly. The third time, although some clouds passed by they were not dense enough to allow direct observation of the Sun. If I had not had exposed film to view through, I would not have known that the eclipse occurred at all: no other environmental changes were manifested.
Lunar eclipses are impossible to ignore. The Earth's shadow is large. The Moon often rises red, blue light being absorbed by our atmosphere. The Moon darkens almost to full black. The entire event takes hours.
Every Lunar eclipse is followed by a Solar eclipse. However, as noted above, the path of totality is narrow and the duration of darkness is just minutes. You would have to be a pretty good sprinter to cover the mile from your field to your home to the village to the church in time to pray before it would all be over. And people 200 miles away might not be aware that anything happened at all.
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