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Thread: Astronomy

  1. #406
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    Happy Winter Solstice. Here is some information about what people do around Stonehenge during this time that I found by accident: http://www.telegraph.co.uk/news/scie...tice-2015.html

    I didn't realize there were things like noctiluscent clouds. From the pictures I can see how they might confuse them with regular clouds early in the morning or late in the evening.

  2. #407
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    Yes, they are, of course, ordinary regular clouds, but very high in the sky (at 85 to 76 km high) and so very cold, forming ice crystals, which reflect direct sunlight. They were only discovered (or publicly noticed) in 1885.

    At first I didn't "notice" these clouds as being noctilucent, they were just pretty. But when I thought about it further and looked up noctilucent clouds it confirmed to me that this is what they are. The sun shines on them from below the horizon so it takes a specific combination of circumstances to see them. And then to classify them as a distinct phenomena. Now I know, I will keep an eye out for them in future.

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    The Kuiper Belt is where many of the minor planets (like Ceres, Pluto, Makemake, and Haumea and some of the Moons like Neptune's Triton and Saturn's Phoebe) are thought to have originated. Ceres is the most recent planet thought to have originated here.

    We clearly know relatively little about the Kuiper belt and KBOs (Kuiper Belt Objects). See this extract from the Wikipedia item on it:

    The Kuiper belt /ˈkaɪpər/ or /'køypǝr/[1] (as in Dutch), sometimes called the Edgeworth–Kuiper belt, is a circumstellar disc in the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun.[2] It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive.[3][4] Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. Although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, are also thought to have originated in the region.[5][6]

    See the posts of 12 16 2015 on this, earlier in this thread.

    The Wikipedia article on KBOs continues and shows by its language how little we know of the Kuiper Belt:

    The Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not actually predict its existence. In 1992, 1992 QB1 was discovered, the first Kuiper belt object (KBO) since Pluto.[7] Since its discovery, the number of known KBOs has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are thought to exist.[8] The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the belt is dynamically stable, and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago;[9] scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.[nb 1]

  4. #409
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    I might as well continue with the references to the last and most distant area of solar influence, the Oort Cloud. The language degenerates further as the Oort Cloud is described in hypothetical language, like "thought to be..." and "thought to comprise..." and "astronomers conjecture that...". The Oort Cloud is thought to define the outer limits of the gravitational dominance of the sun, and the origins of a whole class of comets (like Halley's) that pass close to Earth at intervals.

    This is a quote from the item:

    The Oort cloud is thought to comprise two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane.

    Astronomers conjecture that the matter composing the Oort cloud formed closer to the Sun and was scattered far into space by the gravitational effects of the giant planets early in the Solar System's evolution.[3] Although no confirmed direct observations of the Oort cloud have been made, it may be the source of all long-period and Halley-type comets entering the inner Solar System, and many of the centaurs and Jupiter-family comets as well.[6]


    It is striking how little we know of this area of space within the sun's sphere of influence!

  5. #410
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    We don't know as much as I thought we did either, but I am glad they hesitate to say something definitely. They could have said, "The Oort cloud comprises two regions." I would have been more gullible and believed them. I am probably still too gullible.

    One thing I didn't realize until reading the various links in this thread is that objects can be pushed away from the Sun by the gravitational effects of larger planets. I previously imagined that everything was doing its best to avoid falling into the Sun or one of the planets.

  6. #411
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    The Oort Cloud has continued to fascinate me. It is so far way that even Voyager 1 (which is the furthest out of the Voyager probes) will not reach it for another 300 years:

    Space probes have yet to reach the area of the Oort cloud. Voyager 1, the fastest[53] and farthest[54][55] of the interplanetary space probes currently exiting the Solar System, will reach the Oort cloud in about 300 years[4][56] and would take about 30,000 years to pass through it.[57][58] However, around 2025, Voyager 1's radioisotope thermoelectric generators will no longer supply enough power to operate any of its scientific instruments, preventing any meaningful exploration by Voyager 1. The other four probes currently escaping the Solar System either are already or are predicted to be non-functional when they reach the Oort cloud, however it may be possible to find an object from the cloud that has been knocked into the inner solar system.

    One proposal for exploration is to use a craft powered by a solar sail that would take around 30 years to reach its destination.[59] In the 1980s there was a concept for probe to reach 1000 AU in 50 years called TAU, among its missions would be to look for the Oort cloud.[60]


    Other probes are in preparation, notably the Whipple Mission, and the Whipple Discovery Mission.

  7. #412
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    I didn't realize the Oort Cloud was that far away. The solar sail technology looks promising.

    Looking at the morning sky, I see that Jupiter and Venus are getting further and further apart in the sky. Supposedly Saturn is above the horizon but I haven't seen it yet.

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    Mercury is also above the horizon at this time, after sunset. An opportunity to see it. Planet shine with a steady light, they don't twinkle like stars. See http://earthsky.org/tonight/mercury-...y-after-sunset

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    The headline at the 7.30 pm SVT news last night was that we may get to see the Northern Lights all over Sweden. I doubt we will from our home at least, just a glimpse at 5 am on the northern horizon. See this Space.com item.

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    Based on the link it is interesting that these sunspot cluster eruptions affect us in two ways. First there is the "flare", which I assume comes at the speed of light and causes an "ionization event" which may cause radio blackouts. The second is the "coronal mass ejection" which takes time to reach us and appears as the northern lights a few days later.

    I have been able to see Saturn in the morning sky, but not yet Mercury in the evening sky because of clouds.

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    I am gradually learning about the aurora (borealis and australis). We never see the full glory of curtains of shimmering blue, shifting and changing, not at our latitudes. You have to be inside the polar circle when the conditions are right. At 61 degrees north we see the light in the northern sky but no more. the australis must be even harder to see, as there is no continental land even at 60 degrees south, apart from Antarctica. There is an old Aberdeen song about the northern lights.

  12. #417
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    Here's a YouTube link to that song: https://www.youtube.com/watch?v=-SNODgG0Fd4

    While watching the Sun rise this morning it occurred to me that the planets and finally the Moon are the last that can be seen in the sky as it brightens. The stars have long disappeared. The planets reflect the Sun and are nearer to us than the stars and it seems to make sense that they should remain visible longer than the stars. When I turned my back to the Sun I could see the entire landscape slowing increase its ability to reflect the Sun. Sharp shadows began to appear. I know all that is obvious. I just didn't quite realize it before.
    Last edited by YesNo; 01-02-2016 at 09:56 AM.

  13. #418
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    The u-tube link has shots in it that make it clear that the northern lights (as shifting blue curtains) can be seen from Aberdeen, at 58 degrees north. So they can be seen in Southern Norrland at 61 degrees north, you just have to catch it at the right time, presumably staying up all night in clear weather. That was very useful to know.

  14. #419
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    The SOCIETY for POPULAR ASTRONOMY

    Electronic News Bulletin No. 413 2016 January 3


    GIANT COMETS COULD ENDANGER EARTH

    RAS

    A team of astronomers reports that the discovery of hundreds of giant comets in the outer regions of the Solar System over the last two decades implies that those objects pose a much greater hazard to life than asteroids. The giant comets, termed centaurs, have unstable orbits crossing the paths of the massive outer planets Jupiter, Saturn, Uranus and Neptune. The planetary gravitational fields can occasionally deflect the objects towards the Earth. Centaurs are typically 50 to 100 kilometres across, or larger, and a single such body has more mass than the entire population of Earth-crossing asteroids found to date. Calculations of the rate at which centaurs enter the inner Solar System indicate that one will be deflected onto a path crossing the Earth's orbit about once every 40,000 to 100,000 years. Whilst in near-Earth space, they are expected to disintegrate into dust and larger fragments, flooding the inner Solar System with cometary debris and making impacts on our planet inevitable.

    Known severe upsets of the terrestrial environment and interruptions in the progress of ancient civilizations, together with our growing knowledge of interplanetary matter in near-Earth space, indicate the arrival of a centaur around 30,000 years ago. That giant comet would have strewn the inner planetary system with debris ranging in size from dust all the way up to lumps several kilometres across. Specific episodes of environmental upheaval around 10,800 BCE and 2,300 BCE, identified by geologists and palaeontologists, are also consistent with the new understanding of cometary populations. Some of the greatest mass extinctions in the distant past, for example the death of the dinosaurs 65 million years ago, may similarly be explained on the giant-comet hypothesis. In the last three decades scientists have invested a lot of effort in tracking and analysing the risk of a collision between the Earth and an asteroid. But we need to look beyond our immediate neighbourhood too, and look out beyond the orbit of Jupiter to find centaurs. Those distant comets could be a serious hazard. The researchers have also uncovered evidence from disparate fields of science in support of their model. For example, the ages of the sub-millimetre craters identified in lunar rocks returned in the Apollo programme are almost all younger than 30,000 years, indicating a vast enhancement in the amount of dust in the inner Solar System since then.
    Last edited by Dreamwoven; 01-03-2016 at 06:09 AM.

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