Category Archives: Astronomy

Over the past eight years, I have been asked to submit astronomical evidence for court cases all over Australia.

Normally when we think of evidence in court, we think of eyewitnesses,DNAor police reports. Often, this evidence requires an expert to explain it to be able to communicate the findings and data to the members of the court to make an informed decision. These experts are typically in medicine, engineering, psychology, or other fields.

Expert astronomers usually are not what one pictures in court, but that is exactly what I do.

The first time I was asked by police to do it came as a bit of a surprise. I had never thought about applying astronomy to the courtroom. Once the first group knew I can do it, more and more requests came in, from colleagues in the same police force or division, or investigators having seen my evidence elsewhere.

Related: Who Owns the Moon? Law & Outer Space Treaties

Now, I'm asked to submit evidence for roughly 12 cases per week. Usually this requires submitting astatement of evidenceto the court. But sometimes I am asked to attend court and explain what the evidence means.

When I'm needed as an expert in court, it tends to be for matters of consequence. My evidence is either critical to a part of the case, or the case itself is fairly major and all the details are being checked and verified.

But what exactly am I providing evidence for?

Most court evidence from an astronomer involves calculating the positions and lighting from an astronomical body the sun or moon. Luckily, thetools we useto calculate the positions of celestial bodies are very accurate, and can be calculated hundreds to thousands of years into the past or future.

An obvious example is when someone claims the sun was in their eyes, causing a glare, and they get into a car accident. Someone needs to say where the sun was, its position, and how it aligned with the street and direction of travel. At certain times and in certain directions, the sun may indeed hinder someone's vision.

There is also the situation where someone sees something, but it happened around sunrise or sunset. An expert is needed to say what the lighting level was as there are very clear definitions based on the sun's position below the horizon, and how much you can see. For instance, what if the event occurred five minutes after sunset? The light level depends on the time of year, the location and other factors. It is not a clear-cut case of daytime versus nighttime.

The moon can feature in court evidence as well. Especially in dark locations away from city lights, an astronomer can provide evidence on how much light the Moon provided on a given night.

There are also historical cases or times when people note the view or phase of the moon as a way of defining when something happened. The full moon has a precise definition, but the day before or after may appear to look like a full moon, despite it not technically being full.

Of course, like any part of science, there are limits to what I can say. If someone was looking through a window how refractive was the window? Were there clouds blocking the moon or sun? It is up to other experts, and other parts of the legal system to sort out these factors.

Just like many fields, space technology is changing, and so too is its impact on law and crime. Satellites are being used more and more in cases to help track things as they happen. For example,the space technology company Maxaroperates some of the highest-resolution commercial satellites to image Earth. For a small cost, people can task these satellites to look at certain areas and/or times.

Lately, we have seen the impact of satellites on Russia's war in Ukraine, and how they have been instrumental in looking at troop movements, and even evidence of some of the alleged war crimes.

Satellite images have been used for a range of criminal investigations, such aspeople smugglingorillegal mines.

They are also being used in Australia for criminal matters. This is yet another situation where an expert is needed to explain the satellite imagery and what it may mean, or even help access it altogether.

Working as an expert witness has given me hope, because I see the extent to which the justice system will sometimes go to get all the details right like taking into account the phase of the moon or the position of the sun. It is also the perfect example of the importance of experts in our society.

In science, we are actively encouraging people to go to sources of accurate and trustworthy information, especially in an era of rife misinformation.

Through experts, fields like space and astronomy can impact people's lives directly even in the court room.

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Where was the sun? Here's why astronomers are more useful in ... - Space.com

A supermassive black hole started feasting on surrounding matter resulting in one of the most dramatic "switching on" events ever seen.

Transients are astronomical events or objects that change in brightness over short periods of time, and the one powered by this greedy black hole J221951 is one of the brightest transients ever recorded. The position of the black hole corresponds with the center of a previously observed galaxy, just where a supermassive black hole would be expected to sit. However, astronomers still aren't sure exactly what is causing the transient event witnessed in J221951.

"Our understanding of the different things that supermassive black holes can do has greatly expanded in recent years, with discoveries of stars being torn apart and accreting black holes with hugely variable luminosities," team member and University of Belfast astronomer Matt Nicholl, said in a statement. "J221951 is one of the most extreme examples yet of a black hole taking us by surprise."

Related: Star survives spaghettification by black hole

The nature of what the supermassive black hole located around 10 billion light-years away is consuming is currently unknown, but it is possible that J221951 represents a star that has ventured too close to the black hole being violently ripped apart by tidal forces arising from its immense gravity in a process called spaghettification.

This occurrence, called a tidal disruption event (TDE), would see some of the stellar material from the destroyed star fall to the surface of the black hole while other matter is funneled to the poles of the black hole before being blasted out at near light-speeds, generating intense electromagnetic radiation.

The spaghettification of an unfortunate star isn't the only possible mechanism that could be causing the black hole in question to give rise to this bright transient event, however. Another possibility is that J221951 is the result of the nucleus at the heart of a galaxy switching from a dormant to an active state.

Active galactic nuclei (AGNs) are bright areas at the heart of galaxies that blast out enough light to drown out the combined light of every star in the rest of that galaxy. They are also powered by supermassive black holes.

"Continued monitoring of J221951 to work out the total energy release might allow us to work out whether this is a tidal disruption of a star by a fast-spinning black hole or a new kind of AGN switch on," Nicholl added.

Kilonovas are a type of transient event that occurs during the merger of two neutron stars or a neutron star and a black hole, which releases bright bursts of electromagnetic radiation. Kilonovas initially have a blue coloration, then fade to red over a period of several days. The transient J221951 also appeared blue, but it didn't change to red or fade rapidly as a kilonova would. The nature of this transient was determined by follow-ups with space-based facilities like the Hubble Space Telescope and ground-based observatories like the Very Large Telescope (VLT) located in the Atacama Desert of Northern Chile.

"The key discovery was when the ultraviolet (UV) spectrum from Hubble ruled out a galactic origin. This shows how important it is to maintain a space-based UV spectrograph capability for the future," team member and Mullard Space Science Laboratory at University College London researcher Paul Kuin said.

With a source located 10 billion light-years away, the team realized that J221951 must be one of the brightest events ever seen. They will now work to better understand its cause.

"In the future, we will be able to obtain important clues that help distinguish between the tidal disruption event and active galactic nuclei scenarios," Oates said. "For instance, if J221951 is associated with an AGN turning on, we may expect it to stop fading and to increase again in brightness, while if J221951 is a tidal disruption event, we would expect it to continue to fade.

"We will need to continue to monitor J221951 over the next few months to years to capture its late-time behavior."

The team presented their findings on Tuesday, July 4, at the National Astronomy Meeting 2023 in Cardiff, U.K.

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Hungry black hole 'switches on' as astronomers watch in surprise - Space.com

Nancy Grace Roman (16 May 1925 25 December 2018) not only laid the groundwork for our understanding of how galaxies grow but also founded NASAs space astronomy programme, becoming the mother of Hubble.

Romans love of the stars was evident from an early age, and she set up an astronomy club for her friends when she was just 10.

However, when she told her guidance counsellor she wanted to be a professional astronomer, she was asked, "What lady would take mathematics instead of Latin?"

Ignoring this discouragement, she went on to attain her degree from Swarthmore University before moving to the University of Chicagos Yerkes Observatory for her PhD.

Here she studied the motions of stars which formed in the same cluster as the Plough, but which had drifted apart over time.

Later, Roman expanded this research to all Sun-like stars visible to the naked eye and soon noticed that where stars orbited in the Milky Way was connected to their metallicity.

Metals (meaning anything heavier than helium in astronomy) are only formed inside stars, so if a star contains a lot of metal it must have been born after several generations of previous stars had already produced them.

Younger metal-rich stars tended to move in circular orbits near our Galaxys centre, while older metal-poor stars were further out.

This connection was the first clue towards understanding how the Milky Way grows over time, providing the foundation for modern studies of galactic evolution.

Her work also developed a method of gauging stellar metallicities by comparing their brightness at blue and ultraviolet wavelengths, which is still used today.

Despite these landmark discoveries, Yerkes Observatory refused to grant a woman a permanent position, so in 1954 Roman moved on to the Naval Research Laboratory in Washington DC to work in the emerging field of radio astronomy.

Here, she mapped out the Milky Way in new wavelengths, became head of microwave spectroscopy and consulted on the Vanguard satellite programme.

With radio astronomy still in its infancy, the instrumentation was inadequate for Romans needs, and she didnt want to retrain as an electronics engineer to build her own.

So in 1959 she moved on to the National Aeronautics and Space Administration, NASA, as the head of observational astronomy, just one year after the agency had been established.

This new role effectively brought an end to her research, but with it Roman became the first woman to hold an executive office at NASA, giving her overall responsibility for the growing agencys space-based observatories.

Initially many ground-based astronomers were stubbornly opposed to using remote satellites, but Roman worked tirelessly to convince them of the benefits of observing above Earths atmosphere.

Believing the best way for the US to glean these benefits was for NASA to oversee all major space observatories, Roman was initially the sole voice in deciding which projects would get funded.

Though many of her colleagues advocated for NASA to build a large space telescope, she dismissed the plans as premature, instead electing to fund a series of smaller satellite observatories.

Only in 1968, after a decade of success proved NASAs capability, did Roman return to the idea of a bigger mission, though it took another three years of feasibility studies and funding before she could finally establish the Large Telescope Steering Group.

It would take dozens of institutions 20 years to complete the project, but the telescope launched in 1990, renamed the Hubble Space Telescope.

Although Roman was heavily involved in overseeing the mammoth projects early years, she retired from NASA in 1979 as chief of astronomy, returning occasionally as a consultant.

She continued outreach work as part of her own lifelong mission to champion the inclusion of women in astronomy.

Her vision and many legacies, both scientific and cultural, continue to shape astronomy to this day.

While she may not currently be a household name, Roman will soon be much better-known, as an infrared telescope named in her honour is set to launch in 2027.

The Nancy Grace Roman Space Telescope will have a 2.4m mirror the same size as that of the Hubble Space Telescope but its Wide Field Instrument will have a field of view 100 times that of Hubbles infrared camera.

It will use this huge view to create a 3D map of galaxies, galaxy clusters and distant supernovae to measure how matter is distributed throughout the Universe.

These observations will compliment those by ESAs Euclid mission in the quest to trace dark energy, the mysterious force that appears to be accelerating the expansion of the Universe.

The telescope will even be able to map out otherwise invisible dark matter using a method called microlensing.

When light from a distant galaxy passes another massive object, its path is bent slightly, becoming stretched and distorted.

These distortions can then be analysed to reveal how matter is distributed throughout the cosmos.

Lensing also happens when a planet passes in front of its host star, and the telescope will monitor 100 million stars in the hopes of spotting a stars brightness fluctuating as an exoplanet passes in front.

Most excitingly, this technique should be able to reveal small rocky worlds in habitable orbits, similar to our own Earth.

This article originally appeared in the July 2023 issue of BBC Sky at Night Magazine.

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Life of astronomer Nancy Grace Roman (16 May 1925 25 ... - Sky at Night Magazine

If you're looking for a great pair of binoculars for astronomy, Amazon has got you covered. With impressive 80mm objective lenses and powerful 20x magnification, these SkyMaster binoculars from Celestron are now 55% off in the Prime Day deals, making them just $90.86.

These binoculars are ideal for stargazing, as their large aperture enables exceptional light-gathering capability, resulting in brighter and more detailed celestial views. With their high magnification, you can easily observe the moon's craters, the moons of Jupiter and even some deep-sky objects like star clusters and nebulas. Additionally, their wide field of view allows you to take in the wonders of the night sky and explore constellations with ease. We regularly recommend Celestron's binoculars in our buying guides check out best binoculars, binocular deals, best compact binoculars and best night vision binoculars.

An objective lens cap, rain guard, carrying case, neck strap and lens cloth are all included it also has a tripod adapter, which is particularly useful for extended periods of viewing. We wouldn't necessarily recommend these if you're a total beginner as they have such high magnification which can be tricky to get used to if you're new to the hobby, but if you're looking for your 'next' set of binos or you want to get into astronomy and already have a tripod to keep everything stable, we think they'd be great, especially now that they're such a low price.

But they're also good beyond astronomy, the Celestron Skymaster 20x80 binoculars are also fantastic for terrestrial viewing. They provide stunning clarity and sharpness, making them perfect for wildlife observation, birdwatching and scenic landscape viewing.

Don't forget, if you want to make the most of Amazon Prime Day 2023, check out our Amazon Prime Day hub for a roundup of the best discounts and deals on telescopes, binoculars, cameras, star projectors, drones, lego and much more.

Key Specs:Large multi-coated 80mm objective lenses to let ample light in with 20x magnification for stunning celestial viewing. They're also waterproof, have Bak4 prisms and a comfortable 18mm eye relief, making them good for anyone who wears glasses. They weigh 75oz (2.126kg).

Consensus:For astronomy lovers, we can't recommend these enough, especially for the low price.

Buy if:You're a keen astronomer and want a pair of binos with high magnification for those long nights of stargazing.

Don't buy if:You're a beginner or you want a more 'general' day-to-day pair of binos.

Alternative models: For an even cheaper pair of binos that are more suited to general use (camping trips, sporting events etc) check out the Celestron Outland X 10x42. They're not as beefy and powerful as the SkyMaster, but if that's not what you're looking for, the Outland pair are also on offer for just under $50 for Prime Day. Or if you want even more magnification, the Celestron SkyMaster 25x100 are also 50% off, now sitting at around $250.

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These Celestron SkyMaster binoculars are less than half price in the ... - Space.com

PRESS RELEASE

Published July 11, 2023

[City, Date] -- WhiteStarOutdoors.com, a leading online resource for astronomy enthusiasts, is proud to release its comprehensive guide highlighting the crucial role of amateur astronomers in raising awareness and inspiring interest in the mysteries of the universe.

The focus, of the website, is to encourage and support amateur astronomers worldwide in their efforts to promote astronomy education and exploration.

Amateur astronomers are invaluable in bridging the gap between professional, scientific research and the general public. Through their passion, dedication, and accessible outreach activities, they bring the wonders of the universe closer to people of all ages and backgrounds.

WhiteStarOutdoors.com recognizes and celebrates the immense impact of amateur astronomers in fostering a sense of wonder, scientific curiosity, and lifelong learning about the cosmos.

The guide by WhiteStarOutdoors.com outlines critical strategies and practical tips for amateur astronomers to effectively raise awareness and inspire interest in the mysteries of the universe.

Amateur astronomers play a significant and multifaceted role in the field of astronomy. Their passion, curiosity, and dedication contribute to advancing scientific knowledge, promoting public engagement, and inspiring the next generation of space enthusiasts.

Here are some key aspects that highlight the crucial role of amateur astronomers. It covers a wide range of topics, including:

Observational Research:

Amateur astronomers make valuable contributions to observational astronomy. Equipped with their telescopes and astrophotography equipment, they observe celestial objects, monitor variable stars, track asteroids, and discover comets. These observations provide additional data points for scientific research, complementing the work of professional astronomers and expanding an understanding of the universe.

Public Outreach Events and Education:

One of the most vital roles of amateur astronomers is to inspire and educate the public about astronomy. They organize stargazing events, public lectures, workshops, and star parties, allowing people to observe celestial phenomena firsthand. By sharing their knowledge, explaining complex concepts in accessible language, and answering questions, they ignite curiosity and nurture a sense of wonder about the universe. Suggestions for organizing stargazing events, public lectures, and workshops to engage the local community and ignite curiosity about astronomy.

Online Presence and Social Media:

With the advent of digital platforms, amateur astronomers have a powerful tool to reach a global audience. Many enthusiasts maintain blogs, websites, and social media accounts to share their experiences, astrophotography, and knowledge. Through these online platforms, they can connect with fellow enthusiasts, inspire others to pursue astronomy and contribute to disseminating scientific information. Utilizing social media platforms, blogs, and websites to share knowledge, showcase astrophotography, and connect with a global audience interested in astronomy.

Collaboration with Schools and Institutions:

Collaborative initiatives with schools, colleges, and scientific institutions to provide educational resources, hands-on activities, and mentorship opportunities for aspiring astronomers. Amateur astronomers often collaborate with schools, colleges, and scientific institutions to support astronomy education. They provide resources, conduct workshops, and mentor aspiring astronomers. By sharing their expertise, they contribute to fostering scientific literacy, encouraging young minds to explore the wonders of the universe, and potentially nurturing future professional astronomers.

Citizen Science Projects:

Participating in citizen science initiatives, such as observing variable stars, tracking asteroids, or contributing to light pollution surveys, to contribute valuable data and engage in scientific research. Amateur astronomers actively participate in citizen science projects, contributing to scientific research in collaboration with professional institutions. They engage in activities such as documenting meteor showers, observing transient events like supernovae, or cataloging deep-sky objects. Through their involvement, they contribute to data collection, analysis, and the overall progress of scientific knowledge.

Supporting Amateur Astronomer Communities:

Establishing or joining local astronomy clubs, attending star parties, and organizing group observations to foster community and mutual learning among amateur astronomers.

Advancements in Technology and Innovation:

Amateur astronomers are often early adopters of new technologies and innovations in astronomy. They contribute to developing and refining equipment, software, and techniques used in observational astronomy and astrophotography. Their feedback, experiments, and discoveries have led to advancements benefiting the astronomy community.

Amateur astronomers are vital ambassadors of the universe, bridging the gap between scientific research and the public. Their enthusiasm, knowledge, and commitment help ignite interest in astronomy, foster scientific literacy, and promote a deeper understanding the cosmos. Their contributions are invaluable in shaping a more informed and awe-inspired society.

Amateur astronomers can inspire the next generation of scientists, astronomers, and space enthusiasts. Their efforts catalyze scientific literacy, promote a deeper understanding, and encourage responsible stewardship of the Earth.

For more information and to access the comprehensive guide on the crucial role of amateur astronomers, visit WhiteStarOutdoors.com.

About WhiteStarOutdoors.com:

WhiteStarOutdoors.com is a leading online resource for astronomy enthusiasts, providing informative articles, stargazing tips, equipment reviews, and engaging content related to the mysteries of the universe.

With a mission to inspire and educate, WhiteStarOutdoors.com aims to foster a global community of astronomy enthusiasts, from beginners to advanced observers.

The focus at White Star Outdoors, is understanding astronomers' unique needs and preferences. And, in affiliation with High Point Scientific, their expert team has curated this list to cater to a wide range of budgets and experience levels, ensuring that every stargazer can find a telescope that suits their requirements.

For more information on each telescope, including detailed specifications and pricing, please visit website at https://whitestaroutdoors.com.

White Star Outdoors [emailprotected]

13 Belmont Road Australia

COMTEX_436680330/2824/2023-07-11T04:54:07

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Amateur Astronomers' Crucial Role in the Scientific Discovery of the ... - Digital Journal

Depictions of the Milky Way show a coiling pattern of spiral arms filled with stars extending outward from the center.

Similar patterns have been observed in the swirling clouds of gas and dust surrounding some young stars planetary systems in the making. These so-called protoplanetary disks, which are the birthplaces of young planets, are of interest to scientists because they offer glimpses into what the solar system may have looked like in its infancy and into how planets may form in general. Scientists have long thought that spiral arms in these disks could be caused by nascent planets, yet none had been detected until now.

In a paper published in Nature Astronomy, University of Arizona researchers report the discovery of a giant exoplanet, dubbed MWC 758c, that may be generating the spiral arms in its infant planetary system. The UArizona astronomers also propose possibilities as to why scientists have struggled to find this planet in the past, as well as how their methods may apply to detecting other concealed planets in similar circumstances.

Our study puts forward a solid piece of evidence that these spiral arms are caused by giant planets, said Kevin Wagner, lead author of the paper and a postdoctoral researcher at the UArizona Steward Observatory. And with the new James Webb Space Telescope, we will be able to further test and support this idea by searching for more planets like MWC 758c.

The planets star is located about 500 light-years away from Earth and is only a few million years old an embryo compared to our own 4.6-billion-year-old sun. Hence, the system still has a protoplanetary disk, as it takes about 10 million years for the circling debris to either be ejected out of the system, ingested by the star, or formed into planets, moons, asteroids and comets. The prominent spiral pattern in this systems debris was first discovered in 2013, and astronomers were quick to point out the connection to theoretical simulations of forming giant planets.

I think of this system as an analogy for how our own solar system would have appeared less than 1% into its lifetime, Wagner said. Jupiter, being a giant planet, also likely interacted with and gravitationally sculpted our own disk billions of years ago, which eventually led to the formation of Earth.

Astronomers have imaged most of the protoplanetary disks in stellar systems that are visible using current telescopes. Out of about 30 identified disks, around one-third feature spiral arms prominent swirls within the gas and dust particles of the disk.

Spiral arms can provide feedback on the planet formation process itself, Wagner said. Our observation of this new planet further supports the idea that giant planets form early on, accreting mass from their birth environment, and then gravitationally alter the subsequent environment for other, smaller planets to form.

Spiral arms are generated due to the orbiting companions gravitational pull on the material orbiting the star. In other words, the presence of a massive companion, such as a giant planet, was expected to trigger the spiral pattern in the disk. However, previous attempts to detect the responsible planet have turned up empty until now.

It was an open question as to why we hadnt seen any of these planets yet, Wagner said. Most models of planet formation suggest that giant planets should be very bright shortly after their formation, and such planets should have already been detected.

The UArizona researchers were finally able to detect MWC 758c by using the Large Binocular Telescope Interferometer, or LBTI, a UArizona-built instrument connecting the telescopes two 8.4-meter primary mirrors that can observe at longer wavelengths in the mid-infrared range, unlike most other instruments used for observing exoplanets at shorter, or bluer, wavelengths. According to Steve Ertel, a co-author on the paper and LBTI lead instrument scientist, the instrument has a camera that can detect infrared light in a similar manner to NASAs James Webb Space Telescope, or JWST.

Even though the exoplanet is estimated to be at least twice the mass of Jupiter, it was invisible to other telescopes because of its unexpected red color the reddest planet ever discovered, Ertel said. Longer, redder wavelengths are more difficult to detect than shorter wavelengths because of the thermal glow of Earths atmosphere and the telescope itself. The LBTI is among the most sensitive infrared telescopes yet constructed and due to its larger size, can even outperform JWST for detecting planets very close to their stars, such as MWC 758c.

We propose two different models for why this planet is brighter at longer wavelengths, Ertel said. Either this is a planet with a colder temperature than expected, or it is a planet thats still hot from its formation, and it happens to be enshrouded by dust.

If there is a lot of dust surrounding this planet, the dust will absorb shorter wavelengths, or bluer light, making the planet appear bright only at longer, redder wavelengths, said co-author Kaitlin Kratter, a UArizona theoretical astrophysicist. In the other scenario of a colder planet surrounded by less dust, the planet is fainter and emits more of its light at longer wavelengths.

Wagner said large amounts of dust in the planets vicinity may tip off that the planet is still forming, and that it might be in the process of generating a system of moons like the Jovian moons around Jupiter. On the flip side, if the planet follows the colder model, there might be something going on in these early stellar systems that causes planets to form colder than expected, prompting planetary scientists to revise their planet formation models and exoplanet detection strategies.

In either case, we now know that we need to start looking for redder protoplanets in these systems that have spiral arms, Wagner said.

The UArizona astronomers anticipate that once they observe the giant exoplanet with the James Webb Space Telescope, they will be able to make a judgement call as to which of the two scenarios is playing out in the infant system. The team has been granted time to use JWST in early 2024 to complete these observations.

Depending on the results that come from the JWST observations, we can begin to apply this newfound knowledge to other stellar systems, Wagner said, and that will allow us to make predictions about where other hidden planets might be lurking and will give us an idea as to what properties we should be looking for in order to detect them.

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Astronomers Discover Elusive Planet Responsible For Spiral Arms ... - Space Ref

NASA Astronomy Picture of the Day 7 January 2023: 2 Space Stations in Earth Orbit  HT Tech

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NASA Astronomy Picture of the Day 7 January 2023: 2 Space Stations in Earth Orbit - HT Tech

NASA Astronomy Picture of the Day 3 January 2023: Kemble's Cascade of Stars adorns the sky  HT Tech

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NASA Astronomy Picture of the Day 3 January 2023: Kemble's Cascade of Stars adorns the sky - HT Tech

Heres why you should look up at the stars with San Antonios astronomy clubs  San Antonio Report

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Heres why you should look up at the stars with San Antonios astronomy clubs - San Antonio Report

As the stars move across the sky each night people of the world have looked up and wondered about their place in the universe. Throughout history civilizations have developed unique systems for ordering and understanding the heavens. Babylonian and Egyptian astronomers developed systems that became the basis for Greek astronomy, while societies in the Americas, China and India developed their own.

Ancient Greek astronomers' work is richly documented in the collections of the Library of Congress largely because of the way the Greek tradition of inquiry was continued by the work of Islamic astronomers and then into early modern European astronomy. This section offers a tour of some of the astronomical ideas and models from ancient Greece as illustrated in items from the Library of Congress collections.

By the 5th century B.C., it was widely accepted that the Earth is a sphere. This is a critical point, as there is a widespread misconception that ancient peoples thought the Earth was flat. This was simply not the case.

In the 5th century B.C., Empedocles and Anaxagoras offered arguments for the spherical nature of the Earth. During a lunar eclipse, when the Earth is between the sun and the moon, they identified the shadow of the Earth on the moon. As the shadow moves across the moon it is clearly round. This would suggest that the Earth is a sphere.

Given that opportunities for observations of a lunar eclipse do not come along that often, there was also evidence of the roundness of the earth in the experiences of sailors.

When a ship appears on the horizon it's the top of the ship that is visible first. A wide range of astronomy texts over time use this as a way to illustrate the roundness of the Earth. As the image suggests this is exactly what one would expect on a spherical Earth. If the Earth were flat, it would be expected that you would be able to see the entire ship as soon as it became visible.

Lunar eclipses also allowed for another key understanding about our home here on Earth. In 3rd Century B.C., Aristarchus of Samos reasoned he could figure out the size of the Earth based on information available during a lunar eclipse. The diagram at the right illustrates a translation of his work. The large circle is the sun, the medium circle is the Earth and the smallest circle is the moon. When the Earth is in-between the sun and the moon it causes a lunar eclipse and measuring the size of the Earth's shadow on the moon provided part of the information he needed to calculate its size.

Eratosthenes estimated Earth's circumference around 240 B.C. He used a different approach, measuring the shadows cast in Alexandria and Syene to calculate their angle relative to the Sun. There is some dispute on the accuracy of his calculations as we don't know exactly how long the units of measure were. The measurement however was relatively close to the actual size of the Earth. The Greeks were applying mathematics to theorize about the nature of their world. They held a range of beliefs about nature and the world but they were, in many cases, working to ground those beliefs in an empirical exploration of what they could reason from evidence.

In the tradition of Plato and Empedocles before him, Aristotle argued that there were four fundamental elements, fire, air, water and earth. It is difficult for us to fully understand what this meant as today we think about matter in very different terms. In Aristotle's system there was no such thing as void space. All space was filled with some combination of these elements.

Aristotle asserted that you could further reduce these elements into two pairs of qualities, hot and cold and wet and dry. The combination of each of these qualities resulted in the elements. These qualities can be replaced by their opposites, which in this system become how change happens on Earth. For example, when heated, water seemingly turns steam which looks like air.

In Aristotle's Cosmology, each of these four elements (earth, water, fire and air) had a weight. Earth was the heaviest, water less so, and air and fire the lightest. According to Aristotle the lighter substances moved away from the center of the universe and the heaver elements settled into the center. While these elements attempted to sort themselves out, to achieve this order, most of experience involved mixed entities.

While we have seen earth, fire, air and water, everything else in the world in this system was understood as a mixture of these elements. In this perspective, transition and change in our world resulted from the mixing of the elements. For Aristotle the terrestrial is a place of birth and death, based in these elements. The heavens are a separate realm governed by their own rules.

In contrast to the terrestrial, the celestial region of the heavens had a fundamentally different nature. Looking at the night sky the ancient Greeks found two primary kinds of celestial objects; the fixed stars and the wandering stars. Think of the night's sky. Most of the visible objects appear to move at exactly the same speed and present themselves in exactly the same arrangement night after night. These are the fixed stars. They appear to move all together. Aside from these were a set of nine objects that behaved differently, the moon, the sun and the planets Mercury, Venus, Mars, Saturn and Jupiter each moved according to a different system. For the Greeks these were the wandering stars.

In this system the entire universe was part of a great sphere. This sphere was split into two sections, an outer celestial realm and an inner terrestrial one. The dividing line between the two was the orbit of the moon. While the earth was a place of transition and flux, the heavens were unchanging. Aristotle posited that there was a fifth substance, the quintessence, that was what the heavens were made of, and that the heavens were a place of perfect spherical motion.

In Aristotle's words, "In the whole range of time past, so far as our inherited records reach, no change appears to have taken place either in the whole scheme of the outermost heaven or in any of its proper parts." It's important to keep in mind that in Aristotle's time there simply were not extensive collections of observational evidence. Things that looked like they were moving in the heavens, like comets, were not problematic in this model because they could be explained as occurring in the terrestrial realm.

This model of the heavens came with an underlying explanation. The celestial spheres were governed by a set of movers responsible for the motion of the wandering stars. Each of these wandering stars was thought to have an "unmoved mover" the entity that makes it move through the heavens. For many of the Greeks this mover could be understood as the god corresponding to any given entity in the heavens.

Claudius Ptolemy (90-168) created a wealth of astronomical knowledge from his home in Alexandria, Egypt. Benefiting from hundreds of years of observation from the time of Hipparchus and Eudoxus, as well as a set of astronomical data collected by the Babylonians, Ptolemy developed a system for predicting the motion of the stars that was published in his primary astronomical work, Almagest. Ptolemy's success at synthesizing and refining ideas and improvements in astronomy helped make his Almagest so popular that earlier works fell out of circulation. Translated into Arabic and Latin the Almagest became the primary astronomy text for the next thousand years.

The Almagest is filled with tables. In this sense the book is a tool one can use to predict the locations of the stars Compared to earlier astronomy the book is much more focused on serving as a useful tool than as presenting a system for describing the nature of the heavens. Trying to accurately predict the place of the stars over time resulted in creating a much more complicated model.

By the time of Ptolemy Greek astronomers had proposed adding circles on the circular orbits of the wandering stars (the planets, the moon and the sun) to explain their motion. These circles on circles are called epicycles. In the Greek tradition, the heavens were a place of perfect circular motion, so the way to account for perfection was with the addition of circles. This resulted in disorienting illustrations.

To escape the complicated nature of this extensive number of circles, Ptolomy added a series of new concepts. To accurately describe planetary motion, he needed to use eccentric circles. With the eccentric circle the center of the planets orbit would not be Earth but would instead be some other point. Ptolemy then needed to put the epicycles on another set of circles called deferents. So the planets moved on circles that moved on circular orbits. Ptolomy also needed to introduce equants, a tool that enabled the planets to move at different speeds as they moved around these circles. The resulting model was complex, but it had extensive predictive power.

Ptolemy came to represent a mathematical tradition, one focused on developing mathematical models with predictive power. Aristotle came to be known for putting forward the physical model of the heavens. Ptolemy was also interested in deploying his model of the heavens to describe its physical reality. However, his most important work was the mathematical models and data he used for predicting the motion of heavenly bodies. For a long time his name was synonymous with the model of the heavens.

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